(i)     State equations for the reactions that occur at each electrode in a lead-acid battery when it delivers a current.

Positive electrode (cathode):

Negative electrode (anode):

(ii)     State equations for the reactions that occur at each electrode in a nickel-cadmium (NiCad) battery when it delivers a current.

Positive electrode (cathode):

Negative electrode (anode):

Another source of power for portable devices is the fuel cell. Compare fuel cells with lead-acid rechargeable batteries, stating one similarity and two differences.

Similarity:

Differences:

(i)     Lead-acid:

Positive electrode (cathode):

\({\text{Pb}}{{\text{O}}_2}{\text{(s)}} + {\text{SO}}_4^{2 - }{\text{(aq)}} + {\text{4}}{{\text{H}}^ + }{\text{(aq)}} + {\text{2}}{{\text{e}}^ - } \to {\text{PbS}}{{\text{O}}_4}{\text{(s)}} + {\text{2}}{{\text{H}}_2}{\text{O(l)}}\) /

\({\text{Pb}}{{\text{O}}_2}{\text{(s)}} + {\text{HSO}}_4^ - {\text{(aq)}} + {\text{3}}{{\text{H}}^ + }{\text{(aq)}} + {\text{2}}{{\text{e}}^ - } \to {\text{PbS}}{{\text{O}}_4}{\text{(s)}} + {\text{2}}{{\text{H}}_2}{\text{O(l)}}\);

Negative electrode (anode):

\({\text{Pb(s)}} + {\text{SO}}_4^{2 - }{\text{(aq)}} \to {\text{PbS}}{{\text{O}}_4}{\text{(s)}} + {\text{2}}{{\text{e}}^ - }\) /

\({\text{Pb(s)}} + {\text{HSO}}_4^ - {\text{(aq)}} \to {\text{PbS}}{{\text{O}}_4}{\text{(s)}} + {\text{2}}{{\text{e}}^ - } + {{\text{H}}^ + }{\text{(aq)}}\);

Allow e for e^– throughout.

Ignore state symbols.

Award [1 max] if correct equations are given but at the wrong electrodes.

(ii)     NiCad:

Positive electrode (cathode):

\({\text{NiO(OH)(s)}} + {{\text{H}}_2}{\text{O(l)}} + {{\text{e}}^ - } \to {\text{Ni(OH}}{{\text{)}}_2}{\text{(s)}} + {\text{O}}{{\text{H}}^ - }{\text{(aq)}}\);

Negative electrode (anode):

\({\text{Cd(s)}} + {\text{2O}}{{\text{H}}^ - }{\text{(aq)}} \to {\text{Cd(OH}}{{\text{)}}_2}{\text{(s)}} + {\text{2}}{{\text{e}}^ - }\);

Allow e for e^– throughout.

Ignore state symbols.

Award [1 max] if correct equations are given but at the wrong electrodes.

Similarity:

(both) convert chemical energy to electrical energy / (both are) voltaic cells;

Differences:

Award [2 max] for any two.

rechargeable batteries employ reversible reactions while fuel cells have irreversible reactions;

fuel cells work non-stop while rechargeable batteries take time to recharge;

fuel cells need a constant supply of reactants/fuel while rechargeable batteries do not need any other substances;

fuel cells convert energy and rechargeable batteries store energy;

fuel cell products must be constantly removed but not for rechargeable batteries;

fuel cells are less polluting/more expensive/weigh less/last longer (than lead-acid rechargeable batteries);

fuel cells have inert/Pt electrodes/components while lead-acid rechargeable batteries have active/non-inert/Pb and \({\text{Pb}}{{\text{O}}_{\text{2}}}\) electrodes;

fuel cells run at higher temperatures than rechargeable batteries;

fuel cells are less portable than rechargeable batteries / fuel cells require pumps/cooling systems while rechargeable batteries do not;

Award [2 max] if three valid points (one similarity and two differences) are given without comparison and [1 max] if two valid points are given without comparison.

(i)     Poorly answered even by strong candidates. Very few candidates gave the correct equations for the reactions occurring at the electrodes of a lead-acid battery.

(ii) This part-question was also poorly answered with very few candidates scoring one out of the two possible marks.

Many candidates scored at least one mark, but many answers only reflected a shallow understanding of fuel cells and lead-acid batteries. Quite a few candidates are still neglecting to satisfy the demands of the “compare” command term that requires reference to both items in every point of comparison.