Question

resonance energy = experimental heat of formation - calculated heat of formation

• A. 1

Answer 1

The correct option is A 1
Resonance Energy:
A compound with delocalized electrons is more stable than it would be if all of its electrons were localized. The extra stability a compound gains from having delocalized electrons is called delocalization energy or resonance energy.
Example -
The observed heat of combustion of benzene is 150.6 kJ /mole which is lower than the value calculated for any of the resonating(canonical)forms. This difference in energy is known as resonance energy. The greater the resonance energy, the greater is the stabilization due to resonance.
This is also explained by the heat of formation of the compound. The observed heat of formation of carbon dioxide is 1590 KJ per mole which is 125.5 KJ more than the calculated value. In other words, Carbon dioxide requires 125.25 KJ more energy than that is expected to break it up into elements.
Resonance Stabilization Energy:

• One of the important points connected with resonance is that the resonance hybrid is a more stable structure than any of the structures contributing it. Also, resonance makes a molecule more stable and less reactive.
• The increased stability of a resonance hybrid than that of any individual canonical structure is accounted in the following way.
• If it is considered that a resonance hybrid has two resonating structures, I and II, that can each be represented by wave functions, in simple cases, which give the energies of the structures they represent. By combining the two wave functions it is found that a third wave function will also represent the system and that this function corresponds to a lower energy value, and thus higher stability. (As shown in the figure).
• The difference between the energy content of the resonance hybrid and other canonical structure having least energy is called resonance energy. Resonance energy can be calculated from the heat of hydrogenation and heat of combustion values.
• The source of resonance energy is the same as that of any other type of bonding energy. As the energy of a single or double bond arises from the ability of an electron to encompass two nuclei in a molecule instead of one in the case of an atom.
  
• Similarly resonance energy is merely an additional stabilization resulting from the ability of an electron (pi or unshared) to encompass three or more nuclei instead of two. These electrons, encompassing more than two nuclei, are called mobile electrons and are therefore, said to be delocalized electrons.