Question

A possible mechanism for the overall reaction Br2(g) + NO (g) → 2 NOBr (g) is

Step 1: NO (g) + Br2 (g) ⇄ NO Br2 (g) (fast)

Step 2: NO Br2 (g) + NO (g) → 2NOBr (slow)

The reaction is experimentally determined to be second order in NO and first order in Br2 a) Write down the experimentally determined rate law.

b) Is the mechanism consistent with the observed rate law?

Answer 1

a)

Order of NO is 2

Order of Br2 is 1

So rate law is:

rate = k [NO]^2 [Br2]

b)

rate depends on the slowest step

here 2nd step is slowest

So,

rate = k2[NOBr2][NO]

But NOBr2 is intermediate. We will be removing it using step 1

Kc = [NOBr2] / [NO][Br2]

[NOBr2] = Kc [NO][Br2]

put this in rate equation:

rate = k2[NOBr2][NO]

rate = k2Kc [NO][Br2][NO]

rate = k [NO]^2 [Br2]

Since rate law is same as experimentally determined

The mechanism is consistent with the observed rate law