Exothermic and Endothermic Reactions

As a chemical reaction goes forward, it will either release heat energy to its surroundings (exothermic reaction), or it will absorb heat energy from its surroundings (endothermic reaction).

Exothermic Reactions

When a chemical reaction releases heat energy to its surroundings, the reaction is exothermic.

In terms of reactants and products, an exothermic reaction has more heat energy content, or enthalpy (H), in its reactants than in its products.

Therefore, in going from reactants to products, there is a positive net change in enthalpy (+DH).