Chemistry terms in What drives chemical reactions?

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.

The electrode at which oxidation takes place. In a voltaic cell (including a fuel cell) it is the negative electrode.

The mean bond enthalpy of a particular bond taken over a range of compounds that contain it, as listed in the data booklet.

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.

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.

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.

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.

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.

The electrode at which reduction takes place. In a voltaic cell (including a fuel cell) it is the positive electrode.

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.

A liquid fossil fuel made of a mixture of hundreds of hydrocarbons, separated by fractional distillation into fractions such as petrol, kerosene and diesel.

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 ${\text{kJ cm}}^{-3}$; it equals the specific energy multiplied by the density.

A graph of potential energy against reaction coordinate showing the reactants, the transition state and the products, with the activation energy and $\mathrm{\Delta }H$.

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 $H$. Only changes in enthalpy can be measured.

The heat transferred between a system and its surroundings at constant pressure, $\mathrm{\Delta }H=H_{\text{products}}-H_{\text{reactants}}$; negative for exothermic and positive for endothermic changes.

The enthalpy change when an acid and a base react to form one mole of water; about $-57{\text{kJ mol}}^{-1}$ 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 $+26{\text{kJ mol}}^{-1}$ for ammonium nitrate.

The enthalpy change when one mole of gaseous atoms is formed from an element in its standard state, for example $\frac{1}{2}\mathrm{C}\mathrm{l}{\phantom{X}}_2(\mathrm{g})\stackrel{}{\to }\mathrm{C}\mathrm{l}(\mathrm{g})$.

A measure of the dispersal or distribution of matter and/or energy in a system, symbol $S$, in ${\text{J K}}^{-1}$ (or ${\text{J K}}^{-1}{\text{mol}}^{-1}$). 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 $T$: $\mathrm{\Delta }{S}_{\text{surroundings}}=-\mathrm{\Delta }H\mathrm{/}T$.

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.

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 property of a system, $G=H-TS$, that combines its enthalpy and entropy; at constant temperature and pressure a system changes spontaneously towards lower Gibbs energy.

The quantity $\mathrm{\Delta }G=\mathrm{\Delta }H-T\mathrm{\Delta }S$ for a change at constant temperature and pressure, in ${\text{kJ mol}}^{-1}$. A negative value means the change is spontaneous; at equilibrium $\mathrm{\Delta }G=0$.

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).

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.

An equation showing either the oxidation or the reduction part of a redox reaction, with the electrons lost or gained, for example $\mathrm{C}\mathrm{u}{\phantom{X}}^{2+}+2\mathrm{e}{\phantom{X}}^{-}\stackrel{}{\to }\mathrm{C}\mathrm{u}$.

Energy transferred between two bodies because of a difference in temperature, from the hotter to the colder; symbol $Q$, measured in joules.

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.

The breaking of a covalent bond in which each atom keeps one electron of the shared pair, forming two radicals or atoms.

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.

The persistence of a substance that is energetically unstable relative to its products but reacts extremely slowly because the activation energy is high.

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.

The reactant that is completely used up in a reaction, so it determines the amount of product formed; the others are in excess.

One particular way of arranging the particles of a system and distributing their energy. The more microstates available, the higher the entropy.

A gaseous fossil fuel consisting mostly of methane, with small amounts of other alkanes, carbon dioxide and nitrogen.

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 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 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 endothermic process in which green plants use light energy absorbed by chlorophyll to make glucose and oxygen from carbon dioxide and water: $6\mathrm{C}\mathrm{O}{\phantom{X}}_2+6\mathrm{H}{\phantom{X}}_2\mathrm{O}\stackrel{}{\to }\mathrm{C}{\phantom{X}}_6\mathrm{H}{\phantom{X}}_{12}\mathrm{O}{\phantom{X}}_6+6\mathrm{O}{\phantom{X}}_2$.

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.

The x-axis of an energy profile, representing the progress of a reaction from reactants to products; it has no units or scale.

The value $Q$ obtained by substituting the concentrations present at any moment into the equilibrium constant expression; the mixture reacts forwards if $Q<K$ and backwards if $Q>K$.

A species that reduces another species by donating electrons to it; the reducing agent is itself oxidized.

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 energy needed to remove one mole of electrons from one mole of gaseous 1+ ions, for example $\mathrm{M}\mathrm{g}{\phantom{X}}^{+}(\mathrm{g})\stackrel{}{\to }\mathrm{M}\mathrm{g}{\phantom{X}}^{2+}(\mathrm{g})+\mathrm{e}{\phantom{X}}^{-}$; always larger than the first.

In any spontaneous change, the total entropy of a system and its surroundings increases: $\mathrm{\Delta }{S}_{\text{total}}>0$.

The energy released per unit mass of a fuel when it burns, usually in ${\text{kJ g}}^{-1}$; it equals $\mathrm{\mid }\mathrm{\Delta }{H}_{\text{c}}\mathrm{\mid }$ 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 $4.18{\text{J g}}^{-1}{\text{K}}^{-1}$.

A change that takes place without a continuous supply of energy from outside the system. At constant temperature and pressure it has a negative $\mathrm{\Delta }G$; it may still be very slow.

A pressure of 100 kPa, a stated temperature (usually 298 K) and solution concentrations of $1{\text{mol dm}}^{-3}$, used for quoting standard thermodynamic values.

The enthalpy change, $\mathrm{\Delta }{H}^{\ominus }$, when the amounts in an equation react under standard conditions with every substance in its standard state; units ${\text{kJ mol}}^{-1}$.

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, $S^{\ominus }$, of one mole of a substance in its standard state at 100 kPa and a stated temperature (usually 298 K), in ${\text{J K}}^{-1}{\text{mol}}^{-1}$. It is always positive, including for elements.

The entropy change, $\mathrm{\Delta }{S}^{\ominus }$, when the amounts in an equation react under standard conditions: $\mathrm{\Delta }{S}^{\ominus }=\sum S^{\ominus }(\text{products})-\sum S^{\ominus }(\text{reactants})$.

The Gibbs energy change, $\mathrm{\Delta }{G}_{\text{f}}^{\ominus }$, 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 pure, most stable form of a substance at 100 kPa and the stated temperature, for example $\mathrm{H}{\phantom{X}}_2\mathrm{O}(\mathrm{l})$, $\mathrm{O}{\phantom{X}}_2(\mathrm{g})$ and graphite at 298 K.

A property, such as enthalpy, whose value depends only on the present state of a system and not on how that state was reached.

Everything outside the system, including the solvent, the container, the thermometer and the air.

The part of the universe being studied, usually the reactants and products of a reaction.

A balanced equation with state symbols followed by its enthalpy change, which refers to the amounts in moles shown in the equation.

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 sum of the entropy changes of a system and its surroundings, $\mathrm{\Delta }{S}_{\text{total}}=\mathrm{\Delta }{S}_{\text{system}}+\mathrm{\Delta }{S}_{\text{surroundings}}$; it is positive for a spontaneous change.

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.