For the mechanism below answer questions 9 10 and 11 Step 1

For the mechanism below, answer questions 9, 10 and 11: Step 1 (fast, equilibrium) Step 2 (slow) Step 3 (fast) Which species is an intermediate in the reaction? I2 2I 2 ----> H2l H2l I 2 HI none O HI H2 O 12

Solution

Answer is I.

An intermediate in a multi-step reaction is created as a product in one step, and then used as a reactant in a follow-on step. So the only choices here for an intermediate are I and H2I. What you really care about ultimately is the rate of the slowest step in the overall reaction. This is the rate-determining step, which determines the rate of the overall reaction. The rate-determining step is step 2. To calculate the rate of step 2, you need to know the concentrations of the reactants in step 2. But I is not a reactant you put into the reaction mixture at the beginning. I is formed in step 1, so you don\'t know its concentration. The good news is that you can calculate the concentration of I. Set up an equation for the rate of change in the concentration of I, and because I is an intermediate, you can set this rate of change to zero. You can then solve that equation for the concentration of I. From there, you can find the rate of step 2, which as we said before, is the rate of the overall reaction.

As you figured out, H2I is technically an intermediate also. But declaring H2I as an intermediate and going through the equations I described above doesn\'t help you solve for the overall reaction rate. The reason for that is that H2I is not a reactant in the rate-determining step. So the best way to think about this is that both I and H2I are intermediates, but I is the important intermediate because it is a reactant in the rate-determining step.