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

This was a common question with standard level where there was concern about the use of algebraic notation rather than actual numerical data. Algebraic notation has been used since November 2010 so candidates should be familiar with this type of question.

While one comment in HL agreed with this sentiment, the other said it was “good to use pronumerals”. In the event, it was the fifth easiest question; nearly 91% of candidates gave the correct answer and less than 6% gave B.