Photochemical Reactions

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There are many reactions that are affected by electromagnetic radiations. Chemical reactions, which occur in the presence of visible and ultraviolet light are called photochemical reactions.

Photochemical reactions occur only when photons of radiant energy (hn) are absorbed by the reactant. Absorption of this energy provides the necessary energy needed to cross the activation barrier and form products. A wide variety of reactions can be initiated photochemically.

Photochemical reactions are governed by two laws:

i) Grothus Draper Law

ii) Einstein-Stark Law.

Grothus-Draper Law

states that only the radiation actually absorbed by the reacting system can initiate a reaction. The light which simply passes through the reaction medium does not initiate the reaction. Absorption of a photon is the first essential step in a photo chemical reaction.

Einstein-Stark law

states that one quantum is absorbed by the molecule responsible for the primary photochemical process. After absorption, the excited molecule can then undergo various reactions. It can form a product, it can catalyze a chain reaction, or it can get de-excited.

If on absorption of one photon, one product molecule is formed, then the efficiency of the photochemical reaction is one. If a chain reaction takes place then the quantum efficiency can be high. The efficiency of a photochemical reaction is measured in terms of quantum yield.

Quantum yield, which is denoted by F be defined as the ratio of number of molecules reacted and number of photons absorbed. F is experimentally determined.

The basic steps involved in a photochemical reaction can be best understood with examples.

Example 17:

When a mixture of hydrogen and chlorine is exposed to sunlight, it reacts violently.

This reaction is believed to occur by the following mechanism.

This is the first step. It is called the initiation step. These chlorine atoms are very reactive and react with hydrogen molecule (step 2).

The highly reactive H atom further reacts with another molecule of Cl₂ to give HCl and Cl atom.

In this step the highly energetic chlorine is regenerated. In fact, steps (ii) and (iii) are repeated again and again. This is called the propagation step. In the process, the concentration of chlorine atoms go up and the reaction becomes violent. The reaction terminates when excess chlorine atoms combine with each other to form Cl₂ molecules. This is the termination step of the photochemical reaction.

Formation and dissociation ozone in the ozone layer

Example 18:

The formation of ozone and its dissociation in the atmosphere maintain the oxygen-ozone balance in the ozone layer. Both the reactions occur in the presence of sunlight. The reactions involved are given below:

Destruction of ozone in the ozone layer

Example 19:

The ozone in the ozone layer protects the earth from harmful ultraviolet radiation by trapping these radiations. This ozone layer has been thinning gradually and poses potential health hazards for the future. The thinning of ozone layer has been attributed to the presence of chlorofluorohydrocarbons like CFCl₃ and CF₂Cl₂ in the atmosphere. These chemicals have been used as aerosol propellants and cooling mixtures in refrigerators. How these chemicals affect ozone concentration is illustrated with the equations given below.

Chlorine atom being highly reactive reacts with ozone (O₃).

The monoxide of chlorine further reacts with another molecule of O₃

The chlorine atom so obtained reacts with another ozone molecule. Hence, steps (ii) and (iii) are repeated again and again and, leads to the depletion of concentration of ozone.

Phenomenon of vision

Example 20:

This important phenomenon is the outcome of a photochemical reaction that occurs very fast (~10^-12 s). The pigment in the eye called rhodopsin absorbs light in the near UV and visible region. On absorption of light, a sequence of reactions take place. The actual chemical in rhodopsin that gets excited is retinal. Excited retinal undergoes isomerization and the energy is stored as chemical energy. Shortly after that, retinal is regenerated and the energy released is used to send a signal to the brain. This gives us the sensation of vision.