The standard enthalpy change of formation for \({\text{C}}{{\text{H}}_{\text{3}}}{\text{OH(g)}}\) at 298 K is \( - 201{\text{ kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}\) and for \({{\text{H}}_{\text{2}}}{\text{O(g)}}\) is \( - 242{\text{ kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}\). Using information from Table 11 of the Data Booklet, determine the enthalpy change for this reaction.

The standard entropy for \({\text{C}}{{\text{H}}_{\text{3}}}{\text{OH(g)}}\) at 298 K is \({\text{238 J}}\,{{\text{K}}^{ - 1}}{\text{mo}}{{\text{l}}^{ - 1}}\), for \({{\text{H}}_{\text{2}}}{\text{(g)}}\) is \({\text{131 J}}\,{{\text{K}}^{ - 1}}{\text{mo}}{{\text{l}}^{ - 1}}\) and for \({{\text{H}}_{\text{2}}}{\text{O(g)}}\) is \({\text{189 J}}\,{{\text{K}}^{ - 1}}{\text{mo}}{{\text{l}}^{ - 1}}\). Using information from Table 11 of the Data Booklet, determine the entropy change for this reaction.

Calculate the standard change in free energy, at 298 K, for the reaction and deduce whether the reaction is spontaneous or non-spontaneous.

\(\Delta H_{{\text{reaction}}}^\Theta  = \Sigma \Delta H_{\text{f}}^\Theta {\text{(products)}} - \Sigma \Delta H_{\text{f}}^\Theta {\text{(reactants)}}\)

\( = [(1)( - 85) + (2)( - 242)] - [(2)( - 201)]\);

\( =  - 167{\text{ (kJ/kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}{\text{)}}\);

Award [1] for (+)167.

\(\Delta S_{{\text{reaction}}}^\Theta  = \Sigma {S^\Theta }{\text{(products)}} - \Sigma {S^\Theta }{\text{(reactants)}}\)

\( = [(1)(230) + (2)(189)] - [(2)(238) + (1)(131)]\);

\( = 1{\text{ (J}}\,{{\text{K}}^{ - 1}}{\text{/J}}\,{{\text{K}}^{ - 1}}{\text{mo}}{{\text{l}}^{ - 1}}{\text{)}}\);

\(\Delta G_{{\text{reaction}}}^\Theta  = (\Delta {H^\Theta } - T\Delta {S^\Theta }) = ( - 167) - (298)(0.001)\);

Award [1] for correct substitution of values.

\( =  - 167{\text{ kJ/}} - 167000{\text{ J}}\);

Units needed for mark in (c) only.

Accept –167 kJ\(\,\)mol^–1 or –167000 J\(\,\)mol^–1.

spontaneous;

Award marks for final correct answers throughout in each of (a), (b) and (c).

In (a) the most common mistakes included: failure to consider the correct amount of moles of products/reactants, incorrect identification of values or wrong use of convention. It also should be noted that the correct units of \(\Delta {H^\Theta }\) here in the answer will be kJ, since \(n\) is used in the equation, as explained in previous subject reports.

Part (b) was another question where the vast majority of candidates scored full marks.

Free energy calculations (c) continues to prove problematic for many candidates. Candidates very often lost the first mark due to wrong use of units. ECF allowed them to score the second. In contrast most candidates showed a clear understanding of the relationship between the sign of \(\Delta {G^\Theta }\) and spontaneity.