Elementary Kinetics

Rate Equations

We found out in the last section that reaction rates depend on the concentration of the reactants. In other words, the rate is proportional to the reactant concentrations:

This is usually written as:

where k is a proportionality constant known as the rate constant.

For elementary reactions (reactions that go straight from reactant to product, without any intermediate steps), the rate equation can be determined from the stoichiometry of the reactants of a reaction.

For example, for the hypothetical reaction:

The rate equation (also called the rate law) for the reaction is:

Despite this handy trick, you still can’t get away from lab work to figure out the rate of a reaction! That rate constant has to be determined by experimentation. Remember from collision theory that molecules have to bump into each other, both with enough energy and in the right orientation, for a chemical reaction to occur. The rate constant is the "fudge factor" that accounts for how well that actually happens in the specific reaction you are interested in, so that: