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.