Photochemical Reaction


The Photochemical Reaction is quite different from a dark reaction, as, in a dark reaction, the thermal reaction concerns the ground state of electrons present within the molecule of a chemical compound. The Photochemical Reaction is responsible for playing a crucial role in defining the nature of chemical species. The chemical species include pollutant species present within the atmosphere. However, the Photochemical Reaction takes place due to the solar radiation within the atmosphere.

Defining Photochemical Reaction

Photochemical Reaction is nothing but a chemical reaction that takes place when triggered by light radiation from a light source. This results in the electrons within a molecule of a compound getting excited as a result of the absorption of heat energy.

However, in this reaction, detailed characterisation is considered as there occur significant changes in the primary events of Photochemical Reaction.

Figure 1: Multiple colourful close up photochemical reaction in glass vial under UV light in a dark laboratory

These stages show the pathways for the chemical reactions that are taking place. Photochemical Reaction, supports the sustenance of life on earth. The renowned scientist named Trommsdorff was the first one to have been able to describe photochemical reactions. He conducted this reaction through the experimentation of the two crystals of satonin when they are exposed to the light of the sun. this was proved as the crystals turned into a yellow colour followed by bursting out.

Types of Photochemical Reaction

Varied types of reactions are noticed in the aspect of photochemical reactions that includes, photo dissociation, which is expressed as

$$\mathrm{AB + h
u \:\rightarrow\:A^* + B^*}$$

The next type of reaction is rearrangements of photo-induced or better known as isomerism, that is expressed as

$$\mathrm{A+ h
u \:\rightarrow\: B}$$

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The next reaction is known as photo addition that is expressed as

$$\mathrm{A + B + h
u \:\rightarrow\:AB + C}$$.

The other reaction is photo substitution that is

$$\mathrm{A + BC +\:\rightarrow\:AB + C}$$.

Lastly, the reaction of photo redox that is expressed as

$$\mathrm{A + B + h
u\:\rightarrow\:A^- + B^+}$$.

Application of Photochemical Reaction

The reaction of Photochemical Reaction is quite essential as it is used for several industrial purposes. This reaction is majorly used in the making of Benzyl Chloride. For the production of the organic molecules that are synthetic in nature, this reaction is effective. In the preparation of varied drugs for anti-malarial, photochemical reaction is used.


Several examples are noticed for photochemical reactions, such as in the process of photosynthesis that takes place in plants when they make use of the sunlight for conversion of the carbon dioxide into products consisting of oxygen as well as glucose. Other examples include the ozone layer protecting the earth from ultraviolet radiation. The harmful UV rays that cause harm to DNA and skin cancer are due to the photochemical reactions that take place.

Laws Associated with Photochemistry

The laws that is associated with Photochemistry caters to Grotthuss-Draper law and Stark–Einstein law. In Grotthuss-Draper law, states that light needs to be absorbed by chemical substances.

However, the reaction is chemical in nature. Based on the Stark-Einstein Law, each light photon is to be absorbed by the stated chemical system. In this system, one or more gets activated by the reaction of photochemistry.

Occurrence of Photochemical Reaction in Photosynthesis

The process of photosynthesis refers to as the final property that is crucial to the sustenance of life on earth. This process is photochemical in nature that is conducted by green plants that involve green plants, for example, algae and seaweeds. In certain specific bacteria, the process is noticeable where the conversion of carbon dioxide to carbohydrates occurs as a result of the absorption of solar energy. Plants are known to make conversion of light energy into chemical energy. This energy gets stored in the plants in the form of carbohydrates by the effective usage of water as well as carbon dioxide, thereby releasing oxygen as a by-product.

More to this, the life of animals is sustained through the consumption of both oxygen and carbohydrates. However, both these products are prepared by the plants with the help of this reaction. This results in the active excitement of the electrons resulting in the transformation.


In this tutorial, the focus has been given on understanding the reaction of photochemical that occurs within nature. Most commonly the reactions occur when the electrons within the molecules get excited due to the absorption of heat energy in the form of light. In this reaction, sunlight is quite essential and in this reaction, molecules of the chemical compounds get segregated as well as modified thereby mixing with one another and resulting in the formation of new products.


Q1. What is defined as Photochemical dissociation?

Ans. Photochemical dissociation also known as photolysis or photodecomposition is stated as a chemical reaction where a breakdown of photons occurs for a chemical compound. In this reaction, one to more photons tends to react by interacting with a target molecule. However, in the case of visible lights, there is no limitation to photo dissociation.

Q2. What initiates the reaction of photo dissociation?

Ans. The chemical reaction of photo dissociation tends to get initiated by the absorption of heat in the form of light rays. This results in the absorption of the molecules of light that is referred to as the development of the state of excitation. This state consists of varied chemical as well as physical properties that are quite different from the properties that were exhibited by the original molecules.

Q3. What is defined as the primary photochemical process?

Ans. The primary photochemical process is stated as photolysis; however, the intermediate consequence as a result of light absorption is the reaction to photochemical. However, the secondary reactions are known as chemical shifts that occur subsequently.