Which term describes the internal energy stored in the reactants of a chemical reaction?

The term that accurately describes the internal energy stored in the reactants of a chemical reaction is enthalpy. Enthalpy is a thermodynamic quantity that represents the total heat content of a system, and it encompasses the internal energy as well as the energy associated with the pressure and volume of the system. In the context of a chemical reaction, the enthalpy of the reactants gives insight into the energy available for the reaction to occur, allowing us to understand the heat changes that accompany chemical transformations.

Kinetic energy pertains to the energy of motion and is not relevant to the internal energy stored as part of the reactants. Thermal energy refers to the energy that comes from the temperature of a substance, while activation energy is the minimum energy required for a reaction to occur, not the stored energy within the reactants themselves. Thus, among the given options, enthalpy best describes the internal energy stored in the reactants.