The following mechanisms are proposed for the reaction: CO + NO2 → CO2 + NO, at low temperature:
Mechanism 1: 2NO2 ⇌ N2O4 (fast), N2O4 + 2CO → 2CO2 + 3NO (slow)
Mechanism 2: 2NO2 → NO3 + NO (slo

Question

The following mechanisms are proposed for the reaction: CO + NO2 → CO2 + NO, at low temperature:
Mechanism 1: 2NO2 ⇌ N2O4 (fast), N2O4 + 2CO → 2CO2 + 3NO (slow)
Mechanism 2: 2NO2 → NO3 + NO (slow), NO3 + CO → NO2 + CO2 (fast)
Which of the above mechanism is consistent with the observed rate law: +d[CO2]/dt = K[NO2]^2?

Prashant Kotian · Accepted Answer

Correct answer: 2. The rate law is determined by the slowest step. In Mechanism 1, the slow step gives Rate = k'[N2O4][CO]^2. Substituting the fast equilibrium [N2O4] = Keq[NO2]^2 yields Rate = k'Keq[NO2]^2[CO]^2, which does not match. In Mechanism 2, the slow step yields Rate = k[NO2]^2, which perfectly matches the observed rate law. Therefore, correct answer is 2.

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