Consider the following reaction between nitrogen monoxide and oxygen.

\[{\text{2NO(g)}} + {{\text{O}}_{\text{2}}}{\text{(g)}} \to {\text{2N}}{{\text{O}}_{\text{2}}}{\text{(g)}}\]

The reaction occurs in two steps:

     Step 1: \({\text{NO(g)}} + {\text{NO(g)}} \rightleftharpoons {{\text{N}}_{\text{2}}}{{\text{O}}_{\text{2}}}{\text{(g)}}\)     fast

     Step 2: \({{\text{N}}_{\text{2}}}{{\text{O}}_{\text{2}}}{\text{(g)}} + {{\text{O}}_{\text{2}}}{\text{(g)}} \to {\text{2N}}{{\text{O}}_{\text{2}}}{\text{(g)}}\)     slow

What is the rate expression for this reaction?

A.     Rate \( = k{{\text{[NO]}}^{\text{2}}}\)

B.     Rate \( = k{\text{[NO][}}{{\text{O}}_{\text{2}}}{\text{]}}\)

C.     Rate \( = k{{\text{[NO]}}^{\text{2}}}{\text{[}}{{\text{O}}_{\text{2}}}{\text{]}}\)

D.     Rate \( = k{\text{[NO][}}{{\text{O}}_{\text{2}}}{{\text{]}}^{\text{2}}}\)

C

There were two comments. One suggested a “quantum change in difficulty” and the other commented that “students are not normally taught how to eliminate the intermediate from the rate expression for the slow step”. In the event, 82.71% of the candidates scored correctly, the sixth “easiest” question on the paper.