A proposed reaction mechanism is kinetically plausible only if it can reproduce the experimentally observed rate law as well as the overall stoichiometry.

Because the rate law of an elementary step follows from the reactants in that step, a mechanism can sometimes be converted into a predicted overall rate law.

A rate-determining elementary step

Consider

$$\mathrm{NO_2+NO_2\rightarrow NO_3+NO}\qquad\text{slow}$$

followed by

$$\mathrm{NO_3+CO\rightarrow NO_2+CO_2}\qquad\text{fast}.$$

Adding the steps gives

$$\mathrm{NO_2+CO\rightarrow NO+CO_2}.$$

If the first step is much slower than the subsequent consumption of its intermediate, the overall rate is controlled approximately by that first elementary event:

$$\boxed{r\approx k_1[NO_2]^2}.$$

An experimental law proportional to $[NO_2]^2$ would therefore be consistent with this simple mechanism, whereas an observed law proportional to $[NO_2][CO]$ would contradict the slow-first-step model.

A rate-determining step is a step whose kinetics strongly control the overall rate under the stated conditions. Calling one step “the slow step” is an approximation, not a universal theorem: several steps can contribute to the observed kinetics.

Fast pre-equilibrium before a slow step

A predicted final rate law should normally be written in terms of measurable reactants and products, not an unobserved intermediate.

Suppose

$$\mathrm{A+B\xrightleftharpoons[k_{-1}]{k_1}I}\qquad\text{fast}$$

followed by

$$\mathrm{I+C\xrightarrow{k_2}P}\qquad\text{slow}.$$

The slow elementary step gives

$$r=k_2[I][C].$$

If the first reversible step remains close to dynamic equilibrium, its forward and reverse rates are approximately equal:

$$k_1[A][B]\approx k_{-1}[I].$$

Therefore

$$[I]\approx\frac{k_1}{k_{-1}}[A][B].$$

Substituting into the slow-step rate gives

$$\boxed{r\approx \frac{k_1k_2}{k_{-1}}[A][B][C]}.$$

The intermediate has disappeared from the observable rate law.

Consistency is not proof

A mechanism must satisfy at least two checks:

1. its elementary steps sum to the overall balanced reaction;
2. the rate law derived from it agrees with experiment.

Passing these checks makes a mechanism consistent with the data, not uniquely proven. Different microscopic mechanisms can sometimes produce the same macroscopic rate law, so additional evidence—such as detection of intermediates, isotope effects or spectroscopic observations—may be required.

When no single step is clearly rate determining and a fast pre-equilibrium is not justified, the concentration of a reactive intermediate can often be eliminated using a steady-state approximation.