Photochemistry

Light that can break molecular bonds is most effective at inducing photochemical reactions. The energy required to break a molecular bond ranges from approximately 150 kiloJoules per mole to nearly 1000 kJ/mol, depending on the bond. Visible light, having wavelengths ranging from 400-700 nanometers, corresponds to energies ranging from approximately 300-170 kJ/mol, respectively. Note that this is enough energy to dissociate relatively weak bonds such as the single oxygen (O-O) bond in hydrogen peroxide (HOOH), which is why hydrogen peroxide must be stored in a light-proof bottle.

Ultraviolet light, having wavelengths ranging from 200-400 nm, corresponds to higher energies ranging from approximately 600-300 kJ/mol, respectively. Ultraviolet light can dissociate relatively strong bonds such as the double oxygen (O=O) bond in molecular oxygen (O₂) and the double C=O bond in carbon dioxide (CO₂); ultraviolet light can also remove chlorine atoms from compounds such as chloromethane (CH₃Cl). The ability of ultraviolet light to dissociate these molecules is an important aspect of the stability—and destruction—of ozone molecules in the upper atmosphere.