Question

Heat of dissociation of benzene to elements is 5535 KJ$mol^{-1}$. The bond enthalpies of C-C, C=C and C-H are 347.3, 615.0 and 416.2 KJ respectively. Resonance energy of benzene is:
A. 1.51KJ
B. 15.1KJ
C. 151KJ
D. 1511 KJ

Answer 1

Hint: The amount of heat evolved in the reaction when one mole of the bonds in a molecule are broken to obtain the atoms or ions from the molecule is known as heat of dissociation. For example: The amount of heat evolved when one mole of the methane molecule breaks.

Complete step by step answer:
New products are always formed in a spontaneous reaction. Some processes involve the evolution of energy while some involve the absorption of energy, energy can be given in the form of heat, light and radiations in order to carry out the reaction.
Given: The heat of dissociation of the benzene to elements = 5535 KJ $Mol^{-1}$
The change in the enthalpy of the reaction when one mole of the bonds in an atom is broken to obtain the atoms in the gaseous phase is known as Enthalpy of atomisation or enthalpy of dissociation.
Also the bond enthalpies of C-C, C=C and C-H are 347.3, 615.0 and 416.2 KJ respectively.
In the structure of benzene there are:
a. 3 C-C bonds
b. 3 C=C bonds
c. 6 C-H bonds
This is evident from the structure of the benzene given:

Thus the energy of the formation of benzene can be calculated as:
= $3 C-C bonds+3 C=C bonds+6 C-H bonds $

$ = 3 \times 347.3 + 3 \times 615.0 + 6 \times 416.2$

$= 5384.1kJmo{l^{ - 1}} $

The resonance energy of the benzene= Heat of dissociation- Heat of formation
$ = 5535 - 5384.1$
$ = 151kJmo{l^{ - 1}} $

So, the correct answer is Option C.

Note: Such changes are always taken against a standard and 1 mole number of atoms is considered as a standard to contrast and compare the appropriate reactions in the field of physical chemistry. These changes are easy to quantify if calculated for the 1 mole number of atoms.