Stark-Einstein Law of Photochemical Equivalence
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-In a photochemical reaction, each molecule of the reacting substance absorbs a single photon of radiation causing the reaction and is activated to form the products.
The law of photochemical equivalence is shown in the above Figure where a molecule 'A' absorbs a photon of radiation and gets activated. The activated molecule (A*) then decomposes to yield B.
A + h𝜈 → A*
A* → B
overall Reaction-
A+ h𝜈 → B
In practice, we use molar quantities. That is, one mole of A absorbs one mole of photons or one einstein of energy, E. The value of E can be calculated by using the expression -
E = Nh𝜈 = Nhc/λ = (2.859/λ) x 105 Kcal/mol
Grothus-Draper Law
Share
