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Reaction Mechanisms and Catalysis

Most reactions do not happen in a single collision. A reaction mechanism is the step-by-step sequence of bond-breaking and bond-forming events that takes reactants to products — the actual route, not just the overall equation.

Elementary Steps and Molecularity

Each step in a mechanism is an elementary step: a single molecular event whose rate law follows directly from its stoichiometry. Molecularity counts how many reactant particles collide in that step:

Unimolecular: one particle reacts, e.g., A → products; rate $= k[\text{A}]$.
Bimolecular: two particles collide, e.g., A + B → products; rate $= k[\text{A}][\text{B}]$.
Termolecular: three particles collide at once — rare, because three-way collisions are unlikely; rate $= k[\text{A}]^2[\text{B}]$ for 2A + B.

The overall equation is the sum of the elementary steps, with anything appearing on both sides cancelled out.

The Rate-Determining Step

The slowest step in a mechanism is the rate-determining step (RDS) — the bottleneck that sets the speed of the whole reaction, like the slowest car in a single-lane convoy. The observed rate law is the rate law of the RDS, so a mechanism is only acceptable if its RDS predicts the experimentally measured rate law.

Example: The reaction of nitrogen dioxide with carbon monoxide has this proposed mechanism:

Step 1 (slow): $\text{NO}_2 + \text{NO}_2 \rightarrow \text{NO}_3 + \text{NO}$

Step 2 (fast): $\text{NO}_3 + \text{CO} \rightarrow \text{NO}_2 + \text{CO}_2$

Add the steps and cancel NO₃, which appears on both sides, and one NO₂:

$$\text{NO}_2 + \text{CO} \rightarrow \text{NO} + \text{CO}_2$$

Step 1 is the slow step, so the rate law comes from step 1 alone:

$$\text{rate} = k[\text{NO}_2]^2$$

Note that CO does not appear in the rate law at all — it only reacts in the fast step, after the bottleneck. NO₃ is an intermediate: made in step 1, used up in step 2, and absent from the overall equation.

Intermediates vs Catalysts

An intermediate is produced in one step and consumed in a later one — it never appears in the overall equation, and it is absent at the start. A catalyst is the opposite: it is consumed in an early step and regenerated in a later one, so it also cancels from the overall equation, but it is present at the start. Spot the difference by asking which came first: made-then-used means intermediate; used-then-remade means catalyst. A catalyst speeds a reaction up by providing a new pathway with a lower activation energy — it changes the mechanism, not the thermodynamics, so it never changes the equilibrium position or the overall enthalpy change.

Homogeneous, Heterogeneous, and Enzyme Catalysis

Homogeneous catalysis: the catalyst is in the same phase as the reactants — e.g., aqueous H⁺ catalyzing ester hydrolysis.
Heterogeneous catalysis: the catalyst is in a different phase, usually a solid surface where reactants adsorb, react, and desorb. The catalytic converter in a car uses platinum, palladium, and rhodium on a ceramic support to convert CO, unburned hydrocarbons, and NO_x into CO₂, H₂O, and N₂. The Haber process runs N₂ + 3H₂ → 2NH₃ over an iron catalyst at high temperature and pressure — without it, the reaction is far too slow to be useful.
Enzyme catalysis: biological catalysts — proteins whose active sites bind a specific substrate and stabilize the transition state, lowering the activation energy enormously. Most enzymes work at body temperature and are highly selective; a small change in pH or temperature can denature them and destroy the activity.

In every case the idea is the same: the catalyst opens a lower-energy route to the same products, so more molecules have enough energy to react and the rate goes up.

Practice Problems

A mechanism has a slow step A + B → C, then a fast step C + B → D. What is the rate law, the overall equation, and the intermediate?
For the reaction 2H₂O₂ → 2H₂O + O₂, the proposed steps are: (1) H₂O₂ + I⁻ → H₂O + IO⁻ (slow); (2) H₂O₂ + IO⁻ → H₂O + O₂ + I⁻ (fast). What is the rate law? Identify the catalyst and the intermediate.
An elementary step is 2A + B → products. What is its molecularity, and what is its rate law?
A proposed mechanism has a slow first step A → B, then a fast step B + C → D. What is the rate law, and what is B?