Though the concept of oxidation number is widely used and at the same time is very helpful in redox chemistry, yet it suffers from certain limitations.
The limitations of the concept of oxidation number are listed down as follows.
- According to the classical approach, many reactions are called as redox reactions. However, it has been found that practically it is not possible to do the same by applying the oxidation number concept. Consider the following reaction.
SiCl4 + LiAlH4 → SiH4 + LiCl + AlCl3
As per the classical approach, silicon is reduced and both lithium and aluminium are getting oxidised. However, according to the concept of oxidation number, none of the species is undergoing any change in oxidation number.
- Many a times, it becomes difficult to classify a reaction as redox reaction according to all the three approaches, viz., classical, electronic and oxidation number approach. An example of such a type of reaction is as follows.
CH3NCO + H2O → CH3NH2 + CO2
To overcome these limitations, a broader concept of redox processes in terms of changes in electron density has been proposed.
You will read this in your higher grades.
Hope the answer helps you,