A proposed mechanism for the reaction Cl2(g) + CHCl3(g)  HCl(g) + CCl4(g) in the atmosphere is Step 1: Cl2(g) 2 Cl(g) (very fast, reversible) Step 2: Cl(g) + CHCl3(g)  HCl(g) + CCl3(g) (slow) Step 3: Cl(g) + CCl3(g)  CCl4(g) (fast) What is the rate law for the overall reaction?

The rate law for the overall reaction is

Explanation:

In a mechanism of the reaction, the slow step in the mechanism determines the rate of the reaction.

For the given chemical reaction:

The intermediate reaction of the mechanism follows:

Step 1:  

Step 2:  

Step 3:  

As, step 2 is the slow step. It is the rate determining step.

Rate law for the reaction follows:

          ......(1)

As, [Cl] is not appearing as a reactant in the overall reaction. So, we apply steady state approximation in it.

Applying steady state approximation for Cl from step 1 and step 3, we get:

Putting the value of [Cl] in equation 1, we get:  

Hence, the rate law for the overall reaction is

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