Thermal Reactions Vs Photochemical Reactions

Thermal Reactions	Photochemical Reactions
Energy of activation is provided by the collisions of molecules.	Energy of activation is gained by the absorption of radiation.
Rate of reaction depends on temperature.	Rate of reaction is independent of temperature and depends on intensity of radiation.
Thermal reaction is accompanied by the decrease in free energy i.e. ΔG is always negative.	Free energy of photochemical reaction may increase as some of radiation energy may be converted into free chemical energy of products i.e. ΔG may be positive or negative.
Intermolecular collisions results in the excitation of all the molecules to almost the same extent.	A single atom or molecule can be promoted to excited state independent of the other species present in the reaction system. So, photochemical activation is highly selective.
To increase the average energy of all molecules, significantly high temperature is required.	Molecules on absorption of radiation are strongly excited and reaction can be carried out easily at low temperature.
These reactions can take place by ionic as well as free radical mechanism.	These reactions generally take place by free radical mechanism.
These reactions are accelerated by the presence of a catalyst.	These reactions may be accelerated by the presence of an additional substance called photosensitizer.