An alcohol in which the carbon bonded to the OH group is bonded to one other carbon atom (or none, as in methanol), such as butan-1-ol. Halogenoalkanes are classified by the same rule.
744 terms from the Chemistry syllabus, each defined in a sentence or two. Every term links to the part of the syllabus where you first meet it.
The range, written as ± in the same unit as the value, within which the true value of a measurement is expected to lie, such as .
The lowest possible temperature, (), at which particles have their minimum possible kinetic energy.
A measure of how much light of a chosen wavelength a solution absorbs; in dilute solution it is proportional to the concentration of the absorbing species.
A continuous spectrum crossed by dark lines at the wavelengths absorbed by atoms as their electrons are promoted to higher energy levels.
How close a measured or calculated result is to the true or accepted value; systematic errors reduce accuracy.
Describes a molecule that can be superimposed on its mirror image, such as one with no chiral carbon; achiral compounds are not optically active.
The deposition of acids formed from sulfur oxides and nitrogen oxides in rain, snow and fog (wet) or as gases and particles (dry). Acid rain has a pH below 5.6.
The equilibrium constant for the ionization of a weak acid in water, ; the larger its value, the stronger the acid.
A weak acid, HInd, whose un-ionized form and conjugate base have different colours; it changes colour over a pH range of roughly .
A buffer made of a weak acid and a salt of its conjugate base, such as ethanoic acid with sodium ethanoate; its pH is below 7.
An oxide, usually covalent and of a non-metal, that reacts with bases to form a salt and water; if soluble it gives an acidic solution, as does.
One of the elements actinium to lawrencium in period 7. In the data booklet actinium sits in group 3 and thorium to lawrencium form the second f-block row printed below the main table.
The minimum energy that colliding particles need for a reaction to occur; on an energy profile, the energy difference between the reactants and the transition state.
An electrode that takes part in the electrode reaction, such as a copper anode that is oxidized and dissolves during electrolysis.
A polymer formed by addition polymerization of monomers with a C=C double bond, such as poly(ethene), PVC and PTFE.
A reaction in which many monomers containing a C=C double bond join into one long saturated chain, with no other product, so the atom economy is 100 %.
A reaction in which two reactants combine to form a single product, as when a molecule adds across the double bond of an alkene.
A single species formed when a Lewis acid and a Lewis base combine through a coordination bond without any atoms being lost, such as from and .
A compound containing a hydroxyl group bonded to a saturated carbon atom, general formula , such as ethanol.
A compound with a carbonyl group at the end of a carbon chain, –CHO, general formula , such as ethanal, .
A base that dissolves in water to form hydroxide ions, , such as sodium hydroxide or aqueous ammonia. Every alkali is a base, but insoluble bases such as are not alkalis.
An element of group 1 other than hydrogen (lithium, sodium, potassium, rubidium, caesium, francium): a soft, reactive metal with one valence electron that forms ions.
An element of group 2 (beryllium, magnesium, calcium, strontium, barium, radium): a reactive metal with two valence electrons that forms ions.
A saturated hydrocarbon with only C–C and C–H single bonds and the general formula (for non-cyclic alkanes).
A hydrocarbon containing a C=C double bond, general formula , such as ethene, .
The functional group –O–R, an oxygen atom bonded to two carbon atoms; it defines the ethers.
A group formed by removing one H atom from an alkane, –, such as methyl, –, or ethyl, –; written R in general formulas.
A hydrocarbon containing a C≡C triple bond, general formula , such as ethyne, .
One of two or more structural forms of the same element in the same physical state, such as diamond, graphite, graphene and fullerenes for carbon; allotropes differ in bonding and so in properties.
A mixture of a metal with one or more other elements (usually metals), made by melting them together; different-sized atoms disrupt the layers, so alloys are usually harder than the pure metal.
A helium nucleus, , of two protons and two neutrons, emitted by some radioactive nuclei; used in the gold-foil experiment.
A compound containing an amido group, –CONH₂, such as ethanamide, .
The group that joins the residues of an acid and an amine in an amide or a polyamide.
The functional group –CONH₂ (or –CONH– when the N carries a carbon group): a nitrogen atom bonded to the carbon of a C=O; it defines the amides.
A compound containing an amino group, such as ethylamine, ; primary, secondary or tertiary by the number of carbon atoms bonded to the nitrogen.
An organic molecule with both an amino group () and a carboxyl group (); amino acids are the monomers of proteins.
The functional group –NH₂ (or –NHR, –NR₂) bonded to carbon; it defines the amines.
A measure of the number of specified particles in a sample, symbol , measured in moles.
Able to act as both a Brønsted–Lowry acid and a Brønsted–Lowry base, by donating or accepting a proton; examples are , and .
Able to react both as an acid and as a base. Aluminium oxide, , is an amphoteric oxide: it reacts with both acids and alkalis.
The substance or solution being analysed in a titration, usually measured into the conical flask with a pipette.
Containing no water; an anhydrous salt is the form left when the water of crystallization has been removed, such as white .
A negatively charged ion, with more electrons than protons; non-metals form anions by gaining electrons.
The electrode at which oxidation takes place. In a voltaic cell (including a fuel cell) it is the negative electrode.
The insoluble residue of less reactive metals, such as silver, gold and platinum, that collects beneath the anode during electrolytic refining.
A base taken to neutralize excess hydrochloric acid in the stomach, such as calcium carbonate, magnesium hydroxide or sodium hydrogencarbonate.
A compound that contains a benzene ring, such as methylbenzene, .
The equation , which describes how the rate constant depends on temperature and activation energy. Its linear form is .
The constant in the Arrhenius equation, which takes into account the frequency of collisions with the proper orientation. It has the same units as .
A graph of against : a straight line with gradient and intercept , used to determine the activation energy and the Arrhenius factor.
The molar mass of the desired product as a percentage of the total molar mass of all the reactants in the balanced equation; it measures how much of the reactants' mass can end up in the desired product.
The number of protons in the nucleus of an atom, symbol ; it identifies the element and gives its position in the periodic table.
A region of space around the nucleus with a high probability of finding an electron; each orbital holds a maximum of two electrons of opposite spin.
Half the distance between the nuclei of two neighbouring atoms of the same element bonded together. It decreases across a period and increases down a group.
Electrons occupy the orbitals of lowest available energy first; up to Z = 36 the order of filling is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p.
The reaction in which water molecules transfer protons to each other, ; it occurs to a very small extent and is endothermic.
The mean bond enthalpy of a particular bond taken over a range of compounds that contain it, as listed in the data booklet.
The mean of the electronegativities of two bonded elements, ; on the bonding triangle a low value indicates metallic bonding and a high value covalent bonding.
The change in concentration (or another measured quantity) divided by the time interval over which it occurs; the gradient of a chord on a graph against time.
The number of elementary entities in one mole, (fixed exactly at ).
Equal volumes of all gases, measured at the same temperature and pressure, contain equal numbers of molecules.
One of the two positions on the vertical axis of a trigonal bipyramid, 180° apart, each with three neighbouring electron domains at 90°.
A method in which a known excess of a reagent is added to a sample and the amount left over is found by titration; the amount that reacted with the sample is found by difference.
Approach of a nucleophile to a carbon atom from the side directly opposite the leaving group, as in the mechanism.
In a reversible reaction, the reaction from right to left as the equation is written, turning products back into reactants.
The equilibrium constant for the reaction of a weak base with water, ; the larger its value, the stronger the base.
The tallest peak in a mass spectrum, from the most abundant ion, assigned a relative abundance of 100.
A buffer made of a weak base and a salt of its conjugate acid, such as ammonia with ammonium chloride; its pH is above 7.
An oxide, usually ionic and of a metal, that reacts with acids to form a salt and water; if soluble it gives an alkaline solution, as does.
A vibration in which the angle between two bonds increases and decreases.
The way assessment criteria are applied: the marker chooses the level whose descriptor fits the work as a whole most fairly, even if not every point of it is met.
Describes an elementary step in which two particles collide and react (molecularity 2).
An ionic compound of only two elements, a metal and a non-metal, named with the cation first and the anion ending in "-ide", such as sodium chloride.
Able to be broken down into simpler substances by microorganisms in the environment.
A fuel similar to diesel, made by reacting vegetable oils or waste cooking oil with methanol to form smaller ester molecules.
Ethanol made by the fermentation of sugars from crops such as sugar cane, maize or sugar beet, used as a fuel on its own or blended with petrol.
A fuel produced from recently living material (biomass), whose carbon was fixed from the atmosphere by photosynthesis over a short period.
A fuel gas, mainly methane and carbon dioxide, produced by bacteria breaking down manure, sewage or food waste in the absence of oxygen.
One of the four regions of the periodic table (s, p, d and f), named after the sublevel that receives the last electron in the Aufbau order.
Vaporization throughout a liquid, with bubbles of vapour forming, at the temperature where the vapour pressure of the liquid equals the external pressure.
The temperature at which a liquid boils at a given external pressure; data tables quote it at 101.325 kPa.
A calorimeter in which a sample burns completely in pure oxygen inside a sealed steel vessel immersed in insulated water, giving accurate enthalpies of combustion.
The angle between two bonds from the same central atom, such as 104.5° for H–O–H in water.
The straight line joining the nuclei of two bonded atoms.
The separation of charge along a polar covalent bond, shown with partial charges or with an arrow pointing to the atom.
The energy needed to break one mole of a particular covalent bond in gaseous molecules to form gaseous atoms; always positive.
The distance between the nuclei of two covalently bonded atoms; more shared pairs between the same two atoms give a shorter bond.
The number of shared electron pairs between two bonded atoms (1 single, 2 double, 3 triple); in a resonance hybrid it can be fractional, such as 1.5 in benzene.
The idea that bonding changes gradually from covalent, through polar covalent, to ionic (and from covalent to metallic) rather than falling into separate categories; most real bonds have mixed character.
A pair of electrons shared between two atoms in a covalent bond.
A triangular diagram (in the data booklet) that classifies the bonding in an element or binary compound as ionic, polar covalent, covalent or metallic by plotting the electronegativity difference of its two elements against their average electronegativity.
An energy cycle for the formation of an ionic compound from its elements by way of gaseous atoms and gaseous ions, used to find a lattice enthalpy or another unknown term.
A concentrated aqueous solution of sodium chloride, electrolysed in industry to make chlorine, hydrogen and sodium hydroxide.
Breaking or shattering when struck rather than bending; ionic solids are brittle because a shifted layer brings ions of the same charge together and they repel.
An orange solution of bromine in water used to test for C=C double bonds: an alkene decolourizes it rapidly, even in the dark, while an alkane does not.
A species that donates a proton, , to another species in a reaction.
A species that accepts a proton, , from another species; it needs a lone pair of electrons to bond to the proton.
The amount of acid or alkali a buffer can absorb before its pH changes significantly; it is larger for more concentrated buffers and falls when a buffer is diluted.
The flat part of a pH curve, before equivalence for a weak acid titrated with a strong base, where both the weak acid and its conjugate base are present and the pH changes only slowly.
A solution that resists a change in pH when small amounts of acid or alkali are added; it contains a weak acid and its conjugate base, or a weak base and its conjugate acid.
A substance formed in a reaction alongside the desired product; unless a use is found for it, it is waste.
A graph of a measured property (such as absorbance) against the known concentrations of standard solutions, used to read off the concentration of an unknown sample.
An insulated container in which a reaction heats or cools a known mass of water or solution so that the heat transferred can be measured.
The measurement of the heat transferred in a chemical or physical change, usually from the temperature change of a known mass of water or solution.
An ion with a negative charge on a carbon atom, which carries a lone pair, such as .
An ion with a positive charge on a carbon atom that has only three bonds, such as . It is planar round the positive carbon; stability increases from primary to secondary to tertiary.
Removing carbon dioxide from the waste gases of power stations or factories, compressing it and storing it permanently, usually in porous rock deep underground.
The total mass of greenhouse gases released by producing and using a product or carrying out an activity, expressed as the equivalent mass of carbon dioxide.
Describes a fuel or activity that adds no net carbon dioxide to the atmosphere, for example because the carbon dioxide released was recently absorbed by photosynthesis. Biofuels are only approximately carbon neutral.
A C=O double bond whose carbon is bonded only to carbon or hydrogen atoms; found as –CHO in aldehydes and C–CO–C in ketones.
The functional group –COOH, a C=O and an OH on the same carbon atom; it defines the carboxylic acids.
A compound containing the carboxyl group, –COOH, general formula , such as ethanoic acid, .
A substance that increases the rate of a reaction by providing an alternative reaction pathway with a lower activation energy, and is chemically unchanged at the end of the reaction.
Loss of activity of a heterogeneous catalyst when a substance binds strongly to its surface and blocks the sites where reactants would bind, as lead does in catalytic converters.
A device in a vehicle exhaust in which platinum, palladium and rhodium on a ceramic honeycomb catalyse the conversion of carbon monoxide, unburnt hydrocarbons and nitrogen oxides into carbon dioxide, water and nitrogen.
The ability of atoms of an element to bond to each other to form chains and rings. Carbon shows it strongly because C–C bonds are strong.
The electrode at which reduction takes place. In a voltaic cell (including a fuel cell) it is the positive electrode.
A positively charged ion, with fewer electrons than protons; metals form cations by losing electrons.
Structural isomers with the same functional group but different carbon skeletons, such as butane and 2-methylpropane.
A reaction in which a reactive intermediate, such as a radical, is regenerated in each cycle of steps, so one initiation event leads to many product molecules.
The charge of an ion relative to its size; small, highly charged ions have a high charge density and attract electrons or other ions strongly.
The position of an atom in a molecule, defined by the atoms and groups around it; in NMR each different hydrogen environment gives one signal.
A representation of a reaction using formulas, with reactants on the left and products on the right of an arrow; when balanced, it has the same number of each kind of atom on both sides.
The study of the rates of chemical reactions, the factors that affect them and the mechanisms by which reactions take place.
The position of an NMR signal relative to the signal of TMS, symbol , in parts per million (ppm); it depends on the chemical environment of the hydrogen atoms.
Describes a molecule (or object) that cannot be superimposed on its mirror image, such as a molecule with one chiral carbon atom.
A carbon atom bonded to four different atoms or groups, marked with an asterisk (). A molecule with one chiral carbon exists as a pair of enantiomers.
The industrial electrolysis of brine, producing chlorine at the anode and hydrogen and sodium hydroxide at the cathode.
A compound of carbon, chlorine and fluorine, such as , once used as a refrigerant and aerosol propellant; in the stratosphere UV light releases chlorine atoms from it that destroy ozone.
A technique that separates the components of a mixture by their different distribution between a stationary phase and a mobile phase that moves through or over it.
The stereoisomer in which the two identical or similar groups lie on the same side of a C=C double bond or on the same face of a ring.
Stereoisomerism caused by restricted rotation about a C=C bond or a ring, possible only when each of the two carbon atoms involved carries two different atoms or groups.
A reaction set up to give a sudden visible change once a fixed amount of reaction has taken place, so the time taken measures the rate (rate is proportional to ).
An arrangement of equal spheres, such as the atoms of many metals, that leaves the least empty space; each atom has twelve nearest neighbours.
A system that can exchange energy but not matter with its surroundings, so no reactant or product enters or leaves.
The number in front of a formula in a balanced equation; the coefficients give the ratio of the amounts, in moles, of the substances that react and form.
The statistic , between 0 and 1, that measures how closely data points fit a trend line; a value of 1 means every point lies exactly on the line.
The number of collisions between particles per unit time in a given volume; it increases with concentration, pressure, surface area and temperature.
The relative orientation of particles when they collide; only some orientations bring the atoms that must bond into contact, so only those collisions can lead to reaction.
The theory that particles react only when they collide with a combined kinetic energy at least equal to the activation energy and with the proper orientation.
Presenting work as individual when it was produced with, or copied from, another student; a form of academic misconduct, unlike permitted collaboration.
Measuring the absorbance of a coloured solution at a chosen wavelength to find its concentration, using a calibration curve of standard solutions.
A circle of the visible colours (red, orange, yellow, green, blue, violet) with their wavelength ranges, printed in the data booklet, in which complementary colours lie opposite each other.
For a fixed amount of gas, , with temperatures in kelvin.
An exothermic reaction of a substance with oxygen. Complete combustion of a compound of carbon, hydrogen and oxygen in excess oxygen gives only carbon dioxide and water.
Finding the empirical formula of an organic compound by burning a weighed sample in excess oxygen and weighing the carbon dioxide and water formed; oxygen is found by difference.
The instruction word at the start of a question part, such as state, explain or deduce, which sets the type and depth of answer expected.
The colour opposite a given colour on the colour wheel. A substance that absorbs light of one colour appears the complementary colour.
A central metal ion surrounded by ligands bonded to it by coordination bonds, such as .
A material made of two or more distinct materials that stay separate within it, combining their properties, such as reinforced concrete or fibreglass.
A substance made of atoms of two or more different elements chemically bonded together in a fixed ratio; its properties differ from those of its elements.
Industrial conditions chosen to balance an acceptable equilibrium yield against an acceptable rate and cost, when the conditions that favour each conflict.
A reaction in which bond breaking and bond making happen at the same time in a single step, with no intermediate, as in the mechanism.
Titres that agree closely, usually within ; the mean of the concordant titres is used in the calculation.
The change of state from gas to liquid, which releases energy (exothermic).
A polymer formed by condensation reactions between monomers that each have two functional groups, with a small molecule (usually water or hydrogen chloride) released as each link forms.
A reaction in which two molecules join to form a larger molecule with the release of a small molecule such as water or hydrogen chloride.
An electron configuration in which the inner electrons are written as the preceding noble gas in square brackets, for example for potassium.
A formula that shows the atoms bonded to each carbon atom in order along the chain without drawing every bond, e.g. for butanoic acid.
The species formed when a base has gained one proton: the conjugate acid of is .
Two species that differ by a single proton, , such as and , or and .
The species left when an acid has lost one proton: the conjugate base of is .
The industrial manufacture of sulfuric acid, in which sulfur dioxide is oxidized to sulfur trioxide, , over a vanadium(V) oxide catalyst at about 450 °C and 1–2 atm.
A bond drawn through the bracket at each end of a repeating unit, showing where the unit joins the next one in the polymer chain.
A spectrum containing all wavelengths over a range with no gaps, such as white light from a filament lamp passed through a prism.
A variable that can take any value within its range, such as temperature, concentration or volume.
A variable that could affect the dependent variable and is therefore kept constant, so that only the independent variable is changed.
The narrowing of the spacing between successive lines of a spectral series at higher frequency, until they merge, because the energy levels get closer together at higher energy.
The frequency at which the lines of a spectral series merge; for the series ending on the ground state it corresponds to ionization of the atom.
A graph of temperature against time for a substance as it cools; a pure substance gives a flat plateau at its freezing point, an impure one a gradual slope.
A covalent bond in which both electrons of the shared pair come from the same atom, as in the fourth N–H bond of .
In a lattice, the number of nearest neighbours of opposite charge around an ion (six in sodium chloride); in a complex ion, the number of coordination bonds to the central ion.
The gradual oxidation of a metal at its surface by substances in its surroundings, such as oxygen and water.
The electrostatic attraction between a shared pair of electrons and the positively charged nuclei of the two bonded atoms.
A lattice of atoms joined by covalent bonds throughout, with no separate molecules, as in diamond, silicon and silicon dioxide.
A covalent bond joining two neighbouring polymer chains; cross-links make a polymer more rigid and stop it softening on heating.
A liquid fossil fuel made of a mixture of hundreds of hydrocarbons, separated by fractional distillation into fractions such as petrol, kerosene and diesel.
The formation of crystals of a solute from a saturated solution, usually by evaporating some solvent and then cooling slowly.
A curved arrow with a full arrowhead, used in mechanisms to show the movement of a pair of electrons, from a lone pair or bond to the atom or bond where the pair ends up.
A saturated hydrocarbon whose carbon atoms form a ring, general formula , such as cyclohexane, ; cycloalkanes are functional group isomers of the alkenes.
A voltaic cell made of a zinc electrode in zinc sulfate solution and a copper electrode in copper(II) sulfate solution, joined by a salt bridge; zinc is the anode.
The IB booklet of equations, constants, the periodic table and data tables (bond enthalpies, enthalpies of formation and combustion, electrode potentials, spectroscopic data and more) that every candidate has in every paper.
An investigation whose data are selected from an existing data set rather than measured by the student; its method describes how the data were chosen, filtered and sampled.
The number of multiple bonds and rings in a molecule; each mole of added in hydrogenation removes one degree of unsaturation.
The spreading of electrons over three or more atoms, so that they are not confined to one bond or one atom; delocalized pi electrons form where p orbitals on adjacent atoms overlap sideways.
The extra stability of a species with delocalized electrons compared with one localized structure; for benzene about , from enthalpies of hydrogenation.
An electron that is not attached to one atom or bond but is shared by many atoms and free to move through a structure, as in metals, graphite and benzene.
The variable that is measured in an experiment, which changes in response to the independent variable.
The change of state directly from gas to solid, which releases energy (exothermic), for example .
In NMR, the decrease in shielding of a hydrogen atom when nearby electronegative atoms or π electrons draw electron density away from it, moving its signal to higher chemical shift.
A solvent such as in which hydrogen atoms are replaced by deuterium, used for NMR so that the solvent gives no signal of its own.
The isotope of hydrogen with one proton and one neutron in its nucleus, or D; heavy water is .
An allotrope of carbon in which each atom is covalently bonded to four others tetrahedrally in a three-dimensional network; very hard and does not conduct electricity.
The net movement of particles from a region of higher concentration to one of lower concentration, caused by their random motion.
Adding solvent to a solution. The amount of solute stays the same, so .
The molecule formed when two amino acids join by a condensation reaction, linked by a peptide (amide) link, with the release of water.
A measure of the separation of positive and negative charge in a molecule; a molecule whose bond dipoles do not cancel has a net dipole moment.
An attraction between the permanent dipole of a polar molecule and the dipole it induces in a neighbouring non-polar particle.
An attraction between the permanent dipoles of neighbouring polar molecules.
Two quantities are directly proportional when doubling one doubles the other; a graph of one against the other is a straight line through the origin.
A variable that can take only certain separate values, such as the number of carbon atoms in a molecule.
A reaction in which a more reactive element takes the place of a less reactive one in a compound, for example .
A redox reaction in which atoms of the same element in one species are both oxidized and reduced, as in , where chlorine goes from 0 to −1 and to +1.
Separation of a liquid from a mixture by boiling it and condensing the vapour in a separate container; used for a solvent from dissolved solids or liquids with very different boiling points.
A wrong option in a multiple-choice question, written so that a particular mistake in reasoning or calculation leads to it.
A covalent bond made of two shared pairs of electrons; shorter and stronger than a single bond between the same atoms.
An NMR signal split into two lines of equal height (1 : 1) by one hydrogen atom on an adjacent carbon atom.
Able to be drawn out into a wire without breaking, a typical property of metals.
The state of a reversible reaction in a closed system in which the forward and backward reactions continue at equal rates, so the concentrations of all species remain constant.
The mass of waste produced per unit mass of product in a process, counting by-products, solvents, excess reagents and losses.
The bubbling seen when a gas is produced in a liquid, for example when a reactive metal reacts with a dilute acid.
A collision in which no kinetic energy is lost; the total kinetic energy of the particles is the same before and after.
The ability of a substance to carry an electric current, which needs mobile charged particles: delocalized electrons in metals and graphite, mobile ions in molten or dissolved ionic compounds.
An arrangement in which oxidation and reduction take place at two separate electrodes joined by an external circuit; voltaic and electrolytic cells are the two types.
A list of redox couples in order of their standard electrode potentials; oxidizing agents become stronger towards the positive end and reducing agents towards the negative end.
A conductor, usually a metal or graphite, through which electrons enter or leave the electrolyte in an electrochemical cell; oxidation or reduction takes place at its surface.
The decomposition of a molten or dissolved ionic compound (an electrolyte) by a direct electric current, with ions gaining or losing electrons at the electrodes.
A substance that conducts electricity when molten or in aqueous solution, because it contains mobile ions, and is decomposed by the current.
An electrochemical cell in which an external dc supply drives a non-spontaneous redox reaction, converting electrical energy into chemical energy; its anode is positive and its cathode negative.
Purifying a metal by electrolysis with the impure metal as the anode and a thin sheet of pure metal as the cathode, in a solution of the metal's ions.
The full range of electromagnetic radiation, from gamma rays (shortest wavelength, highest frequency and photon energy) through X-rays, ultraviolet, visible light, infrared and microwaves to radio waves (longest wavelength).
A subatomic particle with a relative charge of −1 and a negligible mass (about 1/1836 of a proton), occupying the space around the nucleus.
The arrangement of the electrons of an atom or ion in sublevels, written with superscripts for the number of electrons in each, for example for sodium.
A region of electron density around a central atom: a lone pair, a single bond or a multiple bond each count as one domain.
The arrangement of all the electron domains, bonding and non-bonding, around a central atom: linear, trigonal planar or tetrahedral for two, three or four domains.
A property of an electron with two possible values, drawn as up and down arrows; two electrons in the same orbital must have opposite spins.
A measure of the ability of an atom to attract a shared pair of electrons in a covalent bond; it has no units (Pauling scale).
The difference between the electronegativities of two bonded elements, ; the larger it is, the greater the ionic character of the bond.
A reactant that forms a bond to its reaction partner (the nucleophile) by accepting both bonding electrons from it; it is electron-deficient and may be positive, such as and , or neutral, such as and .
An addition reaction in which an electrophile attacks the electron-rich C=C double bond of an alkene and the two parts of the reagent add to the two carbon atoms, giving a saturated product.
A reaction in which an electrophile replaces an atom (usually hydrogen) on an aromatic ring, through a carbocation intermediate, the ring keeping its delocalized π system; for example the nitration of benzene.
Using electrolysis to coat an object with a thin layer of metal; the object is the cathode, the plating metal the anode, and the electrolyte contains ions of the plating metal.
A substance made of only one kind of atom (all with the same atomic number), which cannot be broken down into simpler substances by chemical reactions.
The particle being counted in a mole: an atom, molecule, ion, electron, formula unit or other specified particle or group of particles.
A single molecular event in a reaction mechanism: one collision, or one particle breaking apart, in which bonds are broken and formed.
The light emitted by excited atoms, seen as bright lines of specific wavelengths on a dark background; each line comes from electrons falling between two energy levels.
A formula that gives the simplest whole-number ratio of the atoms of each element in a compound.
One of a pair of stereoisomers that are non-superimposable mirror images of each other. Enantiomers rotate plane-polarized light by equal angles in opposite directions.
The point in a titration at which the indicator changes colour (or a self-indicating titrant gives a permanent colour); ideally it coincides with the equivalence point.
Describes a change that absorbs energy from the surroundings, such as melting, vaporization or sublimation; the surroundings get colder and is positive.
A diagram showing two or more routes between the same initial and final states, used with Hess's law to calculate an unknown enthalpy change.
The energy released per unit volume of a fuel when it burns, for example in ; it equals the specific energy multiplied by the density.
One of the fixed (quantized) energies that an electron in an atom can have; electrons can move between levels but cannot have energies in between.
A graph of potential energy against reaction coordinate showing the reactants, the transition state and the products, with the activation energy and .
The additional warming caused by increased concentrations of greenhouse gases in the atmosphere as a result of human activities, mainly the burning of fossil fuels.
The heat content of a system at constant pressure, symbol . Only changes in enthalpy can be measured.
The heat transferred between a system and its surroundings at constant pressure, ; negative for exothermic and positive for endothermic changes.
The enthalpy change when hydrogen adds to the double bonds of one mole of an unsaturated compound to give the saturated compound, for example about for cyclohexene.
The enthalpy change when an acid and a base react to form one mole of water; about for a strong acid with a strong base.
The enthalpy change when one mole of a substance dissolves in a solvent, usually water, to form a dilute solution; it can be exothermic or endothermic, for example for ammonium nitrate.
The enthalpy change when one mole of gaseous atoms is formed from an element in its standard state, for example .
A measure of the dispersal or distribution of matter and/or energy in a system, symbol , in (or ). The more ways the particles can be arranged and their energy distributed, the higher the entropy.
The change in entropy of the surroundings caused by heat transferred from or to a reaction at temperature : .
A biological catalyst: a protein that catalyses a specific reaction in living cells, working best at body temperature and close to neutral pH.
One of the three positions around the middle of a trigonal bipyramid, at 120° to each other; lone pairs occupy these positions because they have only two neighbouring domains at 90°.
The constant given by the equilibrium law for a reaction at a particular temperature; its size shows the extent of reaction, and it changes only with temperature.
The fraction for written from a balanced equation: products over reactants, each concentration in square brackets raised to the power of its coefficient.
At equilibrium at a given temperature, the product concentrations raised to the powers of their coefficients, divided by the reactant concentrations raised to the powers of theirs, is a constant, .
The mixture of reactants and products present in a system at equilibrium, whose composition stays constant while conditions are unchanged.
The point in a titration at which the reactants have been added in exactly the mole ratio of the balanced equation.
Hydrogen atoms in the same chemical environment, related by the symmetry of the molecule or by free rotation; they give a single NMR signal and do not split each other.
A line drawn through a plotted point on a graph to show the range of its uncertainty.
The marking rule that a wrong answer used correctly in a later step does not lose the later marks, so correct method on a wrong earlier value still scores.
A compound containing the ester group, R–COO–R′, made from a carboxylic acid and an alcohol and named alkyl alkanoate, such as ethyl ethanoate.
The functional group –COO–: a C=O whose carbon is also bonded to an oxygen atom that is bonded to another carbon atom.
The group that joins the residues of an acid and an alcohol in an ester or a polyester.
A compound with an oxygen atom bonded to two carbon atoms, R–O–R′, general formula , such as methoxymethane, .
Vaporization from the surface of a liquid at any temperature below its boiling point, as the most energetic particles escape; it cools the remaining liquid. Also used to remove a solvent from a solution.
A reactant present in more than the amount needed to react with the limiting reactant, so some of it remains when the reaction is complete.
An arrangement of electrons with more energy than the ground state, in which one or more electrons have been promoted to a higher energy level.
Describes a change that releases energy to the surroundings, such as freezing, condensation or deposition; the surroundings get warmer and is negative.
A valence shell with more than eight electrons around a central atom, as in (10) and (12); possible only for atoms of period 3 and beyond.
The amount or mass of product actually obtained from a reaction.
How far a reaction has gone towards products when equilibrium is reached; measured by the size of .
The wire and any meter or device that connect the two electrodes of a cell; electrons flow through it from the anode to the cathode.
Extending a line or trend on a graph beyond the measured data, for example extending a cooling line back to the time of mixing to correct for heat loss.
The charge carried by one mole of electrons, ; it is the Avogadro constant multiplied by the charge of one electron.
The liquid or solution that passes through the filter paper during filtration.
Separation of an insoluble solid from a liquid or solution by passing the mixture through filter paper, which holds back the solid particles.
The part of an IR spectrum below about , with a complex pattern of absorptions unique to each compound; it is used to identify a compound by matching with a reference spectrum.
The energy change when one mole of electrons is added to one mole of gaseous atoms to form one mole of gaseous 1− ions: . First electron affinities are usually exothermic.
The minimum energy needed to remove one mole of electrons from one mole of gaseous atoms in their ground state, , in .
Describes a reactant for which the rate is proportional to its concentration: doubling the concentration doubles the rate. Its concentration falls with a constant half-life.
A curved arrow with half an arrowhead, used in mechanisms to show the movement of one electron, as in homolytic fission.
A test in which a compound is heated in a non-luminous flame; the colour of the flame, produced by the emission spectrum of the metal, identifies metal ions such as lithium, sodium, potassium, calcium and copper.
Removing sulfur dioxide from the waste gases of power stations before release, usually by reaction with a slurry of calcium oxide or calcium carbonate: .
The charge an atom in a Lewis formula would have if all bonding pairs were shared equally: (valence electrons minus lone-pair electrons minus half the bonding electrons).
The group of ions or atoms given by the formula of a substance that is not made of molecules, such as one and one in .
In a reversible reaction, the reaction from left to right as the equation is written, turning reactants into products.
A fuel formed over millions of years from the buried remains of plants and other organisms: coal, crude oil or natural gas. Fossil fuels are non-renewable.
Distillation through a packed fractionating column, in which vapour repeatedly condenses and re-evaporates, to separate miscible liquids with similar boiling points; the most volatile component distils first.
A positive ion formed when a molecular ion breaks up; it gives a peak at the of the molecular ion minus the mass of the neutral fragment lost.
The breaking of a molecular ion in a mass spectrometer into a positive fragment ion, which is detected, and a neutral species (often a radical), which is not.
A substitution reaction that proceeds by a radical chain mechanism (initiation, propagation, termination), such as the reaction of an alkane with chlorine or bromine in UV light.
The change of state from liquid to solid, which releases energy (exothermic).
The number of waves passing a fixed point per second, symbol , in or Hz; it is inversely proportional to wavelength, .
A substance that releases useful energy when it reacts, usually by burning in oxygen.
A voltaic cell in which a fuel, such as hydrogen or methanol, and an oxidant, usually oxygen, are supplied continuously to two electrodes, converting chemical energy directly into electrical energy.
A formula that shows every atom in a molecule and every bond between the atoms, usually drawn flat with bonds at 90°.
A molecular allotrope of carbon made of closed cages of carbon atoms, such as ; the molecules are held together only by London forces.
An atom or group of atoms in a molecule that gives the compound its characteristic chemical properties; organic compounds are classified by their functional groups.
Structural isomers with different functional groups, such as ethanol and methoxymethane ().
The part of an IR spectrum above about , where absorptions can be assigned to particular bonds and functional groups using a table of characteristic wavenumbers.
Coating iron or steel with a layer of zinc. The zinc is oxidized in preference to the iron, so it protects the iron even where the coating is scratched.
The constant in the ideal gas equation, .
A formula in terms of n that fits every member of a homologous series, such as for the alkanes.
A property of a system, , that combines its enthalpy and entropy; at constant temperature and pressure a system changes spontaneously towards lower Gibbs energy.
The quantity for a change at constant temperature and pressure, in . A negative value means the change is spontaneous; at equilibrium .
The warming caused by a given mass of a gas over a stated period, usually 100 years, relative to the same mass of carbon dioxide (GWP = 1).
A single layer of graphite, one atom thick: a very strong, light, flexible sheet that conducts electricity and heat well.
An allotrope of carbon made of layers of hexagonal rings, each atom bonded to three others with one delocalized electron; the layers are held by London forces, so it is soft and conducts electricity.
A method of analysis in which the amount of a substance is found by weighing, for example by precipitating it as an insoluble solid that is filtered, dried and weighed.
The design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances, for example by high atom economy, safer solvents, catalysts, lower energy use and renewable feedstocks.
The warming of the Earth's surface and lower atmosphere caused by gases that absorb infrared radiation emitted by the surface and re-emit some of it back towards the surface.
A gas in the atmosphere that absorbs infrared radiation, such as carbon dioxide, methane, water vapour, dinitrogen monoxide, ozone and CFCs.
The lowest-energy arrangement of the electrons in an atom or ion.
A vertical column of the periodic table, numbered 1–18. Elements in the same group have the same number of valence electrons and similar chemical properties.
The industrial synthesis of ammonia, , using an iron catalyst at about 400–450 °C and about 200 atm, with unreacted gases recycled.
One half of a voltaic cell: an electrode in contact with an electrolyte, such as a metal dipping into a solution of its own ions, where either oxidation or reduction occurs.
An equation showing either the oxidation or the reduction part of a redox reaction, with the electrons lost or gained, for example .
The point in a titration at which half the volume needed for equivalence has been added. For a weak acid, there, so .
The time taken for the concentration of a reactant to fall to half of its value. It is constant for a first-order reactant.
An element of group 17 (fluorine, chlorine, bromine, iodine, astatine): a reactive non-metal with seven valence electrons that exists as diatomic molecules and forms ions.
A Lewis acid catalyst such as or that accepts an electron pair from a halogen molecule, polarizing it so that it can substitute into a benzene ring.
A reaction that introduces one or more halogen atoms into a molecule, for example the chlorination of methane to chloromethane.
The functional group –X, where X is F, Cl, Br or I, bonded to carbon; it defines the halogenoalkanes.
A compound in which one or more H atoms of an alkane are replaced by halogen atoms, general formula , such as bromoethane.
Energy transferred between two bodies because of a difference in temperature, from the hotter to the colder; symbol , measured in joules.
A graph of temperature against time for a substance heated at a constant rate; the flat parts (plateaus) show changes of state, during which the temperature is constant.
Repeated heating, cooling and weighing of a sample until two consecutive masses agree, showing that a change such as the loss of water is complete.
The logarithmic form of the expression used for buffers: .
The enthalpy change for a reaction is independent of the route taken between the initial and final states; a consequence of the conservation of energy.
A catalyst in a different phase from the reactants, usually a solid on whose surface gases or dissolved substances react, such as iron in the Haber process.
An equilibrium involving species in more than one phase, such as .
A mixture whose composition is not uniform, with two or more regions (phases) of different composition, for example sand in water or oil and water.
The breaking of a covalent bond in which both bonding electrons stay with one fragment, forming a cation and an anion, for example .
A voltmeter that lets almost no current flow, so that no reaction occurs in the cell and the maximum potential difference between its electrodes is measured.
An apparatus with two graduated tubes joined to a central reservoir, used to electrolyse water and collect and measure the hydrogen and oxygen formed.
A catalyst in the same phase as the reactants, such as in esterification.
An equilibrium in which all the reactants and products are in the same phase, for example all gases or all in one solution.
A mixture with a uniform composition and properties throughout (one phase), so that any sample has the same ratio of components, for example a salt solution, air or brass.
A family of compounds with the same functional group and general formula, in which successive members differ by ; members have similar chemical properties and a gradual trend in physical properties.
The breaking of a covalent bond in which each atom keeps one electron of the shared pair, forming two radicals or atoms.
Electrons occupy the orbitals of a sublevel singly, with parallel spins, before any orbital gains a second electron.
One of a set of equivalent orbitals formed by mixing s and p orbitals on an atom; each has one large lobe pointing in a definite direction.
The mixing of atomic orbitals on one atom to form an equal number of new, equivalent hybrid orbitals used to form sigma bonds and hold lone pairs.
An ion surrounded by water molecules attracted to it: the oxygen atoms face a cation and the hydrogen atoms face an anion.
A salt whose crystals contain water of crystallization, such as blue copper(II) sulfate pentahydrate, .
The addition of water across a C=C double bond to form an alcohol, as in with an acid catalyst.
The ion , a hydrogen atom with an extra electron. Supplied by reducing agents, it adds to the partially positive carbon of a carbonyl group, reducing it.
A compound that contains carbon and hydrogen only, such as methane, , or ethene, .
A compound of carbon, hydrogen and fluorine, such as , used in place of CFCs; it contains no chlorine so does not destroy ozone, but many are strong greenhouse gases.
An attraction between a hydrogen atom covalently bonded to N, O or F and a lone pair on an N, O or F atom of a neighbouring molecule; the strongest intermolecular force.
The addition of hydrogen across a carbon–carbon double or triple bond, using a metal catalyst such as nickel; it is a reduction that lowers the degree of unsaturation.
A reaction in which a bond is broken by reaction with water (or hydroxide ions), as when a halogenoalkane is converted into an alcohol.
The functional group –OH covalently bonded to a carbon atom that carries no C=O; it defines the alcohols. It is not the hydroxide ion.
A provisional, testable explanation of an observation or relationship, from which a specific prediction about the results can be made.
A table of the initial amounts, the changes (in the ratio of the coefficients) and the equilibrium amounts of each species, used to solve equilibrium problems.
A model gas whose particles are in continuous random motion, have negligible volume, exert no intermolecular forces and collide perfectly elastically; it obeys exactly.
: the relationship between the pressure (Pa), volume (), amount (mol) and absolute temperature (K) of an ideal gas.
Describes liquids that do not mix and form separate layers, such as oil and water.
Combustion in a limited supply of oxygen. For an organic compound it produces carbon monoxide and/or carbon (soot) as well as water, and releases less energy than complete combustion.
A central atom with fewer than eight valence electrons in a molecule, as for Be in (four) and B in (six).
The variable that is deliberately changed in an experiment to see its effect on another variable.
A dipole created in an atom or molecule when a nearby dipole distorts its electron cloud.
The shift of electron density along σ bonds towards a more electronegative atom or away from an electron-releasing group such as an alkyl group. Electron-withdrawing chlorine atoms make chloroethanoic acids stronger than ethanoic acid.
An electrode, usually graphite or platinum, that conducts electricity but does not itself take part in the reaction at its surface.
A technique that measures which frequencies of infrared radiation a sample absorbs; each absorption comes from a bond vibration, so the spectrum shows which bonds and functional groups are present.
The instantaneous rate at the start of a reaction (), found from the gradient of the tangent to the curve at .
Finding orders of reaction by measuring the initial rate in several runs at the same temperature, changing the concentration of one reactant at a time.
The first step of a radical chain reaction, in which radicals are formed from a molecule with none, usually by homolytic fission in UV light or heat: .
The rate of reaction at one particular moment, equal to the gradient of the tangent to a concentration–time curve at that time.
A stepped line on an NMR spectrum whose step heights are proportional to the areas under the signals, and so to the relative numbers of hydrogen atoms in each environment.
A property that does not depend on the amount of substance, such as temperature, density or electrode potential.
Forces of attraction between separate molecules (or atoms of a noble gas), such as London (dispersion) forces, dipole–dipole forces and hydrogen bonds; much weaker than covalent bonds.
Reading a value from a graph between the measured data points, within the range over which the relationship has been tested.
An alloy in which much smaller atoms of the added element occupy the spaces between the metal atoms, as carbon does in steel.
Two quantities are inversely proportional when doubling one halves the other, so their product is constant; a graph of one against the reciprocal of the other is a straight line through the origin.
The turning inside out of the arrangement of groups round a carbon atom during an reaction, so that the product has the opposite three-dimensional arrangement to the reactant.
A charged particle formed when an atom or a group of atoms loses or gains electrons; the number of protons does not change.
, equal to at 298 K; it applies to every aqueous solution and increases with temperature.
The electrostatic attraction between oppositely charged ions.
A compound made of cations and anions held together by ionic bonding in a giant lattice, typically formed between a metal and a non-metal.
An equation that shows only the ions and other species that take part in a reaction, leaving out spectator ions; both atoms and charges must balance.
A giant three-dimensional regular arrangement of alternating cations and anions, each surrounded by ions of the opposite charge and held by non-directional electrostatic attraction.
The radius of an ion; cations are smaller than their atoms and anions are larger than their atoms.
The complete removal of one or more electrons from an atom or ion, forming a positive ion; drawn on an energy-level diagram as an upward arrow to .
Describes a vibration that changes the dipole moment of a molecule; only IR-active vibrations absorb infrared radiation, so and do not absorb while and do.
Describes atoms or ions with the same number of electrons, and so the same electron configuration, such as , , and .
One of two or more compounds with the same molecular formula but different structures.
Atoms of the same element (same number of protons) with different numbers of neutrons, and so different mass numbers; they have the same chemical properties but different physical properties.
An isotope, often a heavier or radioactive one, used to follow atoms through a reaction or process, because it reacts identically but can be detected by its mass or radiation.
The rules, agreed by the International Union of Pure and Applied Chemistry, for giving systematic names to organic and inorganic compounds.
A structure of benzene as a six-membered ring of carbon atoms with alternating single and double bonds; it describes one resonance structure, not the real molecule.
The SI unit of temperature. The Kelvin scale starts at absolute zero and has degrees of the same size as Celsius degrees: .
A compound with a carbonyl group bonded to two carbon atoms, general formula (n ≥ 3), such as propanone, .
The energy a particle has because of its motion, . The average kinetic energy of the particles in a substance is proportional to its absolute temperature.
A model in which all matter is made of particles in constant random motion, with attractions between them; it explains the properties of solids, liquids and gases and changes of state.
The persistence of a substance that is energetically unstable relative to its products but reacts extremely slowly because the activation energy is high.
One of the elements lanthanum to lutetium in period 6. In the data booklet lanthanum sits in group 3 and cerium to lutetium form the first f-block row printed below the main table.
The standard enthalpy change when one mole of a solid ionic compound is separated into its gaseous ions, e.g. ; it is positive and measures the strength of the ionic bonding.
Energy cannot be created or destroyed, only transferred or converted from one form to another, so the total energy of a system and its surroundings is constant.
In a chemical reaction, mass is neither created nor destroyed: the total mass of the products equals the total mass of the reactants, because atoms are only rearranged.
If a system at equilibrium is subjected to a change in conditions, the position of equilibrium shifts in the direction that partly opposes the change.
A secondary cell with a lead negative electrode and a lead(IV) oxide positive electrode in sulfuric acid; both electrodes form lead(II) sulfate on discharge, and each cell gives about 2 V.
The atom or group that leaves a substrate in a substitution reaction, taking the bonding pair with it, such as a halide ion from a halogenoalkane.
An electron-pair acceptor. Examples include , , and transition element ions such as ; many Lewis acids have no proton to donate.
An electron-pair donor: a species with a lone pair that it shares to form a coordination bond, such as , , or . Every Brønsted–Lowry base is also a Lewis base.
A diagram of a covalently bonded species that shows all its valence electrons: bonding pairs between the atoms and lone pairs on the atom that owns them, as dots, crosses or lines.
A molecule or ion with a lone pair of electrons that forms a coordination bond to a central metal ion, such as , , or .
A reaction in which the ligands of a complex ion are replaced by other ligands, such as .
A restriction on how widely the conclusion of an investigation applies, set by the range of the data, the system studied or the assumptions made.
The reactant that is completely used up in a reaction, so it determines the amount of product formed; the others are in excess.
The straight line or smooth curve drawn as close as possible to all the plotted points, with the points scattered evenly either side of it.
A spectrum containing only certain separate wavelengths, seen as sharp lines; the pattern of lines is characteristic of each element.
Rearranging a non-linear relationship so that a graph of two processed quantities should be a straight line, such as plotting against for the Arrhenius equation.
A value of a quantity taken from a reliable published source, such as a data booklet or a data table, used to judge the accuracy of an experimental result.
A secondary cell in which lithium ions move through an organic electrolyte between a graphite electrode and a metal oxide electrode such as ; it gives about 3.7 V.
A number in a name that gives the position of a functional group, double bond or substituent on the parent chain, as the 2 in propan-2-ol.
A scale on which the logarithm of a quantity is plotted, so that equal steps represent equal factors (such as ×10); used for data spanning several orders of magnitude, such as ionization energies.
An attraction between a temporary dipole and the dipole it induces in a neighbouring particle; it acts between all molecules and increases with the number of electrons and the contact area.
A pair of valence electrons that belongs to one atom only and is not shared in a bond.
The shine of a freshly cut or polished metal surface, which reflects most visible light falling on it.
A small peak one mass unit above the molecular ion peak, caused by molecules containing a carbon-13 atom; its height is about 1.1 % of the molecular ion peak per carbon atom.
A very large molecule containing thousands of atoms or more, such as a polymer, a protein or DNA.
A property of a bulk sample that can be observed or measured, such as colour, pressure, density or pH; at equilibrium these stay constant.
A medical imaging technique that uses the NMR signals of hydrogen nuclei in water and fat to build images of soft tissue, without ionizing radiation.
A group of electron energy states with the same principal quantum number ; level holds a maximum of electrons.
The product formed in the greater amount when a reaction can give more than one product; the other is the minor product. In additions to unsymmetrical alkenes it forms through the more stable carbocation.
Able to be hammered or pressed into a new shape without breaking; metals are malleable because their non-directional bonding lets layers of cations slide.
When HX or water adds to an unsymmetrical alkene, the hydrogen atom adds to the double-bond carbon that already has more hydrogen atoms. It is explained by the greater stability of the more substituted carbocation.
The examiners' list of the points that earn marks for each question part, with notes on which alternative answers to accept or reject; one tick (✓) is one mark.
The mass of solute per unit volume of solution, in ; equal to the molar concentration multiplied by the molar mass.
The total number of protons and neutrons in the nucleus of an atom, symbol ; always a whole number.
A technique that ionizes the particles of a sample and separates the ions by their mass-to-charge ratio; it is used to detect the isotopes of an element and measure their relative abundances and isotope ratios.
The output of a mass spectrometer: relative abundance against mass-to-charge ratio, . For an element each peak is one isotope and its height is the relative abundance.
The quantity plotted on the horizontal axis of a mass spectrum; for ions with a charge of 1+ it equals the mass of the ion.
A graph of the number of particles against kinetic energy for a sample at one temperature: it starts at the origin, has an asymmetric peak and a tail that never meets the energy axis.
The change of state from solid to liquid, which absorbs energy (endothermic).
The temperature at which a solid turns into a liquid; a pure substance melts sharply at a fixed melting point, an impure one over a range of lower temperatures.
An electrolytic cell for the chlor-alkali process in which a membrane that lets only cations through separates the anode and cathode compartments, so chlorine and sodium hydroxide are kept apart.
The curved surface of a liquid in a narrow tube. For water and aqueous solutions the volume is read at the bottom of the curve, with the eye level with the mark.
A compound of hydrogen with a metal, such as or , in which hydrogen has an oxidation state of −1.
The electrostatic attraction between a lattice of positive metal ions (cations) and delocalized electrons.
How readily an element behaves as a metal: losing electrons to form cations and forming basic oxides. It increases down a group and decreases across a period.
The regular, closely packed three-dimensional arrangement of cations in a metal, surrounded by a sea of delocalized electrons.
An element with properties intermediate between those of metals and non-metals: boron, silicon, germanium, arsenic, antimony and tellurium.
A feature of an investigation's procedure, such as poor control of a variable or imprecise measurement, that reduces the quality of its data.
A fragment of plastic less than 5 mm across, formed when larger plastic items break up or made deliberately small; microplastics persist in oceans, soils and organisms.
One particular way of arranging the particles of a system and distributing their energy. The more microstates available, the higher the entropy.
Describes liquids that mix completely in any proportion to form a single phase, such as ethanol and water.
Two or more elements or compounds that are not chemically bonded together and are present in no fixed ratio; each component keeps its own properties and they can be separated by physical methods.
The phase in chromatography that moves through or over the stationary phase, carrying the components of the mixture with it, usually a solvent or a gas.
The re-marking of a sample of internally assessed work by the IB, so that every school's marks are brought to the same standard.
The amount of solute per unit volume of solution, , in ; written with square brackets, e.g. .
The mass of one mole of a substance, symbol , in ; numerically equal to its relative formula mass. .
The volume occupied by one mole of a gas, ; for an ideal gas at STP it is .
The SI unit of amount of substance. One mole contains exactly elementary entities (the Avogadro constant).
The ratio of the amounts, in moles, of two substances in a reaction, given by their coefficients in the balanced equation.
A formula that gives the actual number of atoms of each element in one molecule of a compound; a whole-number multiple of the empirical formula.
The arrangement of the atoms (not the lone pairs) around a central atom, such as bent, trigonal pyramidal or tetrahedral.
The positive ion formed when a whole molecule loses one electron in a mass spectrometer, such as or ; its peak is at an equal to the relative molecular mass of that molecule.
The number of reacting particles (molecules, atoms, ions or radicals) that take part in a single elementary step.
A small molecule that joins with many others of the same kind (or of a second kind) to form a polymer.
An acid that can donate one proton per molecule, such as , or ; sulfuric acid, which can donate two, is diprotic.
The international agreement of 1987 that phased out the production of CFCs and other ozone-depleting substances.
The kinetic energy possessed by the largest number of particles in a sample, at the peak of the Maxwell–Boltzmann distribution; it is slightly lower than the mean kinetic energy.
An NMR signal split into five or more lines, from hydrogen atoms with four or more hydrogen atoms on adjacent carbon atoms.
A signal from hydrogen atoms with hydrogen atoms on the adjacent carbon atom(s) is split into lines.
A gaseous fossil fuel consisting mostly of methane, with small amounts of other alkanes, carbon dioxide and nitrogen.
A polymer made by living organisms, such as starch, cellulose, proteins, DNA and natural rubber.
The reaction of an acid with a base to form a salt and, usually, water; for strong acids and bases in water the ionic equation is .
An uncharged subatomic particle in the nucleus with a relative mass of 1.
A mixture of concentrated nitric acid and concentrated sulfuric acid, which generates the nitronium ion and is used to nitrate benzene below about 50 °C.
The linear cation (O=N⁺=O), the electrophile in the nitration of benzene, formed from concentrated nitric and sulfuric acids.
An element of group 18 (helium, neon, argon, krypton, xenon, radon): a very unreactive, monatomic gas with a full outer main energy level.
An electron configuration the same as that of a noble gas, with a full outer level (eight electrons, or two for the helium arrangement); main-group ions usually have one.
Bonding in which the attraction acts equally in all directions, as in ionic and metallic bonding, so each particle is not bonded to particular neighbours.
A molecule with no net dipole moment, because its bonds are non-polar or its bond dipoles cancel by symmetry, such as and .
A finite energy source that forms far more slowly than it is used, or not at all, such as coal, crude oil, natural gas and nuclear fuels.
The total positive charge of the nucleus, equal to the number of protons; a greater nuclear charge attracts the electrons more strongly.
The absorption of radio-frequency radiation by nuclei such as in a strong magnetic field, which flips the nuclear spin from the lower-energy to the higher-energy orientation.
The model of the atom as a tiny, dense, positively charged nucleus containing almost all the mass, with electrons occupying the much larger space around it; proposed by Rutherford in 1911 from alpha-particle scattering.
The notation for an atom or ion, with the mass number at the top left, the atomic number at the bottom left and any charge at the top right.
A particle in the nucleus: a proton or a neutron.
A reactant that forms a bond to its reaction partner (the electrophile) by donating both bonding electrons; it has a lone pair and may be neutral, such as and , or negative, such as .
A reaction in which a nucleophile donates an electron pair to form a new bond to an atom (usually a δ+ carbon) while another bond to that atom breaks and releases a leaving group, as in .
The very small, dense, positively charged centre of an atom, containing its protons and neutrons and almost all of its mass.
The fall in the pH of seawater as it absorbs more carbon dioxide from the air, forming carbonic acid; it reduces the carbonate ions available to shell-building organisms.
The arrangement of six electron domains around a central atom, pointing to the corners of an octahedron with all neighbouring domains at 90°, as in and many complex ions.
The tendency of atoms to gain, lose or share electrons until they have eight electrons in their valence shell (two for hydrogen).
A system that can exchange both energy and matter with its surroundings, for example a reaction that releases a gas into the air.
The ability of a substance to rotate the plane of plane-polarized light; shown by a single enantiomer (or an unequal mixture) of a chiral compound.
A diagram showing each orbital as a box and each electron as an arrow whose direction shows its spin, arranged in order of sublevel energy.
The power to which the concentration of a reactant is raised in the rate equation. It can only be found by experiment.
A compound of carbon in which carbon atoms are usually bonded to hydrogen and to each other. Simple carbon compounds such as and the carbonates are normally classed as inorganic.
A result that lies well outside the pattern of the other repeats or data points; it may be excluded from a mean only with a stated reason.
The sum of the orders with respect to all the species in the rate equation.
The yield of a multistep synthesis from its first starting material to its final product; it equals the product of the yields of the individual steps.
The extra voltage, beyond the value predicted from electrode potentials, needed to make an electrode reaction take place at a useful rate; it is large for the formation of oxygen.
The loss of electrons by a species, or an increase in its oxidation state.
A number, written with its sign first (+2, −1), showing the charge an atom would have if the compound were composed of ions. An uncombined element has an oxidation state of zero.
A species that oxidizes another species by accepting electrons from it; the oxidizing agent is itself reduced.
The ion , formed when a water molecule accepts a proton; it is how a hydrogen ion exists in aqueous solution, so and are used interchangeably.
A negative ion containing oxygen and another element, such as nitrate , sulfate or manganate(VII) .
The region of the stratosphere, mostly 15–35 km up, where ozone absorbs most of the Sun's harmful ultraviolet radiation; its thinning by chlorine atoms from CFCs is called ozone depletion.
A dumbbell-shaped atomic orbital with two lobes on opposite sides of the nucleus; a p sublevel has three, lying along the x, y and z axes.
An error in reading a scale caused by viewing it from an angle rather than straight on, so the reading appears higher or lower than it is.
The acid (or base) from which a salt can be made by neutralization: sodium ethanoate has the parent acid and the parent base .
The longest continuous chain of carbon atoms that contains the functional group; it gives the stem of the name.
A small charge, or , on an atom in a polar bond, smaller than the charge of an electron.
Reaction of a weak acid with less than the equivalent amount of strong base (or of a weak base with too little strong acid), leaving a mixture of the weak species and its conjugate: a buffer.
The SI unit of pressure, equal to a force of one newton acting on one square metre: .
An orbital can hold a maximum of two electrons, and they must have opposite spins.
The amide link, , that joins amino acid residues in peptides and proteins.
The mass of each element in 100 g of a compound: the number of atoms of the element in the formula times its , divided by the of the compound, times 100.
The difference between an experimental result and the accepted value, divided by the accepted value and multiplied by 100; a measure of the accuracy of the result.
The percentage of the molecules of a weak acid or base that are ionized at equilibrium, for an acid . It increases as the solution is diluted.
The absolute uncertainty of a value divided by the value itself and multiplied by 100; it shows how large the uncertainty is compared with the measurement.
The experimental yield as a percentage of the theoretical yield: percentage yield .
A horizontal row of the periodic table. The period number is the highest occupied main energy level of the atoms in it.
The arrangement of the elements in order of increasing atomic number into periods (rows) and groups (columns), so that elements with similar outer electron configurations and properties fall in the same group.
The repeating pattern of physical and chemical properties of the elements across periods and down groups, caused by the repeating pattern of outer electron configurations.
A compound containing an O–O single bond, such as or , in which oxygen has an oxidation state of −1.
A logarithmic measure of acidity: , so . A fall of one pH unit is a tenfold increase in .
A graph of the pH of the solution in a titration against the volume of titrant added; its steep section marks the equivalence point.
An instrument whose glass-membrane probe measures pH precisely, usually to two decimal places; it is calibrated with buffer solutions of known pH before use.
A region of a sample with uniform composition and properties, separated from other regions by a boundary.
A benzene ring attached to the rest of a molecule, –, as in methylbenzene, .
A single packet of electromagnetic energy; its energy is proportional to the frequency of the radiation ().
The endothermic process in which green plants use light energy absorbed by chlorophyll to make glucose and oxygen from carbon dioxide and water: .
A covalent bond formed by the sideways overlap of two parallel p orbitals, with its electron density above and below the bond axis; a double bond contains one and a triple bond two.
A few trial measurements made before the main investigation to check the range, timing, precision and safety of the method.
. The smaller the pKa, the stronger the acid; one unit corresponds to a tenfold change in .
. The smaller the pKb, the stronger the base. For a conjugate pair at 298 K, .
The constant linking the energy of a photon to the frequency of the radiation, .
Light whose oscillations all lie in a single plane, produced by passing ordinary light through a polarizing filter.
A synthetic polymer material that can be moulded into shape, usually when heated.
A small molecule added to a polymer that fits between the chains, weakening the forces between them and making the material more flexible, as in flexible PVC.
A finely divided form of platinum coated onto a platinum electrode to give a large surface area, on which the equilibrium between hydrogen gas and hydrogen ions is reached quickly.
The negative logarithm to base 10 of the hydroxide ion concentration: . At 298 K, .
A covalent bond in which the shared pair is attracted more strongly by one atom, because the atoms differ in electronegativity, giving partial charges and .
A molecule with a net dipole moment, because its bond dipoles do not cancel, such as and .
A solvent whose molecules have a permanent dipole, such as water; polar solvents can attract and surround ions, so many ionic compounds dissolve in them.
An instrument that passes plane-polarized light through a sample and measures the angle and direction through which the sample rotates the plane of polarization.
How easily the electron cloud of an atom or molecule is distorted; it increases with the number of electrons and size, and strengthens London forces.
A condensation polymer whose repeating units are joined by amide links, formed from a diamine and a dicarboxylic acid (or diacyl dichloride) or from amino acids; examples are nylon-6,6, Kevlar and proteins.
An ion made of two or more atoms joined by covalent bonds, such as ammonium, , or sulfate, .
A condensation polymer whose repeating units are joined by ester links, formed from a diol and a dicarboxylic acid or from a hydroxy acid; examples are PET and PLA.
A macromolecule made of many repeating subunits joined by covalent bonds, formed from small molecules called monomers.
Structural isomers with the same carbon skeleton and functional group but the group in a different position, such as propan-1-ol and propan-2-ol.
The composition of an equilibrium mixture: an equilibrium that lies to the right contains mainly products, one that lies to the left mainly reactants.
Energy stored in a substance because of the arrangement of its particles, in its bonds and the attractions between particles. Lower potential energy means greater stability.
A fast, reversible step that comes before the rate-determining step in a mechanism and keeps up a small, steady concentration of an intermediate.
An insoluble solid that forms when two solutions are mixed, such as from silver nitrate and sodium chloride solutions.
How close repeated measurements are to one another, or how finely an instrument can be read; random errors reduce precision.
In electrolysis, the conversion of one ion or molecule rather than another into a neutral product at an electrode; it depends on electrode potentials, concentration and the electrode material.
Force per unit area, , measured in pascals (). In a gas it results from collisions of particles with the walls of the container.
An alcohol in which the carbon bonded to the OH group is bonded to one other carbon atom (or none, as in methanol), such as butan-1-ol. Halogenoalkanes are classified by the same rule.
A voltaic cell whose redox reaction cannot be reversed in practice, so it is used once and discarded; an alkaline zinc–manganese(IV) oxide cell is an example.
A halogenoalkane in which the carbon bonded to the halogen is bonded to one other carbon atom (or none), such as 1-bromobutane, .
Values calculated from raw data, such as means, rates, concentrations or enthalpy changes, with their propagated uncertainties.
Combining the uncertainties of measured values to find the uncertainty of a calculated result: absolute uncertainties add for sums and differences, and fractional uncertainties add for products and quotients.
A step in a chain reaction in which a radical reacts with a molecule to form a new molecule and a new radical, so the number of radicals does not change.
A subatomic particle in the nucleus with a relative mass of 1 and a relative charge of +1; the number of protons is the atomic number.
A technique that measures the radio-frequency radiation absorbed by hydrogen nuclei in a magnetic field; it shows the number of hydrogen environments, their chemical shifts, the relative numbers of hydrogen atoms and their neighbours.
A single element or compound, containing only one kind of particle, with a fixed composition and sharp, fixed melting and boiling points.
Observations that describe a property without measuring it, such as a colour change, effervescence or the formation of a precipitate.
Data recorded as measured values with units, such as a volume of gas or a temperature.
An NMR signal split into four lines (1 : 3 : 3 : 1) by three hydrogen atoms on adjacent carbon atoms.
Stopping the reaction in a sample taken from a reaction mixture, by rapid cooling, large dilution or removing a catalyst or reactant, so that its composition at that moment can be measured.
A mixture of equal amounts of the two enantiomers of a chiral compound; it is optically inactive because their rotations of plane-polarized light cancel.
A species with an unpaired electron, shown with a dot, for example ; radicals are formed by homolytic fission and are usually very reactive.
An isotope with an unstable nucleus that decays, emitting radiation, such as carbon-14.
An unpredictable error that makes repeated measurements scatter either side of the true value; its effect is reduced by repeating measurements and taking a mean.
The constant of proportionality, , in the rate equation. It is fixed for a given reaction at a given temperature, increases with temperature, and its units depend on the overall order.
An equation, found by experiment, that links the rate of a reaction to the concentrations of the species that affect it, in the form rate .
The change in concentration of a particular reactant or product per unit time, in ; it is always positive.
The slowest elementary step in a reaction mechanism. It has the largest activation energy and determines the rate of the whole reaction.
Measurements exactly as read from the instruments, before any calculation, recorded with their units and uncertainties.
The x-axis of an energy profile, representing the progress of a reaction from reactants to products; it has no units or scale.
A species formed in one elementary step of a mechanism and used up in a later step. It sits in an energy minimum between two transition states and does not appear in the overall equation.
The sequence of elementary steps by which reactants are converted into products. The steps must add up to the overall stoichiometric equation.
The value obtained by substituting the concentrations present at any moment into the equilibrium constant expression; the mixture reacts forwards if and backwards if .
A list of metals in order of how easily they are oxidized to their positive ions; a metal higher in the series displaces a lower one from a solution of its ions.
An actual gas, whose particles have a finite volume and attract one another; it deviates from ideal behaviour most at high pressure and low temperature.
Purification of a solid by dissolving it in the minimum volume of hot solvent, cooling so that the pure compound crystallizes while impurities stay dissolved, then filtering, washing with cold solvent and drying.
The oxidized and reduced forms of one species that are converted into each other by gaining or losing electrons, such as or .
A reaction in which oxidation and reduction occur together: one species loses electrons (its oxidation state increases) and another gains them (its oxidation state decreases).
A titration in which the titrant is an oxidizing or reducing agent, such as acidified potassium manganate(VII) used to find the amount of iron(II) in a sample.
A species that reduces another species by donating electrons to it; the reducing agent is itself oxidized.
The gain of electrons by a species, or a decrease in its oxidation state.
A half-cell of known, stable potential against which the potentials of other half-cells are measured; the standard hydrogen electrode is the primary reference.
Continuous boiling of a reaction mixture with a vertical condenser above it, so that vapours condense and return to the flask; it allows prolonged heating without loss of volatile substances.
The weighted mean mass of the atoms of an element, taking into account the abundance of each isotope, compared with one-twelfth of the mass of an atom of carbon-12. Symbol ; it has no units.
The sum of the relative atomic masses of all the atoms in the formula of a substance. Symbol ; it has no units.
The mass of an atom of one isotope relative to one-twelfth of the mass of an atom of carbon-12; very close to its mass number.
The relative formula mass of a substance made of molecules: the sum of the relative atomic masses of the atoms in one molecule. It has no units.
A comparison of substances by their potential energy: the one lower in energy is more stable. Products more stable than reactants means the reaction is exothermic.
An energy source that is replenished naturally on a human timescale, such as solar, wind, hydroelectric, tidal and geothermal energy and biofuels.
The smallest section of a polymer chain that repeats along its length; it is drawn in brackets with continuation bonds through the brackets and outside.
A single, focused question that an investigation sets out to answer, naming the independent variable and its range, the dependent variable and how it is measured, and the chemical system studied.
The insoluble solid left on the filter paper after filtration.
A number from 1 to 7 printed on a plastic item, usually inside a recycling symbol, that identifies the polymer it is made of so that plastics can be sorted for recycling.
The smallest change in a quantity that an instrument can show, such as 0.01 g on a balance or 0.1 °C on a digital thermometer.
The real structure of a species with resonance: a single structure that is an average of its resonance structures, with delocalized electrons and equivalent bonds of intermediate order.
One of two or more valid Lewis formulas for a species that differ only in the positions of electrons, not of atoms; they are joined by a double-headed arrow, ↔.
The inability of the groups on two bonded atoms to turn relative to each other, because the atoms are joined by a double bond (rotation would break the π bond) or are part of a ring.
In chromatography, the distance moved by a component divided by the distance moved by the solvent front, both measured from the baseline; always less than 1.
A reaction in which the products can react to re-form the original reactants; shown with the symbol .
A written review, made before practical work, of the hazards of each substance and procedure, the precautions that reduce the risk, and how waste will be disposed of.
The corrosion of iron or steel in the presence of both oxygen and water, forming hydrated iron(III) oxide, (rust).
A spherical atomic orbital centred on the nucleus; each main energy level has one s orbital, and its size increases with .
Protecting iron from corrosion by connecting it to a more easily oxidized metal, such as zinc or magnesium, which is oxidized instead of the iron.
An ionic compound formed when the hydrogen of an acid is replaced by a metal ion or an ammonium ion, for example in neutralization.
A tube or strip of filter paper containing a solution of an unreactive salt, such as , that joins the two half-cells of a voltaic cell; its ions move to keep each solution electrically neutral and complete the circuit.
The reaction of an ion from a salt with water, in which a proton is transferred and or forms, so that the solution is acidic or basic; for example .
Describes a compound in which all the carbon–carbon bonds are single bonds, such as an alkane.
A solution that contains the maximum amount of dissolved solute at a given temperature; in contact with undissolved solute it is at dynamic equilibrium.
A way of writing a number as a value between 1 and 10 multiplied by a power of ten, such as .
The enthalpy change when one mole of gaseous 1− ions each gain one electron, for example ; always endothermic because of repulsion.
The energy needed to remove one mole of electrons from one mole of gaseous 1+ ions, for example ; always larger than the first.
In any spontaneous change, the total entropy of a system and its surroundings increases: .
Describes a reactant for which the rate is proportional to the square of its concentration: doubling the concentration multiplies the rate by four.
An alcohol in which the carbon bonded to the OH group is bonded to two other carbon atoms, such as butan-2-ol. Halogenoalkanes are classified by the same rule.
A voltaic cell whose redox reaction can be reversed by supplying electrical energy, so it can be recharged and reused; examples are the lead–acid and lithium-ion cells.
Data measured by others and taken from a published source, such as a database, rather than measured by the investigator.
A halogenoalkane in which the carbon bonded to the halogen is bonded to two other carbon atoms, such as 2-bromobutane, .
The molecular geometry of a species with four bonding pairs and one lone pair (five electron domains), the lone pair equatorial, as in .
Describes a titration in which a reactant changes colour itself at the end point, so no indicator is added; purple manganate(VII) is decolourized until the first permanent pink appears.
A solid, such as silicon or germanium, whose electrical conductivity lies between that of a conductor and an insulator and increases as the temperature rises.
A series of dilutions, each by the same factor, used to make very dilute solutions or a set of standards accurately.
The reduction in the attraction between the nucleus and an outer electron caused by repulsion from electrons in inner main levels.
In NMR, the reduction in the magnetic field felt by a nucleus caused by the electrons around it; more shielded hydrogen atoms absorb at lower chemical shift.
A covalent bond formed by the head-on overlap of two orbitals, with its electron density concentrated along the bond axis; every single bond is a sigma bond.
The digits of a value that carry meaning about its precision: all non-zero digits, zeros between them and trailing zeros after a decimal point; has three.
A covalent bond made of one shared pair of electrons.
An NMR signal that is not split, from hydrogen atoms with no hydrogen atoms on the adjacent carbon atoms.
A formula that shows the carbon chain as a zig-zag line with a carbon atom at every corner and line end; H atoms on carbon are omitted, and all other atoms (with any H on them) are written in.
When is very small, the amount of reactant used up is negligible, so the equilibrium concentration of each reactant is taken as equal to its initial concentration.
A two-step nucleophilic substitution: the C–X bond breaks heterolytically in the slow step to give a carbocation, which then bonds to the nucleophile; rate . Typical of tertiary halogenoalkanes.
A one-step (concerted) nucleophilic substitution in which the nucleophile attacks the carbon from the side opposite the leaving group as the C–X bond breaks; rate . Typical of primary halogenoalkanes.
The substance that dissolves in a solvent to form a solution.
A homogeneous mixture formed when a solute dissolves in a solvent; a solution in water is an aqueous solution, shown by (aq).
The process in which solute particles become surrounded by solvent particles and spread through the solvent; used to separate a soluble component of a mixture from an insoluble one.
The substance, usually a liquid, in which a solute dissolves to form a solution.
The mixing of one s and one p orbital to give two sp hybrid orbitals at 180°, leaving two p orbitals for pi bonds; found in atoms with two electron domains.
The mixing of one s and two p orbitals to give three hybrid orbitals in a plane at 120°, leaving one p orbital for a pi bond; found in atoms with three electron domains.
The mixing of one s and three p orbitals to give four hybrid orbitals pointing to the corners of a tetrahedron, 109.5° apart; found in atoms with four electron domains.
The energy released per unit mass of a fuel when it burns, usually in ; it equals divided by the molar mass.
The energy needed to raise the temperature of 1 g of a substance by 1 K; for water it is .
An ion that is present in a reaction mixture but is unchanged by the reaction, so it is left out of the ionic equation.
An instrument that uses a slit and a prism or diffraction grating to separate light into its wavelengths, so that the lines of a spectrum can be seen and their wavelengths measured.
The splitting of an NMR signal into several lines by the hydrogen nuclei on adjacent atoms, whose spins slightly change the magnetic field it feels.
The separation of the five d orbitals of a transition element ion into two sets of different energy, apart, caused by the lone pairs of the ligands in a complex. Light absorbed as an electron moves between the sets gives the complex its colour.
A change that takes place without a continuous supply of energy from outside the system. At constant temperature and pressure it has a negative ; it may still be very slow.
The molecular geometry of a species with four bonding pairs and two lone pairs opposite each other (six electron domains), as in ; the bond angles are 90°.
The molecular geometry of a species with five bonding pairs and one lone pair (six electron domains), as in .
The reference conditions 298.15 K (25 °C) and 100 kPa; the molar volume of an ideal gas at SATP is about .
The potential difference between the electrodes of a voltaic cell under standard conditions with no current flowing: . It is positive for a spontaneous reaction.
A pressure of 100 kPa, a stated temperature (usually 298 K) and solution concentrations of , used for quoting standard thermodynamic values.
The potential difference between a half-cell and the standard hydrogen electrode under standard conditions (298 K, 100 kPa, ), quoted for the reduction half-equation; symbol .
The enthalpy change, , when the amounts in an equation react under standard conditions with every substance in its standard state; units .
The enthalpy change when one mole of a substance is completely burned in excess oxygen under standard conditions; always negative.
The enthalpy change when one mole of a substance is formed from its elements in their standard states under standard conditions; zero for an element in its standard state.
The entropy, , of one mole of a substance in its standard state at 100 kPa and a stated temperature (usually 298 K), in . It is always positive, including for elements.
The entropy change, , when the amounts in an equation react under standard conditions: .
The Gibbs energy change, , when one mole of a compound is formed from its elements in their standard states under standard conditions; zero for an element in its standard state.
The reference half-cell, assigned a potential of exactly 0.00 V: hydrogen gas at 100 kPa bubbled over a platinized platinum electrode in a solution with at 298 K.
A solution whose concentration is accurately known, made by dissolving an accurately weighed pure solute and making it up to an exact volume in a volumetric flask.
The pure, most stable form of a substance at 100 kPa and the stated temperature, for example , and graphite at 298 K.
The reference conditions 273.15 K (0 °C) and 100 kPa, at which the molar volume of an ideal gas is .
A property, such as enthalpy, whose value depends only on the present state of a system and not on how that state was reached.
A symbol in brackets after a formula showing the state of the substance: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
The phase in chromatography that does not move, such as the water held in the fibres of chromatography paper or the silica layer on a TLC plate.
A formula that shows the three-dimensional arrangement of bonds: ordinary lines in the plane, solid wedges towards the viewer and hashed wedges away from it.
One of two or more compounds with the same constitution (the same atoms, connectivity and bond multiplicities) but a different arrangement of their atoms in space.
A reaction in which a particular stereoisomer of the reactant gives one particular stereoisomer of the product, such as the reaction, which inverts the configuration.
The slowing of a reaction because bulky groups get in the way of an approaching species, as when three methyl groups block backside attack on a tertiary halogenoalkane.
The quantitative relationship between the amounts of reactants and products in a chemical reaction, as given by the balanced equation.
The layer of the atmosphere from about 10 to 50 km above the Earth's surface, which contains the ozone layer.
A vibration in which the length of a bond increases and decreases; stretching vibrations absorb infrared radiation at wavenumbers characteristic of the bond.
An acid that ionizes (dissociates) completely in aqueous solution, such as , , , and .
A base that is completely ionized in aqueous solution, such as the group 1 hydroxides.
The very strong, very short-range attraction between nucleons that holds the nucleus together despite the repulsion between its protons.
Compounds with the same molecular formula but a different connectivity of atoms; they are chain, position or functional group isomers.
A particle smaller than an atom; in chemistry, the proton, neutron and electron.
A group of orbitals of equal energy within a main energy level, labelled s, p, d or f, holding a maximum of 2, 6, 10 or 14 electrons.
The change of state directly from solid to gas without melting, which absorbs energy (endothermic), for example .
A reaction in which one atom or group in a molecule is replaced by another atom or group, as when a chlorine atom replaces a hydrogen atom of methane.
An alloy in which atoms of the added element, of similar size, take the places of some of the metal atoms in the lattice, as in brass.
The organic molecule that is attacked in a reaction, such as the halogenoalkane in a nucleophilic substitution.
A collision between reactant particles that leads to reaction, because the particles have energy and the proper orientation.
The energy needed to remove each further mole of electrons, one at a time, from one mole of gaseous ions, for example for the third.
Everything outside the system, including the solvent, the container, the thermometer and the air.
A polymer made industrially, such as poly(ethene), PVC, nylon or PET.
The part of the universe being studied, usually the reactants and products of a reaction.
An error that shifts every measurement in the same direction, such as heat loss in calorimetry; repeating the measurement does not reduce it.
The molecular geometry of a species with three bonding pairs and two equatorial lone pairs (five electron domains), as in , with bond angles just under 90°.
A straight line that touches a curve at one point and has the same gradient as the curve there; its gradient gives the rate at that time on a graph against time.
A measure of the average kinetic energy of the particles in a substance, measured in kelvin or degrees Celsius; it does not depend on the amount of substance. The absolute temperature in kelvin is proportional to the average kinetic energy.
A short-lived dipole that forms when the electrons of an atom or molecule are momentarily spread unevenly.
A step in a chain reaction in which two radicals combine to form a molecule, removing radicals and ending a chain.
Describes an elementary step in which three particles collide at the same moment (molecularity 3). Such steps are rare.
An alcohol in which the carbon bonded to the OH group is bonded to three other carbon atoms, such as 2-methylpropan-2-ol. Halogenoalkanes are classified by the same rule.
A halogenoalkane in which the carbon bonded to the halogen is bonded to three other carbon atoms, such as 2-bromo-2-methylpropane, .
, the reference compound for NMR, defined as ; it gives one strong signal from twelve equivalent, highly shielded hydrogen atoms and is inert and volatile.
The maximum amount or mass of product that can be obtained from the limiting reactant, assuming complete reaction with no side reactions and no losses.
The ability of a substance to transfer heat; metals conduct heat well because their delocalized electrons carry kinetic energy rapidly through the lattice.
A balanced equation with state symbols followed by its enthalpy change, which refers to the amounts in moles shown in the equation.
A plastic that softens on heating and can be remoulded, because heating overcomes the intermolecular forces between its chains without breaking covalent bonds.
A polymer whose chains are joined by covalent cross-links, so it does not soften on heating but chars or decomposes instead.
The entropy of a perfect crystal at 0 K is zero, because there is only one way of arranging its particles and their energy.
The solution added from the burette in a titration, usually of known concentration.
A technique in which a solution is added from a burette to a measured volume of another until the reaction between them is just complete, so that an unknown concentration or amount can be found.
The volume of titrant added from the burette to reach the end point: the final burette reading minus the initial reading.
The sum of the entropy changes of a system and its surroundings, ; it is positive for a spontaneous change.
The stereoisomer in which the two identical or similar groups lie on opposite sides of a C=C double bond or on opposite faces of a ring.
An element whose atoms have an incomplete d-sublevel, or which forms at least one stable ion with an incomplete d-sublevel; most form ions with more than one charge.
The highest-energy arrangement of atoms on the path from reactants to products, at the top of the hump on an energy profile, in which bonds are partly broken and partly formed.
The arrangement of five electron domains around a central atom: three equatorial at 120° in a plane and two axial at 90° to that plane, as in .
A covalent bond made of three shared pairs of electrons, as in ; the shortest and strongest bond between two given atoms.
An NMR signal split into three lines (1 : 2 : 1) by two hydrogen atoms on adjacent carbon atoms.
Describes an elementary step in which a single particle breaks apart or rearranges (molecularity 1).
A mixture of indicators that shows a range of colours across the pH scale, used to estimate pH to about the nearest whole unit.
An electron that occupies an orbital on its own, without a second electron of opposite spin.
Describes a compound that contains at least one carbon–carbon double or triple bond, such as an alkene or alkyne.
An alkene whose two double-bonded carbon atoms carry different groups, such as propene, so that an adding molecule HX can give two different products.
An electron in the outermost main energy level of an atom (for transition elements, the 3d electrons too); valence electrons take part in bonding and reactions.
The collective name for London (dispersion), dipole-induced dipole and dipole–dipole forces; it does not include hydrogen bonding.
The change of state from liquid to gas, by evaporation at the surface or by boiling throughout the liquid; it absorbs energy (endothermic).
The pressure exerted by the vapour of a liquid in equilibrium with the liquid in a closed container at a given temperature; it increases with temperature.
The tendency of a substance to vaporize; substances with weak forces between their particles are volatile, while ionic compounds have low volatility.
An electrochemical cell that converts the chemical energy of a spontaneous redox reaction into electrical energy; its anode is negative and its cathode positive.
A flask with a long narrow neck and a single graduation mark, calibrated to contain one precise volume of solution at a stated temperature.
A pipette with a single graduation mark, calibrated to deliver one precise volume of liquid, such as .
The valence shell electron pair repulsion model: electron domains around a central atom repel each other and take up positions as far apart as possible, which predicts the shape of a molecule.
Water molecules held in fixed proportion within the crystal lattice of a salt, shown after a dot in the formula, as in .
The distance between two successive crests (or other identical points) of a wave, symbol ; for electromagnetic radiation, a shorter wavelength means a higher frequency and photon energy.
The reciprocal of the wavelength, , usually in ; it is proportional to the frequency and energy of the radiation and is the x-axis of an IR spectrum.
An acid that ionizes only partially in aqueous solution, so the un-ionized acid is in equilibrium with its ions; examples are carboxylic acids and carbonic acid.
A base that is only partially protonated in aqueous solution, so its solution contains an equilibrium; examples are ammonia and amines.
Finding the mass of solid transferred as the difference between the masses of its container before and after transfer, so solid left in the container causes no error.
Describes a reactant whose concentration does not affect the rate: changing its concentration leaves the rate unchanged.
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