Consider the following enthalpy of combustion data.

\[\begin{array}{*{20}{l}} {{\text{C(s)}} + {{\text{O}}_{\text{2}}}{\text{(g)}} \to {\text{C}}{{\text{O}}_{\text{2}}}{\text{(g)}}}&{\Delta {H^\Theta } = - x{\text{ kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}} \\ {{{\text{H}}_2}{\text{(g) + }}\frac{1}{2}{{\text{O}}_2}{\text{(g)}} \to {{\text{H}}_2}{\text{O(l)}}}&{\Delta {H^\Theta } = - y{\text{ kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}} \\ {{{\text{C}}_2}{{\text{H}}_6}{\text{(g)}} + {\text{3}}\frac{1}{2}{{\text{O}}_2}{\text{(g)}} \to {\text{2C}}{{\text{O}}_2}{\text{(g) + 3}}{{\text{H}}_2}{\text{O(l)}}}&{\Delta {H^\Theta } = - z{\text{ kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}} \end{array}\]

What is the enthalpy of formation of ethane in \({\text{kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}\)?

\[{\text{2C(s)}} + {\text{3}}{{\text{H}}_2}{\text{(g)}} \to {{\text{C}}_2}{{\text{H}}_6}{\text{(g)}}\]

A.     \(\left[ {( - x) + ( - y)} \right] - ( - z)\)

B.     \(( - z) - \left[ {( - x) + ( - y)} \right]\)

C.     \(\left[ {( - 2x) + ( - 3y)} \right] - ( - z)\)

D.     \(( - z) - \left[ {( - 2x) + ( - 3y)} \right]\)

C

There were three G2 comments on this question on Hess’s law, all of which stated that giving x, y and z variables instead of numeric data was confusing. However, candidates do not have the use of a calculator in P1 and hence it is common practice to use algebraic notation for this purpose. This notation has been used previously in P1 (though not always). In addition, this is a very common question and in fact, candidates had no problem whatsoever answering this question, with 80% getting the correct answer, C. The question was the third easiest question on the paper.