Law of Photochemistry

Law of Photochemistry

Law of Photochemistry

There are two basic laws governing photochemical reactions-
1. Grothus-Draper law
2. Stark-Einstein law of Photochemical Equivalence

Grothus-Draper law

This law is also called the Principle of Photochemical Activation or first law of photochemistry. When light falls on a cell containing a reaction mixture, some light is absorbed and the remaining light is transmitted. Obviously, it is the absorbed component of light that is capable of producing the reaction. The transmitted light is ineffective chemically. Early in the 19th century, Grothus and Draper studied a number of photochemical reactions and enunciated a generalisation. This is known as Grothus-Draper law and may be stated as-

Stark-Einstein law of Photochemical Equivalence

Stark and Einstein in 1905 studied the quantitative aspect of photochemical reactions by application of Quantum theory of light. They observed that each molecule taking part in the reaction absorbs only a single quantum (h𝜈) or photon of light. Therefore, this law is also called one molecule-one quantum law. The molecule that gains one photon-equivalent energy is activated and enters into reaction. Stark and Einstein thus proposed a basic law of photochemistry which is named after them. The Stark-Einstein law of photochemical equivalence may be stated as-