What is a Reaction Mechanism?

A mechanism is the sequence of elementary steps by which a complex (overall) reaction takes place. Each step is an elementary reaction with a specific molecularity.

Elementary Step vs Complex Reaction

Rate-Determining Step (RDS)

• The slowest elementary step in the mechanism
• Controls the overall rate (bottleneck analogy)
• The rate law of the overall reaction = rate law of the RDS
• Intermediates must be eliminated from the final rate law

Worked Example: Deriving Rate Law from Mechanism

Reaction: 2NO + $O_{2}$ → 2N$O_{2}$

Proposed mechanism:

1. (fast, equilibrium): 2NO ⇌ N_{2}$$O_{2} with $K_{1}$ = [N_{2}$$O_{2}]/[NO]^{2}
2. (slow, RDS): N_{2}$$O_{2} + $O_{2}$ → 2N$O_{2}$ with rate_{2} = k_{2}[N_{2}$$O_{2}][$O_{2}$]

Derive overall rate law: From step 1: [N_{2}$$O_{2}] = $K_{1}$[NO]^{2}

Substitute into step 2: rate = k_{2} × $K_{1}$[NO]^{2} × [$O_{2}$] = k_obs[NO]^{2}[$O_{2}$]

This correctly gives the experimental rate law: rate = k[NO]^{2}[$O_{2}$] (third order overall).

Rules for Mechanism Writing

1. All elementary steps must sum to the overall reaction
2. Intermediates appear on both sides and cancel out
3. The slow step rate law must match experimental rate law
4. Intermediates in rate law must be eliminated using equilibrium steps