Chemistry terms in How much, how fast and how far?

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.

The equilibrium constant for the ionization of a weak acid in water, $K_{\text{a}}=[\mathrm{H}{\phantom{X}}^{+}][\mathrm{A}{\phantom{X}}^{-}]\mathrm{/}[\mathrm{H}\mathrm{A}]$; the larger its value, the stronger the acid.

The substance or solution being analysed in a titration, usually measured into the conical flask with a pipette.

The equation $k=A{\text{e}}^{-E_{\text{a}}\mathrm{/}RT}$, which describes how the rate constant depends on temperature and activation energy. Its linear form is $\ln k=-E_{\text{a}}\mathrm{/}RT+\ln A$.

The constant $A$ in the Arrhenius equation, which takes into account the frequency of collisions with the proper orientation. It has the same units as $k$.

A graph of $\ln k$ against $1\mathrm{/}T$: a straight line with gradient $-E_{\text{a}}\mathrm{/}R$ and intercept $\ln A$, used to determine the activation energy and the Arrhenius factor.

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.

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.

In a reversible reaction, the reaction from right to left as the equation is written, turning products back into reactants.

Describes an elementary step in which two particles collide and react (molecularity 2).

A substance formed in a reaction alongside the desired product; unless a use is found for it, it is waste.

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.

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.

A reaction set up to give a sudden visible change once a fixed amount of reaction has taken place, so the time taken $t$ measures the rate (rate is proportional to $1\mathrm{/}t$).

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 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.

Measuring the absorbance of a coloured solution at a chosen wavelength to find its concentration, using a calibration curve of standard solutions.

Industrial conditions chosen to balance an acceptable equilibrium yield against an acceptable rate and cost, when the conditions that favour each conflict.

Titres that agree closely, usually within $\pm 0.10{\text{cm}}^3$; the mean of the concordant titres is used in the calculation.

The industrial manufacture of sulfuric acid, in which sulfur dioxide is oxidized to sulfur trioxide, $2\mathrm{S}\mathrm{O}{\phantom{X}}_2(\mathrm{g})+\mathrm{O}{\phantom{X}}_2(\mathrm{g})\stackrel{}{\rightleftharpoons }2\mathrm{S}\mathrm{O}{\phantom{X}}_3(\mathrm{g})$, over a vanadium(V) oxide catalyst at about 450 °C and 1–2 atm.

A variable that could affect the dependent variable and is therefore kept constant, so that only the independent variable is changed.

The variable that is measured in an experiment, which changes in response to the independent variable.

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.

A single molecular event in a reaction mechanism: one collision, or one particle breaking apart, in which bonds are broken and formed.

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.

A biological catalyst: a protein that catalyses a specific reaction in living cells, working best at body temperature and close to neutral pH.

The constant $K$ 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 $K$ 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, $K$.

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.

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.

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 $K$.

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.

In a reversible reaction, the reaction from left to right as the equation is written, turning reactants into products.

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 industrial synthesis of ammonia, $\mathrm{N}{\phantom{X}}_2(\mathrm{g})+3\mathrm{H}{\phantom{X}}_2(\mathrm{g})\stackrel{}{\rightleftharpoons }2\mathrm{N}\mathrm{H}{\phantom{X}}_3(\mathrm{g})$, using an iron catalyst at about 400–450 °C and about 200 atm, with unreacted gases recycled.

The time taken for the concentration of a reactant to fall to half of its value. It is constant for a first-order reactant.

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 $\mathrm{C}\mathrm{O}{\phantom{X}}_2(\mathrm{g})\stackrel{}{\rightleftharpoons }\mathrm{C}\mathrm{O}{\phantom{X}}_2(\mathrm{a}\mathrm{q})$.

A catalyst in the same phase as the reactants, such as $\mathrm{H}{\phantom{X}}^{+}(\mathrm{a}\mathrm{q})$ 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 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.

The variable that is deliberately changed in an experiment to see its effect on another variable.

The instantaneous rate at the start of a reaction ($t=0$), found from the gradient of the tangent to the curve at $t=0$.

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 rate of reaction at one particular moment, equal to the gradient of the tangent to a concentration–time curve at that time.

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.

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 property of a bulk sample that can be observed or measured, such as colour, pressure, density or pH; at equilibrium these stay constant.

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 ratio of the amounts, in moles, of two substances in a reaction, given by their coefficients in the balanced equation.

The number of reacting particles (molecules, atoms, ions or radicals) that take part in a single elementary step.

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.

A system that can exchange both energy and matter with its surroundings, for example a reaction that releases a gas into the air.

The power to which the concentration of a reactant is raised in the rate equation. It can only be found by experiment.

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 experimental yield as a percentage of the theoretical yield: percentage yield $={\displaystyle \frac{\text{experimental yield}}{\text{theoretical yield}}}\times 100$.

The composition of an equilibrium mixture: an equilibrium that lies to the right contains mainly products, one that lies to the left mainly reactants.

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 $\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{l}(\mathrm{s})$ from silver nitrate and sodium chloride solutions.

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.

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, $k$, 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 $=k[\text{A}{]}^m[\text{B}{]}^n$.

The change in concentration of a particular reactant or product per unit time, in ${\text{mol dm}}^{-3}{\text{s}}^{-1}$; 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.

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.

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 reaction in which the products can react to re-form the original reactants; shown with the symbol $\rightleftharpoons$.

A solution that contains the maximum amount of dissolved solute at a given temperature; in contact with undissolved solute it is at dynamic equilibrium.

Describes a reactant for which the rate is proportional to the square of its concentration: doubling the concentration multiplies the rate by four.

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.

When $K$ 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.

The quantitative relationship between the amounts of reactants and products in a chemical reaction, as given by the balanced equation.

A collision between reactant particles that leads to reaction, because the particles have energy $E\ge E_{\text{a}}$ and the proper orientation.

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.

Describes an elementary step in which three particles collide at the same moment (molecularity 3). Such steps are rare.

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 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.

Describes an elementary step in which a single particle breaks apart or rearranges (molecularity 1).

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.

Describes a reactant whose concentration does not affect the rate: changing its concentration leaves the rate unchanged.