Energy produced or absorbed in the process of a chemical reaction. In such a reaction, energy losses or gains usually involve only the outermost electrons of the atoms or ions of the system undergoing change; here a chemical bond of some type is established or broken without disrupting the original atomic or ionic identities of the constituents. Chemical changes, according to the nature of the materials entering into the change, may be induced by heat (thermochemical), light (photochemical), and electric (electrochemical) energies.