KEY2CHEM

Reaction Mechanisms

A reaction mechanism is a detailed sequence of elementary reactions that describe the individual process by which a reaction actually proceeds. This can differ from the overall balanced equation, which only states the initial reactants and final products of a reaction. To be validated, a reaction mechanism must follow three criteria:

1) The sum of the elementary reactions must yield the overall balanced equation.

2) The elementary reactions must be reasonable (primarily unimolecular and bimolecular, since termolecular elementary reactions are quite rare).

3) The reaction mechanism must correlate with the experimentally-determined rate law. Often, there is a slow (rate-determining) step, which then determines the overall rate law.

If a reaction mechanism meets these criteria, it is considered valid (or reasonable). If not, it is not a valid mechanism. Note that a reaction mechanism can never be proven to be the only reasonable pathway; it can only be disproved. The detection of reaction intermediates (species formed in one step of a reaction mechanism and used up in a subsequent step) can provide evidence for a particular mechanism.

Example 1.

Identify the intermediate in the following two-step reaction mechanism.

$$\require{mhchem}\ce{H2O2 + I- -> H2O + OI-}$$

$$\require{mhchem}\ce{OI- + H2O2 -> H2O + O2 + I-}$$

A. $$\require{mhchem}\ce{H2O2}$$

B. $$\require{mhchem}\ce{H2O}$$

C. $$\require{mhchem}\ce{OI-}$$

Solution

C. $$\require{mhchem}\ce{OI-}$$

An intermediate is a product of one elementary reaction and a reactant in a subsequent elementary reaction.

Example 2.

Which statement about reaction mechanisms is true?

A. The reaction mechanism is always one step, which is identical to the overall balanced chemical equation.

B. The reaction mechanism proposes the detailed stepwise sequence of reactions by which a process occurs.

C. The reaction mechanism determines the experimental rate law.

Solution

B. The reaction mechanism proposes the detailed stepwise sequence of reactions by which a process occurs.

The reaction mechanism is a proposed sequence of steps by which the overall process occurs. The reaction mechanism must be consistent with the experimentally-determined rate law, and can be multiple steps to accommodate the experimental findings.

Example 3.

The experimental rate law of a reaction is found to be $$\text{rate} = k[\require{mhchem}\ce{NO2}]^2$$. Which statement about the reaction mechanism for this reaction must be true?

A. The reaction mechanism must contain a single step.

B. The slow step in the reaction mechanism must have a rate law that matches the experimental rate law.

C. The slow step in the reaction mechanism cannot contain $$\require{mhchem}\ce{NO2}$$.

Solution

B. The slow step in the reaction mechanism must have a rate law that matches the experimental rate law.

The slow (or rate-determining) step must have a rate law that matches the experimental rate law.