| Date | May 2011 | Marks available | 1 | Reference code | 11M.2.sl.TZ1.3 |
| Level | SL | Paper | 2 | Time zone | TZ1 |
| Command term | Explain | Question number | 3 | Adapted from | N/A |
Question
Methanol may be produced by the exothermic reaction of carbon monoxide gas and hydrogen gas.
\[\begin{array}{*{20}{l}} {{\text{CO(g)}} + {\text{2}}{{\text{H}}_{\text{2}}}{\text{(g)}} \rightleftharpoons {\text{C}}{{\text{H}}_{\text{3}}}{\text{OH(g)}}}&{\Delta {H^\Theta } = - 103{\text{ kJ}}} \end{array}\]
State and explain the effect of changing the following conditions on the amount of methanol present at equilibrium:
State the equilibrium constant expression, \({K_{\text{c}}}\), for the production of methanol.
increasing the temperature of the reaction at constant pressure.
increasing the pressure of the reaction at constant temperature.
The conditions used in industry during the production of methanol are a temperature of 450 °C and pressure of up to 220 atm. Explain why these conditions are used rather than those that could give an even greater amount of methanol.
A catalyst of copper mixed with zinc oxide and alumina is used in industry for this production of methanol. Explain the function of the catalyst.
Markscheme
\(({K_{\text{c}}}) = \frac{{{\text{[C}}{{\text{H}}_{\text{3}}}{\text{OH]}}}}{{{\text{[CO][}}{{\text{H}}_{\text{2}}}{{\text{]}}^{\text{2}}}}}\);
Do not award mark if incorrect brackets are used or brackets are missing.
amount (of methanol)/product decreases / less methanol;
(forward reaction) exothermic / reverse reaction endothermic / OWTTE;
amount (of methanol)/product increases / more methanol;
3 gas molecules/mol \( \to \) 1 / decrease in volume / fewer gas molecules on right
hand side/products / more gas molecules on left hand side/reactants;
high pressure expensive / greater cost of operating at high pressure;
lower temperature – lower (reaction) rate;
increases rate of forward and reverse reactions (equally) / lowers activation energy/\({E_{\text{a}}}\) (of both the forward and reverse reaction equally) / provides alternative path with lower activation energy/\({E_{\text{a}}}\);
Accept reactants adsorb onto the catalyst surface and bonds weaken resulting in a decrease in the activation energy.
Examiners report
Candidates were able to write an equilibrium expression accurately in part (a).
Candidates had a good understanding of the effect of temperature and pressure on an equilibrium system but some did not make reference to the change in the amount of methanol produced and hence did not always achieved full marks.
In (b) (ii) candidates also failed to achieve full marks if reference was not made to the gaseous nature of the reactants and products. Occasionally candidates incorrectly discussed the effect on the rate of reaction rather than answering the question.
There was much confusion with part (c) and candidates often struggled to express themselves adequately, demonstrating that they did not fully understand the question.
Part (d) on the other hand was extremely well answered with candidates having an excellent understanding of the function of a catalyst.
