In a chemical reaction, the difference between the potential energy of the products and the potential energy of the reactants is defined as the

(1) activation energy

(2) ionization energy

(3) heat of reaction

(4) heat of vaporization

The answer is (3) heat of reaction. The difference in energy between the products and the reactants is the amount of energy either absorbed or liberated during the process (depending on whether the difference is negative or positive).