Consider the equations below.

\[\begin{array}{*{20}{l}} {{\text{C}}{{\text{H}}_{\text{4}}}{\text{(g)}} + {{\text{O}}_{\text{2}}}{\text{(g)}} \to {\text{HCHO(l)}} + {{\text{H}}_{\text{2}}}{\text{O(l)}}}&{\Delta {H^\Theta } = x} \\ {{\text{HCHO(l)}} + \frac{1}{2}{{\text{O}}_2}{\text{(g)}} \to {\text{HCOOH(l)}}}&{\Delta {H^\Theta } = y} \\ {2{\text{HCOOH(l)}} + \frac{1}{2}{{\text{O}}_2}{\text{(g)}} \to {{{\text{(COOH)}}}_2}({\text{s)}} + {{\text{H}}_2}{\text{O(l)}}}&{\Delta {H^\Theta } = z} \end{array}\]

What is the enthalpy change of the reaction below?

\[2{\text{C}}{{\text{H}}_4}{\text{(g)}} + {\text{3}}\frac{1}{2}{{\text{O}}_2}{\text{(g)}} \to {{\text{(COOH)}}_2}{\text{(s)}} + 3{{\text{H}}_2}{\text{O(l)}}\]

A.     \(x + y + z\)

B.     \(2x + y + z\)

C.     \(2x + 2y + z\)

D.     \(2x + 2y + 2z\)

C

On one of the G2’s it was stated that this question was challenging as candidates are not used to dealing with three equations when the enthalpy change asked for specifically involves only two organic compounds. This question is based on AS 5.3.1 which states explicitly that candidates should be able to determine the enthalpy change of a reaction that is the sum of two or three reactions with known enthalpy changes. In this question three reactions were given with corresponding enthalpy change values of x, y and z. Hence, by fairly straight-forward manipulation of the reactions, the final enthalpy change of the given reaction could be determined as \(C = 2x + 2y + z\). The question was answered correctly by 84% of candidates and in fact was found to be the seventh easiest question on the paper with a corresponding discrimination index of 0.32.