JJC H2 Chem 2012 Prelim P3 Soln

Page 1 Mark Scheme for 2012 JC2 Preliminary Examination Paper 3 Minus [1m] overall for P3 for lack of 3 sf. Indicate on Cover page when penalised. General Comments for Q1 Many candidates probably left this question as their last attempted question, as they ran out of time and thus (i) did not manage complete answers or (ii) did not have enough time to calculate their answers or (iii) made a lot of careless mistakes. Marks for this question varies greatly from 4-5 to 17-20. 1. (a) (i) Anode: 2H2(g) ® 4H+(aq) + 4e- Cathode: O2(g) + 4H+(aq) + 4e- ® 2H2O(l) [1m] s.s not penalised [1m] s.s. not penalised · Small handful of candidates still make the usual errors: o Use of reversible arrows o Use of [O] and [R] o Wrong electrodes or no labelling of electrodes · A few wrote equations for alkaline medium · A few chose half eqn for H2O2 instead. (ii) · Requires a heavy storage tank of H2 gas on board a car / takes up more storage space / must be kept under high pressure · Difficult to refill H2 fuel due to lack of infrastructure · H2 gas is MORE explosive than fuels in internal combustion engines. · More expensive due to the expensive catalysts etc · Explosive at high temperatures [1m] for any relevant ans. Do not accept: “explosive” only Common Mistakes: · A few listed advantages instead · A small handful of candidates did not understand what to compare e.g. thinking that the hydrogen-oxygen fuel cell is used together with the internal combustion engine or to supply electricity to the internal combustion engine. (iii) Using 12 2 11 12 2 , PP P PTTT T æö æö= == ç÷ ç÷èøèø 53.29 ×10 373293 Pa= 54.19 ×10 [1m]: correct subst into formula. Accept 293.15 and 373.15 [1m]: correct ans, 3 s.f. · Many candidates solved using PV=nRT, which is a longer method. Common Mistake: · Wrong units for Temperature · Some gave a qualitative answer only, with no calculation.

Page 2 1. (b) (i) Reference electrode (cathode): Hg2Cl2(s) + 2e à 2Hg(l) + 2Cl-(aq) [1m] full arrow (ii) Measuring electrode (anode): H2(g) à 2H+ (aq) + 2e Overall: Hg2Cl2(s) + H2(g) à 2Hg(l) + 2Cl-(aq) + 2H+ (aq) [1m] full arrow Common Mistakes for (i) and (ii): · Did not seem to understand Qn or were simply confused with ‘Hg’ and ‘H’ e.g. o Mixed up ‘Hg’ with ‘H2’ or ‘Hg’ with ‘H’ ! o wrote reduction of chlorine to chloride only o wrote eqns for reaction between hydrogen and chlorine only · Did not notice formula of “Hg2Cl2”. · Did not check their equation for balancing of charges. (iii) At a higher pH, there is lower [H+], · equilibrium in 2H+ (aq) + 2e = H2(g) shifts t o the left (OR eqm shifts to favour oxidation of H2) and thus · E(H+/H2) becomes (more) negative. Thus Ecell becomes more positive. [1m] No penalty for “E,”. Just circle. Common Mistakes: · Thought “higher pH” = “higher [H+]” · Did not write reversible arrow for the eqm