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: The energy of a resonance hybrid is lower than the energy of any of its individual canonical forms due to resonance stabilization.

Explanation:

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 structures is called resonance energy.

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

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

Hence, option 4 is the correct answer.

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