Some reactions do not take place in the dark but take place only in the presence of light or some other radiation. Such reactions are known as ‘photochemical reactions’. For example:
Photosynthesis of HCl gas:
H2 (g) + Cl2 (g) + hν ——————-> 2 HCl (g)
Light is a form of energy. Hence it provides the necessary activation energy for the reaction to take place.
What are the Features of photochemical reactions?
1. Photochemical reactions do not take place in dark but take place in the presence of light by absorbing it.
2. Since different colored radiations in the range of visible light have different frequencies and hence different energies, therefore all radiations may not be able to initiate a particular reaction. For example, a photon of violet light has highest frequency and hence the highest energy. Hence a reaction which is initiated by violet light may not be initiated by other colored radiations 0of visible light. On the other hand, a photon of red light has lowest frequency and energy. Therefore a reaction that can be initiated by all other radiations as well.
3. Temperature has very little effect on the rate of a photochemical reaction. Instead, the intensity of light has marked effect on the rate of photochemical reaction.
4. The free energy change of a photochemical reaction may not be negative.
5. There are many substances which do not react directly when exposed to light. However, if another substance is added, the photochemical reaction starts.
