Ncert Exercises & Solutions

Given below are two statements:

Assertion (A):	Energy of resonance hybrid is equal to the average of energies of all canonical forms.
Reason (R):	Resonance hybrid cannot be presented by a single structure.

1.	Both (A) and (R) are true and (R) is the correct explanation of (A).
2.	Both (A) and (R) are true but (R) is not the correct explanation of (A).
3.	(A) is true but (R) is false.
4.	(A) is false but (R) is true.

Hint: energy of the resonance hybrid is equal to the sum of energies of all canonical forms in a proportion to their contribution towards the resonance hybrid.

Resonance hybrids are always more stable than any of the canonical structures would be, if they existed. The

delocalization of the electrons lowers the orbital energies, imparting stability. The gain in the stability of the resonance

hybrid over the most stable of the canonical structure is called resonance energy.

A canonical structure that is lower in energy makes a relating greater contribution to resonance hybrid.

Thus, the correct assertion will be the energy of the resonance hybrid is equal to the sum of energies of all canonical forms in a proportion to their contribution towards the resonance hybrid. Hence, the Assertion is false but the reason is true.

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