When four moles of aluminium and four moles of iron combine with oxygen to form their oxides, the enthalpy changes are –3338 kJ and –1644 kJ respectively.

     \({\text{4Al(s)}} + {\text{3}}{{\text{O}}_{\text{2}}}{\text{(g)}} \to {\text{2A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{(s)}}\)     \(\Delta H =  - 3338{\text{ kJ}}\)

     \({\text{4Fe(s)}} + {\text{3}}{{\text{O}}_{\text{2}}}{\text{(g)}} \to {\text{2F}}{{\text{e}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{(s)}}\)     \(\Delta H =  - 1644{\text{ kJ}}\)

What is the enthalpy change, in kJ, for the reduction of one mole of iron(III) oxide by aluminium?

\[{\text{F}}{{\text{e}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{(s)}} + {\text{2Al(s)}} \to {\text{2Fe(s)}} + {\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{(s)}}\]

A.     \( + 1694\)

B.     \( + 847\)

C.     \( - 847\)

D.     \( - 1694\)

C

There was one concern about how students of mathematical studies might fare with this question. The arithmetic is straightforward and, in the event, nearly 64% gave the correct answer.