Question

When ethane is burnt in excess of oxygen, the oxidation number of carbon changes by
A. +8
B. +7
C. +3
D. +4

Answer 1

Hint: Ethane is a saturated hydrocarbon with carbon-carbon single bonds. Ethane has two carbon atoms and 6 hydrogen atoms. Ethane reacts with oxygen to give carbon dioxide and water. Calculate the oxidation number of carbons in both reactants and products side and find the change in it by subtracting the oxidation number of carbon in the products side minus reactants side.

Complete step by step answer:
We are given that ethane is burnt in excess of oxygen.
We have to calculate the change in the oxidation number of carbon in this reaction.
Ethane is a colorless, odorless, gaseous saturated hydrocarbon with 2 carbon atoms and 6 hydrogen atoms. Ethane is also known as dimethyl. The chemical formula of ethane is ${C_2}{H_6}$
Ethane reacts with excess oxygen to produce carbon dioxide and water.
$2{C_2}{H_6} + 7{O_2} \to 4C{O_2} + 6{H_2}O$
On the reactants side, the oxidation number of carbon in ${C_2}{H_6}$ is x
$
  2x + 6 = 0 \\
   \Rightarrow x = - 3 \\
 $
In the products side, the oxidation number of carbon in is $C{O_2}$ is y
$
  y + \left( {2 \times - 2} \right) = 0 \\
\Rightarrow y = + 4 \\
 $
The oxidation number of carbon is changed from -3 in ${C_2}{H_6}$ to +4 in $C{O_2}$
The change in oxidation number is ${O_p} - {O_r} = + 4 - \left( { - 3} \right) = + 4 + 3 = + 7$
The change in the oxidation number of carbon when ethane is burnt in excess of oxygen is +7.

The correct option is Option B, +7.

Note:
Ethane is a saturated hydrocarbon, whereas ethene and ethyne are unsaturated hydrocarbons. Do not confuse ethane with ethene or ethyne as the no. of hydrogen atoms and carbon-carbon bonds in these three compounds are different. Be careful while writing the respective formulas.