In an exothermic reaction, the difference between the chemical energy of reactants and products equals the heat released.

Here's why:

* Exothermic reactions release energy: This energy is typically in the form of heat.

* Energy is conserved: The total energy in a system remains constant.

* Chemical energy is stored in bonds: Reactants have a certain amount of chemical energy stored in their bonds.

* Products have less chemical energy: When bonds break and new bonds form, the products have a lower total chemical energy.

The difference between the energy stored in the reactants and the energy stored in the products is the energy that was released. This is the heat of reaction, which is a negative value for exothermic reactions.

Let me know if you'd like more details about exothermic reactions!