Consider the following reactions.

\[\begin{array}{*{20}{l}} {{\text{C}}{{\text{u}}_2}{\text{O(s)}} + \frac{1}{2}{{\text{O}}_2}{\text{(g)}} \to {\text{2CuO(s)}}}&{\Delta {H^\Theta } = - 144{\text{ kJ}}} \\ {{\text{C}}{{\text{u}}_2}{\text{O(s)}} \to {\text{Cu(s)}} + {\text{CuO(s)}}}&{\Delta {H^\Theta } = + 11{\text{ kJ}}} \end{array}\]

What is the value of \(\Delta {H^\Theta }\), in kJ, for this reaction?

\[{\text{Cu(s)}} + \frac{1}{2}{{\text{O}}_2}{\text{(g)}} \to {\text{CuO(s)}}\]

A.     \( - 144 + 11\)

B.     \( + 144 - 11\)

C.     \( - 144 - 11\)

D.     \( + 144 + 11\)

C

One respondent stated that there was too much mathematics required to answer this question. However, candidates simply had to use Hess’s law and were not required to determine the numerical value of the final answer. In fact, the question was the eight easiest question on the paper and 76.61% of candidates got the correct answer C.