Consider the following two equations.

\({\text{2Ca(s)}} + {{\text{O}}_2}{\text{(g)}} \to {\text{2CaO(s)}}\)     \(\Delta {H^\Theta } =  + x{\text{ kJ}}\)

\({\text{Ca(s)}} + {\text{0.5}}{{\text{O}}_2}{\text{(g)}} + {\text{C}}{{\text{O}}_2}{\text{(g)}} \to {\text{CaC}}{{\text{O}}_3}{\text{(s)}}\)     \(\Delta {H^\Theta } =  + y{\text{ kJ}}\)

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

\[{\text{CaO(s)}} + {\text{C}}{{\text{O}}_2}{\text{(g)}} \to {\text{CaC}}{{\text{O}}_3}{\text{(s)}}\]

A.     \(y - 0.5x\)

B.     \(y - x\)

C.     \(0.5 - y\)

D.     \(x - y\)

A

There was concern about the use of algebraic notation rather than actual numerical data. This has been used since November 2010 so candidates should be familiar with this type of question. (In fact, some G2s in the past have suggested it would be better to use algebraic notation!) In the event it was the sixth easiest question; over 81% of candidates gave the correct answer and 9% gave B.