The rate of a chemical reaction or Reaction Rate is the estimation of the alteration in concentration of the reactants or the alteration in concentration of the products over a unit time.
For an example, during the chemical reaction between products A and B as shown in the equation below, the reactants A and B are used up while the concentration of product AB increases. The reaction rate or rate of the chemical reaction can be measured by estimating how fast the concentration of A or B is being used up, or by how fast the concentration of AB is being formed.
A + B —- AB
For the purpose of stochiometry, the above equation can be further represented as:
aA + bB —— cC+ dD