Mechanisms of Chemical Reactions

Mechanisms describe in a stepwise manner the exact collisions and events that are required for the conversion of reactants into products. Mechanisms break up the overall balanced chemical equation into a series of elementary steps.

The following images show one reaction that requires a single collision, and one reaction where there are two elementary steps (2 collisions) needed for the reaction to occur.

Single Step reaction

Multi-Step reaction

Most reactions do not occur in one step, but a series of elementary steps.

Consider the following mechanism for the decomposition of ozone to oxygen:

The above mechanism exhibits a series of elementary steps whose sum is the overall (net) balanced reaction. In the above mechanism, the first reaction is labeled as "slow". This elementary step of the reaction is the called the “rate determining step” because it is the slowest step. This means that the rate of the overall reaction is equal to the rate of the rate determining step.

The Rate Determining Step

Intermediates:

Note the oxygen atom, O, is an intermediate species in the above equation. An intermediate is both formed and consumed in the mechanism and does not appear in the net equation.

Catalysts:

A catalyst speeds up a reaction without being explicit in the overall balanced equation. It does this by providing an alternate mechanism for the reaction that has a lower activation barrier than does the uncatalyzed pathway.

Compare the catalytic and regular mechanisms for the hydrogenation of ethylene to ethane:

The catalyst changes the mechanism of the reaction and in doing so lowers the activation energy. A catalyst that raises the activation energy to slow down a reaction is called a negative catalyst or inhibitor.

The catalyst is not consumed by the reaction, and therefore does not appear in the overall balanced equation.

Problem:

Based on the following reaction mechanism with three elementary steps, write the net reaction equation. Also, identify the intermediates and the catalysts (if any) in the following mechanism.