Determine the value for the enthalpy of hydrogenation of ethene using the values for the enthalpies of combustion of ethene, hydrogen and ethane given in Table 12.

Suggest one reason why John’s answer is slightly less accurate than Marit’s answer and calculate the percentage difference.

\(\Delta H = -1411 + ( - 286) - ( - 1560)\) / correct energy cycle drawn;

\( =  - 137{\text{ kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}\);

Award [1 max] for incorrect or missing sign.

the actual values for the specific bonds may be different to the average values / the combustion values referred to the specific compounds / OWTTE;

(percentage difference) \( = \frac{{(137 - 125)}}{{137}} \times 100 = 8.76\% \);

Accept \(\frac{{(137 - 125)}}{{125}} \times 100 = 9.60\% \).

In part (b) the formula involving enthalpies of formation was often used instead of a correct enthalpy cycle for the combustion. This caused the majority of candidates to score half marks for these questions.

A few candidates could suggest a reason why one answer was slightly less accurate than the other in part (c). Most could correctly calculate the percentage difference. Surprisingly, several candidates calculated part (a) correctly and part (b) incorrectly, and then determined a percentage difference of more than 200% without seeming to notice that this does not reflect two slightly different answers.