Ordinarily, when reactants change to products, the difference in their energy is known as the enthalpy of the reaction, which can be either exothermic, where heat is released, or endothermic, where heat is absorbed.
In a chemiluminescent reaction, the reactants form an intermediate, which is in an electronically excited state. This intermediate is unstable. As it relaxes down to the ground state, the electrons in the intermediate releases energy in the emission of visible light rather than in the form of heat.
This phenomenon is similar to fluorescence. Recall that the electrons in a molecule occupy the ground state. When they absorb energy, such as when exposed to a certain wavelength of light, they become excited and transition to a higher, excited energy level. As the excited state relaxes back down to the ground state, the excess energy is emitted in the form of visible light.
However, it is important to distinguish between the two. The difference is how the electrons of the molecules are excited. In chemiluminescence, energy generated from a chemical reaction excites the electrons…whereas in fluorescence, the electrons are excited by direct absorption of light energy or other
