Question

For the reaction X_2+ Y + Z → XY + XZ, it is found that the rate equation is rate = k [X_2][Y]. Which of the following might explain why the concentration of Z has no effect on the rate?

A. The concentration of Z is very small and the others are very large.
B. Z must react in a step other than the rate determining step.
C. Z is an intermediate.
D. The fraction of molecules of Z that have very high energies is zero.
E. The activation energy for Z to react is very low.

Answer 1

Answer: C. Z is an intermediate.

Explanation: The given reaction is
`X_(2) + Y + Z \rightarrow XY + XZ`

And the rate equation is rate = k [X_2][Y]

From this, we can imply that either the concentration of Z is very small in comparison tot he other reactants or the reactant Z is an intermediate which is available only for the given small amount of time.

There can be various steps in the mechanism for the proposed reaction and Z can also react in any of the steps. It is not necessary that it should react in a step other than the rate determining step.

This might be possible that the activation energy for Z to react is very low But since it has made a new kind of product that is XZ, then the former cannot be true.