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Lattice enthalpy The enthalpy change when one mole of an ionic compound is converted to gaseous ions. Enthalpy of atomisation The enthalpy change when one mole of gaseous atoms isformed from the element in its standard state. First electronaffinity The enthalpy change when one mole of gaseous atoms is converted into gaseous ions with a single negative charge. Enthalpy of solution The enthalpy change when one mole of a solute dissolves in water. Enthalpy of hydration The enthalpy change when one mole of gaseous ions is converted to one mole of aqueous ions. Feasible A reaction for which ΔG 0. Entropy A measure of disorder (randomness). Free energy change (ΔG) ΔG=ΔH-TΔS Rate of reaction The change of the concentration (amount) of a reactant or product with respect to time Order of reaction (with respect to a reactant) The power to which the concentration of a reactant is raised in the rate equation. Order of reaction (overall) The sum of the powers to which the concentration terms are raised in the rate equation. Rate constant The proportionality constant which links the rate of the reaction to the concentrations in the rate equation. Rate determining step The slowest step in the mechanism of a reaction. Brønsted acid Proton donor. Brønsted base Proton acceptor. Kw Kw=[H+][OH−] Ka Ka = [H+][ A-] / [HA] pH pH=-log10[H+] pKw pKw=-log10Kw Buffer solution A solution which resists changes in pH on addition of small amounts of acid or alkali. Monobasic acid Donates one proton per molecule. Asymmetric (chiral) centre An atom which has four different atoms or groups attached. Optical isomers Molecules which exist as non-superimposable mirror images. Optically active A sample which rotates the plane of plane polarised light. Racemic mixture A 50:50 mixture of two optical isomers. Monoester An ester which contains only one ester group. Transesterification A reaction where the alkyl group of an ester is exchanged withthe alkyl group of an alcohol. Biodiesel A fuel, similar to diesel, which is made from vegetable sources. e.g. from the reaction of rape seed oil with methanol.