Chemical Kinetics and Reaction Dynamics brings together the major facts and theories relating to the rates with which chemical reactions occur from both the macroscopic and microscopic point of view. This book helps the reader achieve a thorough understanding of the principles of chemical kinetics and includes:

• Detailed stereochemical discussions of reaction steps
• Classical theory based calculations of state-to-state rate constants
• A collection of matters on kinetics of various special reactions such as micellar catalysis, phase transfer catalysis, inhibition processes, oscillatory reactions, solid-state reactions, and polymerization reactions at a single source.

The growth of the chemical industry greatly depends on the application of chemical kinetics, catalysts and catalytic processes. This volume is therefore an invaluable resource for all academics, industrial researchers and students interested in kinetics, molecular reaction dynamics, and the mechanisms of chemical reactions.

5 Kinetics of Some Special Reactions (p. 115)

5.1 Kinetics of Photochemical Reactions

There are certain reactions in which the activation may be carried out by electromagnetic radiations in visible and ultraviolet region having wavelength approximately between 100 to 1000 nm. These reactions are called photochemical reactions. A photon of radiation, referred to as quantum with energy h? (? being its frequency), is primary unit of radiation. When a photon from high energy electromagnetic radiations such as X- and ?-ray is used, the chemical processes are then called radiolytic reactions. The photochemical reactions are governed by two basic principles, viz. the Grotthus-Draper law and Einstein law of photochemical equivalence.

5.1.1 Grotthuss-Draper Law

This law states that only those radiations which are absorbed can be effective in producing the chemical change.

Although photochemical reaction is a result of absorption of light, it may not always lead to chemical change. Sometimes the absorption of photon may only increase the thermal energy or it may be reemitted (fluorescence).

5.1.2 Einstein Law of Photochemical Equivalence

In 1912, Einstein extended the concept of quantum theory of radiation to photochemical processes and stated that‘each quantum of radiation absorbed by molecule activates one molecule in the primary step of a photochemical process’. This is known as Einstein law of photochemical equivalence. According to this law, if every molecule so activated in primary absorption directly decomposes, the number of molecules taking part in chemical change would be equal to the number of quanta of energy absorbed. However, a molecule activated photochemically may initiate a sequence of reactions so that many reactant molecules through a chain mechanism would undergo chemical change. In such cases the number of molecules undergoing chemical changes will be many times of the quanta of energy absorbed.