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Photochemical Reaction

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Many reactions that do not proceed under ordinary conditions can take place if the reactants are irradiated with visible or ultraviolet light. It may provide the necessary energy to overcome the activation energy which is otherwise thermodynamically impossible. 

In photochemical reactions, activation energy is received by the absorption of photons of light by molecules while the rate of thermal reactions depends on the temperature. In photochemical reactions the number of molecules activated depends on the intensity of light and hence the concentration of activated molecules will be proportional to the light intensity to which the reactant is exposed.

Definition 

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Photochemical Reaction :

"Photochemical reactions are ones in which photons are absorbed by an atom or molecule called a chromophore. The chromophore transforms the solar energy into high energy electron capable of conducting chemical work."

Most of the photochemical reactions are probably powered by ultraviolet radiation because its wavelength is more energetic than those of visible light. Photochemical reactions underlie many chemical and biochemical processes and they are governed by two basic laws.
  • According to Grotthus and Draper - "only light that is absorbed can produce photo chemical change". This absorption of light is known as the primary process in the photochemical reaction.
  • The second law is that of Stark and Einstein, which states that "a molecule absorbs a single quantum of radiation in the primary step of a photochemical reaction."

Characteristics

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The characteristics of photochemical reactions are listed below.
  1. Each molecule taking part in a photo process absorbs only one photon of radiant energy.
  2. Photo processing does not occur in the dark.
  3. Each photochemical reaction requires a definite amount of energy which is characteristic of a particular wavelength of the photon.
  4. The rate of photochemical reaction depends upon the intensity of radiation absorbed.
  5. The $\Delta$G values for light initiated reactions may or may not be negative.
  6. The temperature does not have marked effect on the rate of light initiated reactions.
Some of the important photochemical reactions are
  • Photosynthesis in plants
  • Photography
  • Formation and destruction of ozone layer
  • Photoetching in electronic industry
  • Polymerization reactions
  • Modern printing technology

Process

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The basis of the classification of the variety of possible photochemical reactions is the idea of the existence of  eight types of elementary photochemical processes which depend on the chemical structure of compounds and nature of lower electronic excited states of molecules.

The eight types of photochemical processes are listed below.
  1. Electron transfer
  2. Proton transfer
  3. Hydrogen atom transfer
  4. Photodissociation
  5. Pericyclic reactions that occurthrough a cyclic intermediate state
  6. Geometrical cis-trans photoisomerization
  7. Valence photoisomerization
  8. Photosubstitution
Diagrammatic representation of different photochemical processes is shown below. 

Photochemical Process

Applications

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  1. Photochemistry is attaining an increasing importance in health science, curing some forms of cancers through phototheraphy, repairing tissues and performing microsurgery using lasers.
  2. Other important applications of photochemistry include the use of photolithography to manufacture computer chips and photopolymerization to produce protective coatings for a variety of high value materials such as optical fibers.
  3. One of the "Holy Grails" of photochemistry is the discovery of practical ways to convert sunlight into high grade fuels to replace fossil fuels.
  4. Photophysics especially the use of fluorescence as a sensor is currently of enormous importance for applications in the materials sciences and in biological sciences.