Consider the following reactions.

\[\begin{array}{*{20}{l}} {{{\text{N}}_2}({\text{g)}} + {{\text{O}}_2}{\text{(g)}} \to {\text{2NO(g)}}}&{\Delta {H^\Theta } = + 180{\text{ kJ}}} \\ {2{\text{N}}{{\text{O}}_2}({\text{g)}} \to {\text{2NO(g)}} + {{\text{O}}_2}{\text{(g)}}}&{\Delta {H^\Theta } = + 112{\text{ kJ}}} \end{array}\]

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

\[{{\text{N}}_2}({\text{g)}} + {\text{2}}{{\text{O}}_2}{\text{(g)}} \to {\text{2N}}{{\text{O}}_2}{\text{(g)}}\]

A.     \( - 1 \times ( + 180) +  - 1 \times ( + 112)\)

B.     \( - 1 \times ( + 180) + 1 \times ( + 112)\)

C.     \(1 \times ( + 180) +  - 1 \times ( + 112)\)

D.     \(1 \times ( + 180) + 1 \times ( + 112)\)

C

Two respondents 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 second easiest question on the paper and 82.41% of candidates got the correct answer C.