Consider the following proposed two-step reaction mechanism at temperature T.

     Step 1:     ${\text{2N}}{{\text{O}}_2}{\text{(g)}}\xrightarrow{{{k_1}}}{\text{NO(g)}} + {\text{N}}{{\text{O}}_3}{\text{(g)}}$     Slow

     Step 2:     ${\text{N}}{{\text{O}}_3}{\text{(g)}} + {\text{CO(g)}}\xrightarrow{{{k_2}}}{\text{N}}{{\text{O}}_2}{\text{(g)}} + {\text{C}}{{\text{O}}_2}{\text{(g)}}$     Fast

Which statements are correct?

I.     The overall reaction is ${\text{N}}{{\text{O}}_2}{\text{(g)}} + {\text{CO(g)}} \to {\text{NO(g)}} + {\text{C}}{{\text{O}}_2}{\text{(g)}}$.

II.     Step 1 is the rate-determining step of the reaction.

III.     The rate expression for Step 1 is rate $= {k_1}{{\text{[N}}{{\text{O}}_2}{\text{]}}^2}$.

A.     I and II only

B.     I and III only

C.     II and III only

D.     I, II and III

D

This was reported to be “demanding (makes them think) but good”. It was the sixth hardest question with 63% scoring correctly. The most common error was not to recognize that the overall reaction equation is correct.