Reaction Mechanisms and a Rate-Limiting Step

A single step chemical reaction is called an elementary step. Elementary steps have simple rate laws and can be classified as unimolecular, bimolecular, or termolecular. With unimolecular you have a single reactant producing products.

We could illustrate it like this. Here you have your single reactant producing products. Now, the rate law for this would be R equals the rate constant multiplied by the single reactant. Now with bimolecular you have two reactants producing products.

We can illustrate it like this, A plus A or A plus B, either way they’re both producing products. The rate law would look like, either like this with the reactant being squared or with just the rate constant being multiplied by A being multiplied by B.

Finally, we get termolecular. Here we have reaction’s that would look like this, with combinations of three reactants. Here we see the same three reactants or A plus two B reactants or three different reactants here.

So, the corresponding rate laws would look like this, with A being brought to the third power or the rate constant being multiplied by A being multiplied by B taken to the second power or finally the rate constant being multiplied by reactants A, B, and C.

Now, most chemical reactions occur in multiple elementary steps. For example, in your cell’s glucose gets broken down into pyruvic acid in a series of ten steps called glycolysis. Not all the steps occur at the same rate, some steps are fast while others are slow.

Now, we have something called the rate limiting step, which is the slowest step in the overall reaction. That’s the step that is slowing the whole reaction down. Again, an elementary step is a single step chemical reaction and an elementary step has simple rate laws and can be classified as unimolecular, bimolecular, or termolecular.