Question

In the series ethane, ethylene and acetylene, the $C - H$ bond energy is:
(A) the same in all three compounds
(B) greatest in ethane
(C) greatest in ethylene
(D) greatest in acetylene

Answer 1

As we know that ethane, ethylene and acetylene all have two carbon atoms but the number of hydrogen atoms in case of ethane is more, in case of ethylene is less than ethane and in case of acetylene is very less.

Complete step by step answer:
As the hydrogen atom is less in a chemical formula with the same number of carbon atoms, it indicates that there is multiple bonding. As we can see in the formula, in case of ethane the formula is $C{H_3} - C{H_3}$, in case of ethylene, it is $C{H_2} = C{H_2}$ and in case of acetylene, it is $CH \equiv CH$.
As the number of bonding increases, the hybridization also changes which tells about the s and p character in the molecule which further tells about the electronegativity of molecules as ethane has $s{p^3}$hybridization which tells that total s-character is $25\%$ and total p-character is $75\%$. So, this molecule is less electronegative and the carbon atom is less tightly bonded with hydrogen atom so the $C - H$ bond energy is low in this case.
The ethylene has $s{p^2}$ hybridization which tells that total s-character is $33\%$ and total p-character is $67\%$. So, this molecule is more electronegative than the ethane molecule and the carbon atom is more tightly bonded with hydrogen atom so the $C - H$ bond energy is more in this case than the ethane molecule.
The acetylene has $sp$hybridization due to triple bonding which tells that total s-character is $50\%$ and total p-character is $50\%$. So, this molecule is more electronegative than the ethane and ethylene molecule and the carbon atom is even more tightly bonded with hydrogen atom so the $C - H$ bond energy is greatest in this case.
Therefore, the correct option is option (D).
Note:
More the multiplicity of bonding in the molecule more will be the s-character and more will be the strength of bonding.