- An elementary reaction occurs in a single step with a single transition state.
- An overall reaction occurs over multiple elementary steps.
- The Law of Mass Action states that the rate of an elementary reaction is directly proportional to the concentration of each reactant raised to the power of its coefficient in the balanced reaction.
- The molecularity (e.g. unimolecular, bimolecular, termolecular) describes the total number of reactant molecules that must collide in an elementary reaction.
Rate laws can be determined experimentally using the Method of Initial Rates described in Section 1.2. In some cases it's also possible to predict a rate law. To predict the rate law, we must know the mechanism (steps) of the reaction. If a reaction takes place in a single step it is called an elementary reaction. If a given reaction occurs via multiple steps, then it is called an overall reaction. Each individual step in the mechanism for the overall reaction is an elementary reaction. For example, consider the classification of the generic reactions below:
When a reaction is elementary its mechanism involves all reactants colliding together in a single step that passes through a single transition state. Since every reactant is involved in the collision, the rate of the reaction is directly proportional to the concentration of each and every reactant. For example, if
is an elementary reaction, then its rate is given by
where k is the rate constant, as in Section 1.2. If, instead, the elementary reaction was
then the rate law would be
.
We are able to predict the rate laws for these elementary reactions using the Law of Mass Action, which states that the rate of a reaction is directly proportional to the concentrations of the reactants. As demonstrated in the example above, if the balanced elementary reaction has a coefficient of in front of
, then
's must be involved in the collision, so the rate law will include
.
For an elementary reaction each coefficient, , in the balanced reaction will end up as an exponent in the rate law. The sum of the coefficients gives the total number of reactants and the sum of the exponents gives the reaction order. Thus, for an elementary reaction, the overall order is always equal to the number of reactants in that reaction. Keep in mind, though, that this is a special case that is not universally true. If a reaction is not elementary, its order will not necessarily be the same as the number of reactants.
For elementary reactions, the number of reactants is referred to as the molecularity of the reaction. If there is a single reactant then reaction is unimolecular, if there are two reactants then the reaction is bimolecular, and if there are three reactants then the reaction is termolecular. Elementary reactions with three (termolecular) or more reactants are very rare.
When determining molecularity, each individual reactant counts separately (even if it is the same as another reactant). For example, 2A on the reactant side counts as two separate reactants, as shown in the examples below:
Interactive: