Difference Between Dark and Photochemical Reactions

Thermochemical(Dark) Reactions	Photochemical Reactions
(1) It involves absorption or evolution of heat.	(1) It involves absorption of light.
(2) It can occur in dark as well as in light.	(2) Presence of light is a must.
(3) Temperature has a significant effect on the rate of reaction.	(3) Temperature has very little effect on the rate of reaction.
(4) These reactions are always accompanied by a decrease in free energy.	In these reactions,   free energy may increase or decrease.
(5) Activation energy is obtained by intermolecular collisions.	(5) Activation energy is obtained by a particular type of radiation.
(6) Thermal excitation increases in a random manner. i.e. translational, rotational and vibrational energy of all the molecules increases.	(6) By selecting a particular type of radiation particular atom or molecule can be excited among the mixture.