In the 1930s, Sir Christopher Ingold and coworkers at the University of London studied the kinetics of substitution reactions such as the following.

CH₃Br(aq) + OH^-(aq) CH₃OH(aq) + Br^-(aq)

They found that the rate of this reaction is proportional to the concentrations of both reactants.

Rate = k(CH₃Br)(OH^-)

When they ran a similar reaction on a slightly different starting material, they got similar products.

(CH₃)₃CBr(aq) + OH^-(aq) (CH₃)₃COH(aq) + Br^-(aq)

But now the rate of reaction was proportional to the concentration of only one of the reactants.

Rate = k((CH₃)₃CBr)

These results illustrate an important point: The rate law for a reaction cannot be predicted from the stoichiometry of the reaction; it must be determined experimentally. Sometimes, the rate law is consistent with what we expect from the stoichiometry of the reaction.

2 HI(g) H2(g) + I2(g)

Rate = k(HI)2

Often, however, it is not.

2 N2O5(g) 4 NO2(g) + O2(g)

Rate = k(N2O5)

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