Question

State the oxidation number of carbonyl carbon in methanal and Methanoic acid respectively.

A.0 and 0
B.0 and ${{ + 2}}$
C.$+ 1$ and $+ 2$
D.$+ 1$ and $+ 3$

Answer 1

Oxidation number refers to the total number of electrons an atom gains or loses to form a chemical bond with another atom. The oxidation number is also referred to as the oxidation state and the net charge on the neutral molecule will be zero. Thus the oxidation number of the central atom can be calculated.

Complete step by step answer:
We know that the net charge on a neutral molecule is zero. So let us apply this concept to methanal HCHO
Let the oxidation state of the carbonyl carbon in methanol be x
The oxidation state of HCHO is the sum of the oxidation state of the hydrogen atom, the oxidation state of carbon, the oxidation state of oxygen, and the sum is zero since it is a neutral molecule.
We know the charge of hydrogen = ${{ + 1}}$
The oxidation state of carbon ${{ = x}}$
The oxidation state of oxygen ${{ = - 2}}$
${{2( + 1) + x - 2 = 0}}$
${{2 + x - 2 = 0}}$
${{x = 0}}$
Hence the oxidation state of carbonyl carbon in methanal is 0
${{2( + 1) + y + 2( - 2) = 0}}$
$\Rightarrow {{2 + y - 4 = 0}}$
$\Rightarrow {{y - 2 = 0}}$
$\Rightarrow {{y = + 2}}$
Now considering methanoic acid that HCOOH, we get
The oxidation state of oxygen ${{ = - 2}}$
The oxidation state of hydrogen ${{ = + 1}}$
Let the oxidation state of carbonyl carbon in methanoic acid (HCOOH) be y
Then ${{2( + 1) + y + 2( - 2) = 0}}$

Hence the oxidation state of carbonyl carbon in methanoic acid is ${{ + 2}}$
Thus the oxidation state of carbonyl compounds in methanal and methanoic acid respectively are 0 and ${{ + 2}}$.
Hence the correct option is B.

Note:
We came across two compounds, methanal and methanoic acid. Methanal is the smallest compound of the aldehyde group and the same goes for methanoic acid. Also, the oxidation number of hydrogen is always ${{ + 1}}$ except in metal hydrides and oxygen is always ${{ - 2}}$ except in the case of peroxides.