State the half-equations occurring at each electrode in the hydrogen-oxygen fuel cell in an alkaline medium.

(+) Cathode:

(–) Anode:

Describe the composition of the electrodes and state the overall cell equation of the nickel-cadmium battery.

(+) Cathode:

(–) Anode:

Cell equation:

Compare a fuel cell and a lead-acid battery, with respect to possible concerns about pollution of the environment.

(+) Cathode:

\({\text{2}}{{\text{H}}_{\text{2}}}{\text{O}} + {{\text{O}}_{\text{2}}} + {\text{4e}} \to {\text{4O}}{{\text{H}}^ - }\);

(–) Anode:

\({\text{2}}{{\text{H}}_2} + {\text{4O}}{{\text{H}}^ - } \to {\text{4}}{{\text{H}}_2}{\text{O}} + {\text{4e}}\);

If both equations given but at wrong electrodes award [1].

(+) Cathode:

nickel hydroxide/\({\text{Ni(OH}}{{\text{)}}_{\text{2}}}\);

(–) Anode:

cadmium hydroxide/\({\text{Cd(OH}}{{\text{)}}_{\text{2}}}\);

Cell equation:

\({\text{Cd}} + {\text{2}}{{\text{H}}_2}{\text{O}} + {\text{2NiO(OH)}} \to {\text{Cd(OH}}{{\text{)}}_2} + {\text{2Ni(OH}}{{\text{)}}_2}\);

neither cause pollution when running;

lead/sulfuric acid are pollutants (making or disposing of battery);

production of hydrogen and oxygen for fuel cells causes pollution;

In (a) some candidates managed to give the correct half-equations but often the cathode and anode were reversed.

In (b) this was poorly answered although some candidates managed to write the cell equation without properly describing the cathode and anode.

In (c) most candidates could score 1 mark for stating that Pb and \({{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}\) pollute.