Dr. S. B Maulage Dept of Chemistry

Photochemistry PHOTOCHEMICAL REACTION: A reaction which takes place by absorption of visible and ultraviolet radiations is called a photochemical reaction and the branch of chemistry which deals with the study of photochemical reactions is called Photochemistry.

Interaction of light with matter: When light falls on matter, then reflection, transmission and absorption may takes place. The Photochemistry is related only with the absorption of light by the matter.

Photo-physical processes: The substance absorb light, emit it without undergoing any chemical changes is termed as photo-physical process. Ex. Fluorescence, Phosphorescence, and photo-electric effects etc.   Photochemical changes: In this process, absorption of light brings about some chemical changes in substance. Ex. H2 and Cl2 reacts explosively in presence of sunlight.

Difference between thermal and photochemical reactions: Thermal Reactions Photochemical reactions In thermal reactions, activation energy is derived due to collisions of reactant molecules. In photochemical reactions, activation energy is derived due to absorption of light by which they are irradiated. In thermal reactions, number of activated molecules depend on temperature of reaction. In photochemical reactions, number of activated molecules depend on intensity of light. The thermal reactions takes place by absorption or evolution of heat. Photochemical reactions involve only absorption of light. Thermal reactions can be carried out in ‘dark’ also. Photochemical reactions require presence of light.

The free energy change ∆G is always negative in thermal reactions. The free energy change ∆G of most photochemical reactions is negative but it may positive in some cases. Rate of thermal reactions depends on temperature and independent on intensity of light absorbed. Rate depends only on intensity of light absorbed. Ex. i) N2 + 3H2 → 2NH3 ii) H2 + I2 → 2HI iii) PCl5 → PCl3 + Cl2 Ex. i) 2HBr → H2 + Br2 ii) H2 + Cl2 → 2HCl iii) 2O3 → 3O2

Photochemical Laws: Grothus-Draper Law: “The Gratthus –Drapper law is stated as, When light falls on substance only fraction of incident light which is absorbed by the substance can brings about chemical change, reflected and transmitted do not produce any such changes.”

Limitations: This law state about absorption of light responsible for the photochemical change but it has following limitations, This law is purely qualitative. It does not give any relationship between amount of light absorbed and number of molecules reacted. It is applicable to primary chemical processes and not for secondary processes.

Stark – Einstein’s Law: Statement: It is stated as, each molecule taking part in photochemical reaction absorbs one quantum of light energy. From this law it is clear that for a reaction, number of molecules reacted should be equal to number of quanta of light absorbed.

e.g. Polymerization of anthracene into dianthracene takes place in ultra-violet light by absorbing two quanta of light. 2 C14H10 →→ C28H20 Anthracene Dianthracene

The substance undergoing photochemical reaction absorbs equal number of quanta to the number of molecules reacted. The energy of photon or quantum is given as, E = h υ = h c / λ Where, υ – frequency of light, λ - wavelength of light, c – velocity of light, h – Plank’s constant.

Limitations The law of photochemical equivalence is applicable to primary processes where one molecule get reacted by absorption of one quantum energy but it is failed in such reactions where more number of quanta of energy are required for one molecule to react. The law of photochemical equivalence is not applicable in photo-initiated chain reactions. In such reactions by absorption of one quantum of energy large number of molecules get reacted.

Joblonski Diagram: A Jablonski diagram is a diagram that illustrates the electronic states of a molecule and the transitions between them.