1Summary: Chapter 4© Cambridge University Press IGCSE Chemistry

Chapter 4SummaryYou should know that:

chemical reactions differ from physical changes such as melting or dissolving because new chemical substance(s) are produced!there is no loss or gain in total mass during a chemical reaction (the law of conservation of mass)!reactions can be represented by word equations and balanced chemical equations!balancing an equation involves making sure that there are the same numbers of each type of atom involved on the reactants side !and products side of the equationreactions involve an energy change – with most reactions being exothermic and only a few being endothermic!there is a variety of different types of chemical reaction!synthesis and decomposition represent opposite processes!neutralisation reactions involve the reaction of an acid with a base to produce a salt and water only!insoluble salts can be produced by precipitation reactions!reactive elements can displace other, less reactive, elements from their compounds in displacement reactions!combustion reactions involve reaction with oxygen and include the biologically important reaction known as respiration, while !combustion reactions that produce a fl ame are known as burningredox reactions involve the loss or gain of oxygen by substances – oxidation being the gain of oxygen and reduction being the !loss of oxygensome substances are reducing agents (for example hydrogen, carbon), while others are oxidising agents (for example oxygen, !potassium manganate(VII)).metals conduct electricity whereas non-metals (except graphite) do not!in metallic conductivity, the current is produced by electrons moving through the solid or liquid metal!there is a different type of conductivity, known as electrolytic conductivity, in which ionic compounds conduct when molten or !dissolved in waterelectrolytic conductivity involves the movement of ions and results in a chemical change (electrolysis)!liquids or solutions can be classifi ed as electrolytes or non-electrolytes!an electrolytic cell consists of positive (anode) and negative (cathode) electrodes and an electrolyte!the products of electrolysis appear at the electrodes where ions are discharged!there are various rules that help us to predict the products of electrolysis, but generally metals or hydrogen are always produced !at the negative electrode (the cathode)electroplating can be used to produce a protective and/or decorative layer of one metal on another.!

! equations can be made more informative by including state symbolsfor reactions involving ions (for example neutralisation and precipitation reactions), an equation can be simplifi ed to include only !those ions taking part in the reaction – the ‘spectator ions’ are eliminated from the equationthe defi nitions of oxidation and reduction can be extended to include reactions involving the transfer of electrons – oxidation is !the loss of electrons and reduction is the gain of electronsthe reactions taking place at the electrodes during electrolysis can be regarded as oxidation or reduction reactions because the !ions either lose or gain electrons when they are dischargedelectrolysis is industrially important for the extraction of very reactive metals (for example aluminium) and the production of !sodium hydroxide and chlorineoxidation takes place at the positive electrode (the anode), and reduction at the cathode!the products of electrolysis can depend on the concentration of ions in the solution!the method of electroplating can be adapted to provide a method of purifying (refi ning) copper – in these processes the !electrodes are not inert and the anode actually dissolves.

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