Difference Between Thermochemical and Photochemical Reaction

Difference Between Thermochemical and Photochemical Reaction:

The following are the points of difference between Thermochemical and Photochemical Reaction.

Thermochemical ReactionPhotochemical Reaction
These reactions proceed with the absorption or evolution of heat.These reactions proceed with the evolution or absorption of light.
These reactions can take place in the dark also.The presence of light is essential for these reactions.
The temperature has a significant effect on these reactions and rate of reaction.The temperature has no significant effect on these reactions but the intensity of light affects the rate of reaction.
ΔG is always negative for spontaneous reactions.In photochemical reactions ΔG may or may not be negative for spontaneity. For example, ΔG is +ve for
(i) Photosynthesis in plants
(ii) Ozonization of oxygen.
Thermochemical activation is not of selective nature.Photochemical activation is highly selective, i.e., absorbed photons of light excite only a particular atom, or molecule, or group of atoms, which then becomes the site for the photochemical reactions.

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