State and explain the effect of increasing the pressure on the yield of sulfur trioxide.

State the effects of a catalyst on the forward and reverse reactions, on the position of equilibrium and on the value of \({K_{\text{c}}}\).

When a mixture of 0.100 mol NO, 0.051 mol \({{\text{H}}_{\text{2}}}\) and 0.100 mol \({{\text{H}}_{\text{2}}}{\text{O}}\) were placed in a \({\text{1.0 d}}{{\text{m}}^{\text{3}}}\) flask at 300 K, the following equilibrium was established.

\(2{\text{NO(g)}} + 2{{\text{H}}_2}{\text{(g)}} \rightleftharpoons {{\text{N}}_2}{\text{(g)}} + 2{{\text{H}}_2}{\text{O(g)}}\)

At equilibrium, the concentration of NO was found to be \({\text{0.062 mol}}\,{\text{d}}{{\text{m}}^{ - 3}}\). Determine the equilibrium constant, \({K_{\text{c}}}\), of the reaction at this temperature.

Outline two differences between an electrolytic cell and a voltaic cell.

Electroplating is an important application of electrolysis. State the composition of the electrodes and the electrolyte used in the silver electroplating process.

yield (of \({\text{S}}{{\text{O}}_{\text{3}}}\)) increases / equilibrium moves to right / more \({\text{S}}{{\text{O}}_{\text{3}}}\) formed;

3 gaseous molecules \( \to \) 2 gaseous molecules / decrease in volume of gaseous molecules / fewer gaseous molecules on right hand side;

Do not allow ECF.

rates of both forward and reverse reactions increase equally;

no effect on position of equilibrium;

no effect on value of [3] \({K_{\text{c}}}\);

\({\text{2NO(g)}} + {\text{2}}{{\text{H}}_2}{\text{(g)}} \rightleftharpoons {{\text{N}}_2}{\text{(g)}} + {\text{2}}{{\text{H}}_2}{\text{O(g)}}\)

\({\text{[}}{{\text{H}}_{\text{2}}}{\text{] at equilibrium}} = 0.013{\text{ (mol}}\,{\text{d}}{{\text{m}}^{ - 3}}{\text{)}}\);

\({\text{[}}{{\text{N}}_{\text{2}}}{\text{] at equilibrium}} = 0.019{\text{ (mol}}\,{\text{d}}{{\text{m}}^{ - 3}}{\text{)}}\);

\({\text{[}}{{\text{H}}_{\text{2}}}{\text{O] at equilibrium}} = 0.138{\text{ (mol}}\,{\text{d}}{{\text{m}}^{ - 3}}{\text{)}}\);

\({K_{\text{c}}} = {\text{[}}{{\text{N}}_2}{\text{][}}{{\text{H}}_2}{\text{O}}{{\text{]}}^2}{\text{/[NO}}{{\text{]}}^2}{{\text{[}}{{\text{H}}_2}{\text{]}}^2} = {\text{(0.019)(0.138}}{{\text{)}}^2}{\text{/(0.062}}{{\text{)}}^2}{{\text{(0.013)}}^2} = 5.6 \times {10^2}\);

Award [4] for final correct answer.

Accept any value also in range 557–560.

Do not penalize significant figures.

electrolytic cell converts electrical energy to chemical energy and voltaic cell converts chemical energy to electrical energy / electrolytic cell uses electricity to carry out a (redox) chemical reaction and voltaic cell uses a (redox) chemical reaction to produce electricity / electrolytic cell requires a power supply and voltaic cell does not;

electrolytic cell involves a non-spontaneous (redox) reaction and voltaic cell involves a spontaneous (redox) reaction;

in an electrolytic cell, cathode is negative and anode is positive and vice-versa for a voltaic cell / electrolytic cell, anode is positive and voltaic cell, anode is negative / electrolytic cell, cathode is negative and voltaic cell, cathode is positive;

voltaic cell has two separate solutions and electrolytic cell has one solution / voltaic cell has salt bridge and electrolytic cell has no salt bridge;

electrolytic cell, oxidation occurs at the positive electrode/anode and voltaic cell, oxidation occurs at the negative electrode/anode and vice-versa;

Cathode/negative electrode:

object to be plated;

Allow a specific example here e.g. spoon.

Accept inert metal/graphite.

Do not accept silver halides or their formulae.

Anode/positive electrode:

Silver/Ag;

Electrolyte:

\({{\text{[Ag(CN}}{{\text{)}}_{\text{2}}}{\text{]}}^ - }\);

Allow silver nitrate/AgNO3 / silver cyanide/any other suitable silver salt/solution.

Do not accept AgCl.

In (ii) an overwhelming number of candidates were able to score the first mark but did not refer to the gaseous state and hence lost the second mark.

Part (iv) was another question where candidates easily scored the second and third mark. Although this has been asked a number of times in recent sessions, some candidates still do not state that the rates of both the forward and reverse reactions increase equally.

(b) was considered a very challenging question for candidates, and usually only the better candidates scored all four marks.

In (c) (i) most candidates scored two marks.

Electroplating was a topic only partially understood by candidates, and so only a few candidates obtained all three marks in (v). Often the nature of the electrode was mixed up or in many cases incorrect electrolytes were given.