Final answer:
In a chemical reaction where the products have less energy than the reactants, the reaction is an exothermic reaction, characterized by the release of energy, typically in the form of heat, and a negative enthalpy change.
Step-by-step explanation:
If in a chemical reaction, the products have less energy than the reactants, that reaction is an exothermic reaction. During an exothermic reaction, the bonds in the products form such that they have higher bond energy (stronger bonds) than those broken in the reactants. This results in a release of energy, typically as heat, and the enthalpy change (ΔH) for the reaction is negative. Hence, the products are energetically downhill from the reactants on the energy scale.
Exothermic reactions entail a net flow of energy from the system to the surroundings, which often results in the surroundings getting warmer. The enthalpy of the system decreases, and energy is one of the products of the reaction.
Conversely, endothermic reactions absorb more energy in breaking the bonds of the reactants than is released in forming the bonds of the products, resulting in a net absorption of energy and a higher energy level of the products compared to the reactants.