AS Science Unit 2 Module 2

Acid: A substance which produces H+ ions in solution.

Weak Acid: One that breaks up into ions partially (only a small fraction of the molecules break up) - it is said to be partially ionised.

Strong Acid: One that breaks up into ions completely - it is said to be fully ionised.

Ion: An atom or group of atoms with a charge (+ or -) e.g. H+ and NO3-.

pH: A measure of the concentration of H+ ions - the lower the pH the higher the concentration of H+ ions.

Concentration: A measure of the ratio of dissolved substance to water in a solution. Often measured in g dm-3 or mol dm-3.

Oxidation: A reaction in which the oxidation state of an atom increases during a chemiclal process. This occurs when, for example, oxygen is gained by the atom.

Reduction: A reaction in which the oxidation state of an atom decreases during a chemical process. This occurs when, for example, oxygen is lost by the atom or when hydrogen is gained.

Activation Energy: The minimum energy required to break the bonds in a molecule in order that a reaction can proceed (and new bonds form).

Catalyst: A substance which speeds up the rate of a chemical reaction without being used up - it provides a new pathway with a lower activation energy.

Heterolytic/Heterogenous Catalyst: A catalyst which is in a different physical state to the other substances in the reaction - e.g. when the catalyst is solid and the other substances are gases.

Homolytic/Homogenous Catalyst: A catalyst which is in the same physical state as the other substances in the reaction - e.g. when all the substances are gases.

Chlorofluorocarbons: Small organic molecules which contain chlorine and fluorine atoms bonded to carbon atoms.

Radical: An atom or molecule with one (or more) unpaired electron(s). Most radicals have an odd number of electrons so one of these electrons cannot be part of a pair.

Wavenumber: The number of wavelengths in 1cm. It can be calculated from the wavelength of the wave. (Wavenumber=1/wavelength (cm)).