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.