Question

The oxidation number of $ C $ in $ HCN $ and $ HNC $ respectively are ______.
A. $ + 2, + 2 $
B. $ + 2, + 4 $
C. $ + 4, + 4 $
D. $ - 2, - 2 $

Answer 1

Hint : Oxidation number also known as oxidation state of an element in a molecule is defined as the total number of electrons that element may gain or lose in order to form a chemical bond with another atom. It is represented by the superscript of the symbol of an element.

Complete Step By Step Answer:
In order to calculate oxidation number, we need to understand certain rules which are as follows:
1. The oxidation state of an element in its uncombined state is zero. This is also valid for the elements which exist as diatomic molecules like $ {H_2} $ , $ {O_2} $ , etc.
2. The oxidation state of a mono-atomic ion is the same as the charge present on the ion.
3. For a neutral compound, the sum of oxidation number of all atoms in a molecule is zero. In case of polyatomic ions, the sum of oxidation number of all atoms is equal to the overall charge present on the ion.
4. The oxidation number of an alkali metal i.e., elements of group I is $ + 1 $ while for alkaline earth metals i.e., group II elements, the oxidation number is $ + 2 $ .
5. Generally the oxidation number of oxygen atoms in a compound is $ - 2 $ . However, in the case of peroxides, the oxidation number of the oxygen atom is considered as $ - 1 $ .
6. The oxidation number of a hydrogen atom is usually $ + 1 $ . However, in the case of binary metal hydrides, it exists in its $ - 1 $ oxidation state.
7. The oxidation number of elements of the halogen family is usually $ - 1 $ unless the atoms are combined with oxygen or fluorine atoms.
8. Generally, the oxidation number of nitrogen atoms lies in the range of $ - 3 $ to $ + 5 $ . In amines, imines and nitriles, the oxidation number of nitrogen atoms is $ - 3 $ whereas in nitro and nitrite compounds, the nitrogen atom exists in its $ + 5 $ oxidation state.
 Now, let us calculate the oxidation state of carbon in each given molecule:
Assume the oxidation number of $ C $ be $ x $ .
The general oxidation state for hydrogen and nitrogen atoms is $ + 1 $ and $ - 3 $ respectively.
For $ HCN $ molecule:
 $ x + 1 - 3 = 0 $
 $ \Rightarrow x = + 2 $
For $ HNC $ molecule:
 $ x + 1 - 3 = 0 $
 $ \Rightarrow x = + 2 $
Therefore, the oxidation number of $ C $ in $ HCN $ and $ HNC $ respectively are $ + 2, + 2 $ . Hence, option (A) is the correct answer.

Note :
It is important to note that $ HCN $ named as hydrogen cyanide and $ HNC $ named as hydrogen isocyanide are functional isomers that means they only differ in structure. Also remember that in case of reduction, a decrease in the oxidation state of an atom is observed while in case of oxidation, the oxidation number of an atom increases.