Deviations in the law of photochemical equivalence

Class - I: High quantum yield reactions.

Reactions in which quantum yield is greater than unity are high quantum yield reactions.

The quantum yield of the combination of carbon monoxide and chlorine by the light of a wavelength from 4000 to 4360 Å is 10³.

CO + Cl₂ ➝ COCl₂

The quantum yield of the combination of hydrogen and chlorine by the light of wavelength less than 4500 Å is 10⁴ to 10⁶ 

H₂ + Cl₂ → 2HCl

Photochemical decomposition of H₂O₂ by a wavelength of about 3100 Å has a quantum yield of more than 7.

2H₂O₂ → 2H₂O + O₂   

_{Class - II: Low quantum yield reactions.}

_{Reactions in which quantum yield is less than unity are low quantum yield reactions.
The quantum yield of the combination of hydrogen and bromine to form HBr is about 0.01}
H₂ + Br₂ → 2HBr
The quantum yield of photolysis of ammonia by a wavelength 2100 Å is nearly 0.2
2NH₃ → N₂ + 3H₂

_{Class - III: Small integer quantum yield reactions.}

_{Reactions in which quantum yield is a small integer} like 1,2,3,....... are small integer quantum yield reactions.
The quantum yield for the dissociation of HI by the light of wavelength 2070-2823 Å is 2.
2HI → H₂ + I₂
The quantum yield of the following reaction is 1.
2Fe⁺² + I₂ → 2Fe⁺² + 2I^-
This reaction is carried out in the liquid phase by a wavelength of 5790 Å.

Bodenstein pointed out that photochemical reactions invoice two processes.

(1) Primary Process: It is the process in which each molecule absorbs one quantum of energy (hv). The absorbed energy may give the excited molecule
AB + hv → AB*
or molecule may dissociate into free atoms or free radicals.
AB + hv → A⋅ + B⋅

(2) Secondary Process: It is the process that involves the excited atoms or molecules or free radicals produced in the primary process. This process may occur in dark also. Due to this process, the different number of molecules may undergo a reaction, by absorbing one quanta only. As a result, quantum yield differs from unity. Thus, quantum yield depends upon secondary process, the primary process remains the same for every reaction.

Reasons for high quantum yield :

The primary process gives excited atoms or molecules or free radicals. The secondary process may give some further exciting particles or radicals. Thus, by absorbing one quanta only, a large number of reactants undergo reaction. Thus, the quantum yield will be greater than unity.
In some reactions, such as the combination of H₂ and Cl₂, a chain is set up and quantum yield becomes as high as 10⁵. These reactions are chain reactions.

Reasons for low quantum yield :

If the excited particles formed in the primary process are such that they can not react due to their deactivation by collisions, by fluorescence or by internal arrangements or by coming in contact with inert molecules, the quantum yield becomes low.
The excited particles may recombine to give reactants. Thus, quantum yield becomes low.