Here's why:

* Enthalpy (H) is a thermodynamic property that represents the total energy of a system. It includes internal energy (like the energy of chemical bonds) and the energy associated with pressure and volume.

* Change in enthalpy (ΔH) is the difference in enthalpy between the products and reactants of a reaction.

* Exothermic Reactions: Release heat to the surroundings (ΔH is negative). The products have lower enthalpy than the reactants.

* Endothermic Reactions: Absorb heat from the surroundings (ΔH is positive). The products have higher enthalpy than the reactants.

In summary: The energy difference between the reactants and products, as represented by the change in enthalpy, determines whether heat is released or absorbed during a chemical reaction.