7. Which of the following would be a rate law for the reaction?

Question

The proposed mechanism for a reaction is:

I₂ => 2 I            

I + H₂ => H₂I          slow

H₂I + I => 2 HI       Fast

Which of the following would be a rate law for the reaction?

Detailed explanation please

The answer is: rate = k[I₂]^1/2[H₂]

Answer 1

As mentioned in a previous answer to a similar question, since the slow step is the FIRST step in the proposed mechanism, the rate law will be based on this elementary step. In the slow step proposed, you have 1/2 of an I2 molecule, and so you it can be written as 1/2 I2 and the coefficient will be 1/2. The coefficient for H2 is simply 1. So, the rate law will be rate = k[I₂]^1/2[H₂]