Glossary

anode - the positive electrode.

electrochemical cell – two half cells connected by a salt bridge, creating a potential difference that can be used to produce a current in a complete circuit.

electrochemical series - a list of standard electrode potentials ordered with the more negative electrode potentials at the top and the more positive potentials at the bottom.

electrode – the part of a half cell at which either oxidation or reduction occurs.

electrode potential – a measure of how likely a half cell is to gain electrons. Represented by E (or E^θ under standard conditions) and sometimes called reduction potentials. The electrode potential of a half cell cannot be measured directly but must be calculated by connecting the half cell to a standard hydrogen electrode. Half cells with negative values of E^θ are less likely to gain electrons (i.e. worse reducing agents) than hydrogen.

electrolysis – the process of passing an electric current through a molten or aqueous compound to split it into elements.

electromotive force (e.m.f) – the maximum voltage an electrochemical cell can produce. This can only be observed when negligible current flows.

cathode – the negative electrode.

cryolite – the sub-stance Na₃AlF₆(sodium hexafluo-roaluminate) used to dissolve alumin-ium oxide to make the electrolyte for the extraction of aluminium by elec-trolysis.

displacement reaction - a reaction in which an element is displaced out of a compound by a more reactive element; for example, bromine displaces iodine in: Br₂+2KI → 2KBr+I₂

disproportionation – a reaction in which the same element is both oxidized and reduced; for exam-ple, chlorine in: Cl₂+ H₂O → HCl + HOCl.

half-cell – half of an electrochemical cell, where either oxidation or reduction occurs. It consists of an electrode in contact with a solution of ions. The simplest half cells are a metal electrode immersed in a solution of metal ions.

half-equation – an equation describing either the oxidation or reduction in a redox reaction; for example, Cl₂+ 2e^- → 2Cl^- shows the reduction of chlorine to chloride ions.

hydrogen electrode – the half-cell against which all other electrode potentials are measured. Also known as the standard hydrogen electrode. It consists of a 1 mol/dm3 solution of H+ ions, in which is immersed a platinum-covered electrode, and over which is passed hydrogen gas at standard temperature and pressure. The half equation is 2H⁺_(aq)+ 2e^- → H_2(g), and the standard electrode potential is 0.

oxidation – a reaction in which a substance loses electrons or increases its oxidation state.

oxidation half equation – a half-equation where the reactant loses at least one electron. It is written with the electrons on the right of the arrow, i.e. as oxidation.

oxidation number – a simple way of keeping track of redox reactions, so that it is easy to see which species have been oxidized and which reduced. Also known as an oxidation state.

oxidation state – the charge a particular atom in a compound would have if the compound consisted entirely of separate ions. Also called the oxidation number.

oxidizing agent – the species that accepts electrons, thus oxidizing another species while it is itself reduced.

redox – a chemical reaction that involves oxidation and a reduction.

reduction – a reaction in which a substance gains at least one electron or decreases its oxidation state.

reducing agent – a species that donates electrons, thus reducing another species while it is itself oxidized.

reduction half equation – a half- equation where the reactant gains at least one electron. It is written with the electrons on the left of the arrow, i.e. as a reduction (sometimes known as a reduction potential). Electrodes with negative values of E^θ are better at releasing electrons (better-reducing agents) than hydrogen.

salt bridge – provides electrical contact between solutions without allowing them to mix. The simplest salt bridge is a piece of paper soaked in a saturated salt solution, such as KCl_(aq), KNO_3(aq) or NH₄NO_3(aq). A better version is a glass tube filled with the salt in jelly form.

spectator ion – an ion that does not take part in a reaction, but remains unchanged. Spectator ions may still appear in the overall redox equation for the reaction.

standard electrode potential – the potential difference between a particular electrode and a standard hydrogen electrode when both half cells are under standard conditions.

standard hydrogen electrode – the half-cell against which all other electrode potentials are measured. Also known as the standard hydrogen electrode. It consists of a 1moldm^-3 solution of H⁺ ions, in which is immersed a platinum-covered electrode, and over which is passed hydrogen gas at standard temperature and pressure. The half equation is 2H⁺_(aq) + 2e^- → H_2(g), and the standard electrode potential is 0.

Practice Exercises