Question

The rate law for the reaction: 2H2+2NO--->N2+2H2O is rate=k[H2][NO]^2. Which of the follwing mechanisms can be ruled out on the basis of the observed rate expression? PLEASE EXPLAIN

Mechanism 1: H2+NO--->H2O+N (slow) / N+NO----->N2+O (fast) / O+H2----> N2+O (fast)

Mechanism 2: H2+2NO---->N2O+H2O (slow) / N2O+H2---->N2+H2O (fast)

Mechanism 3: 2NO<----->N2O2 (fast equilibrium) / N2O2+H2------>N2O+H2O (slow) / N2O+H2------>N2+H2O (fast)

PLEASE EXPLAIN WHY.

Answer 1

The rate of the reaction is determined to be

rate = k[H₂][NO]²

We know rate of a reaction depends on the rate of the rate limiting or determining step which is the slowest step.

In mechanism 1, as per the slow step, rate should be = k[H₂][NO], but this is not the case

In mechanism 2, as per the slow step rate = k[H₂][NO]², which is same as that has been mentioned.

Similarly in mechanism 3, as per the shlow step, rate = k[H₂][N₂O₂] = k1[H₂][NO]², which is also same as that mentioned in the question.

So both mechanism 2 and 3 are possible here. Mechanism 1 can be ruled out.