Answer to Question #134690 in Physical Chemistry for Vishal Soni

Question

Describe the various laws of photochemistry.

Answer 1

Ans.

There are two “laws” of photochemistry. The first, the Grotthuss–Draper law (named for the chemists Christian J.D.T. von Grotthuss and John W. Draper), is simply: for light to produce an effect upon matter it must be absorbed. The second, or Stark–Einstein law (for the physicists Johannes Stark and Albert Einstein), in its most modern form is: one resultant primary physical or chemical act occurs per photon absorbed. The quantum yield of a particular species of product is the number of moles of that product divided by the number of einsteins of light (units of 6.02 × 10²³ photons)—or the number of molecules of product per photon—absorbed. In the ideal case the quantum yield, frequently denoted by the Greek letters gamma, γ, or phi, Φ, is unity. In real cases, Φ may approach zero on the one hand—particularly if a back reaction is involved—or it may be of the order of 1,000,000, in which case the primary product may start a chain reaction, as in a clean, dry mixture of hydrogen (H) and chlorine (Cl).

The Bunsen-Roscoe Law of Reciprocity states that a photochemical effect is directly proportional to the total energy dose, irrespective of the time required to deliver the dose. This law is true for chemicals in a test tube, but the response of cells to radiation usually involves a sequence of interacting biological reactions, making a linear "dose x time" relationship highly unlikely. There is no reciprocity when damage is produced, e.g., DNA damage, but there can be reciprocity over a narrow range of doses for photoreceptors that trigger a response, such as phytochrome.