Most reactions are a sequence of elementary steps. The list of steps is the mechanism. A mechanism is a proposal: it must add up to the overall equation and agree with the measured rate law.
Key idea
Why it works
Worked examples
Worked example: Finding an intermediate
Step 1: $\mathrm{NO_2 + NO_2 \to NO_3 + NO}$
Step 2: $\mathrm{NO_3 + CO \to NO_2 + CO_2}$
Check that it adds up, and name any intermediate.
- Add: $$\mathrm{2NO_2} + \mathrm{NO_3} + \mathrm{CO} \to \mathrm{NO_3} + \mathrm{NO} + \mathrm{NO_2} + \mathrm{CO_2}$$
- Cancel $\mathrm{NO_3}$ (both sides) and one $\mathrm{NO_2}$: $\mathrm{NO_2 + CO \to NO + CO_2}$. It matches.
- $\mathrm{NO_3}$ is made in step 1 and used in step 2: an intermediate.
Answer: It adds up; $\mathrm{NO_3}$ is the intermediate.
Worked example: Telling a catalyst from an intermediate
Step 1: $\mathrm{Cl + O_3 \to ClO + O_2}$
Step 2: $\mathrm{ClO + O \to Cl + O_2}$
Find the overall reaction, the catalyst and the intermediate.
- Add and cancel $\mathrm{Cl}$ and $\mathrm{ClO}$: $\mathrm{O_3 + O \to 2O_2}$.
- $\mathrm{Cl}$ is used in step 1 and made again in step 2: a catalyst. One chlorine atom can go around this cycle many thousands of times.
- $\mathrm{ClO}$ is made in step 1 and used in step 2: an intermediate.
Answer: $\mathrm{O_3 + O \to 2O_2}$; Cl is the catalyst and ClO the intermediate.
Common mistake
A mechanism can be ruled out by experiment, but never proven. Chemists test proposals by checking the rate law they predict, by detecting intermediates with spectroscopy, and by labeling atoms with isotopes to see where they end up. The next lessons show how a mechanism predicts a rate law.
- The steps must add up to the overall equation.
- Each step must be a plausible collision (one, two or rarely three particles).
- The rate law it predicts must match the measured one.