Here's why:
* Chemical reactions involve breaking and forming bonds. Breaking bonds requires energy input, while forming bonds releases energy. The overall change in energy can be either a release (exothermic) or absorption (endothermic) of heat.
* Heat is a direct measure of the kinetic energy of molecules. As molecules react, their kinetic energy changes, resulting in a change in temperature. This is the most readily observable effect of energy changes in most chemical reactions.
While other forms of energy like light or electrical energy can be involved in some chemical reactions, heat energy is the most common and fundamental form exchanged.
