Question

Define a photochemical reaction. Give one example.

Answer 1

Hint :Different types of reactions need different conditions to proceed. Photochemical reactions are those chemical reactions which take place in the presence of light (sunlight). Sunlight initiates a reaction by providing energy to the molecule and this leads the molecules to jump in higher excited states.

Complete Step By Step Answer:
We know that different reactions need different environmental conditions to proceed. Some need a certain temperature, pressure, catalysts and sunlight conditions. So the chemical reactions which take place in the presence of sunlight are known as photochemical reactions.
Photochemical reactions are studied under a branch of science which is known as Photochemistry. So, these reactions take place by absorbing light. The molecules gain energy by the absorption of light. In these reactions, generally absorption of ultraviolet light, infrared light and visible lights are absorbed.
We should know that the chemical and physical properties of the molecules present in the excited state are different as compared to the molecules present in the ground state or initial state.
Example:
The most popular example of a photochemical reaction is Photosynthesis. This is the process by which plants make their own food. Plants use the sunlight and water present in the environment to convert the carbon dioxide into simpler sugar (glucose) which can be used as food by them.
 $ 6C{O_2} + 6{H_2}O + sunlight \to {C_6}{H_{12}}{O_6} + 6{O_2} $ .

Note :
There are two laws in photochemistry which are Grotthuss-Draper law and Stark-Einstein law. Grotthuss-Draper law states that light must be absorbed by the molecules for photochemical reactions while Stark-Einstein law states that only one molecule can get excited by absorbing one photon.