WAEC Chemistry 2015 Paper

Concept:Hot concentrated tetraoxosulphate(VI) acid behaves as an oxidising agent.It oxidises aluminium metal while being reduced to sulphur(IV) oxide.Explanation:Aluminium changes from oxidation state $0$ to $+3$, so each aluminium atom loses $3$ electrons.Two aluminium atoms lose a total of $6$ electrons.Each sulphur atom in $\mathrm{H_2SO_4}$ changes from $+6$ to $+4$ in $\mathrm{SO_2}$, gaining $2$ electrons.Therefore, $3$ sulphate units are reduced to $3\mathrm{SO_2}$ to accept the $6$ electrons.The remaining $3$ sulphate units combine with aluminium to form $\mathrm{Al_2(SO_4)_3}$.Total molecules of $\mathrm{H_2SO_4}$ consumed are $6$.Balancing the equation gives:$$2\mathrm{Al_{(s)}} + 6\mathrm{H_2SO_{4(l)}} \to \mathrm{Al_2(SO_4)_{3(aq)}} + 3\mathrm{SO_{2(g)}} + 6\mathrm{H_2O_{(l)}}$$This matches option A.Answer:Option A.