What do chemists express the rates of chemical reaction?

They represented rate of chemical reactions are the rate of change in concentration of products or reactants in a reaction

Rate of a chemical reaction = change in concentration of a reactant or product per unit time.

It can be expressed in two terms:

1) The rate of decrease in concentration of any of the reactants.

2) The rate of increase in the concentration of any of the products.

Explanation:

Rate of reaction is the speed at which reactants are converted into products or the rate at which products are produced from reactants.