Consider the two reactions involving iron and oxygen.

$$\begin{array}{*{20}{l}} {{\text{2Fe(s)}} + {{\text{O}}_2}{\text{(g)}} \to {\text{2FeO(s)}}}&{\Delta {H^\Theta } = - 544{\text{ kJ}}} \\ {{\text{4Fe(s)}} + {\text{3}}{{\text{O}}_2}{\text{(g)}} \to {\text{2F}}{{\text{e}}_2}{{\text{O}}_3}{\text{(s)}}}&{\Delta {H^\Theta } = - 1648{\text{ kJ}}} \end{array}$$

What is the enthalpy change, in kJ, for the reaction below?

$${\text{4FeO(s)}} + {{\text{O}}_2}{\text{(g)}} \to {\text{2F}}{{\text{e}}_2}{{\text{O}}_3}{\text{(s)}}$$

A.     $- 1648 - 2( - 544)$

B.     $- 544 - ( - 1648)$

C.     $- 1648 - 544$

D.     $- 1648 - 2(544)$

A

One respondent stated that it would have been better to represent the four choices A-D as numerical values. However, candidates do not have access to a calculator in P1 and therefore simply had to use Hess’s law without working out the final answer. 67.83% of candidates got the correct answer.