The answer is: Exothermic reaction.
There are two types of reaction:
1) Endothermic reaction (chemical reaction that absorbs more energy than it releases).
2) Exothermic reaction (chemical reaction that releases more energy than it absorbs).
For example, balanced chemical reaction:
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O ΔrH= -1370 kJ/mol.
This reaction is exothermic reaction (heat is released), so products have less potential energy and energy is transerred to surroundings.
Potential energy diagram
During chemical reactions, energy is needed to break the chemical bonds which hold the molecules of the reactants together so that new bonds that yield products can be made. When new products are made in a chemical reaction, energy is always released. Thus energy is needed to break existing bonds and energy is released when new bonds are formed.