To find out and enthalpy of a reaction using bond energy we use the formula

$$\Delta H = \sum\Delta_\mathrm{bond}H_\mathrm{reactants} - \sum\Delta_\mathrm{bond}H_\mathrm{products}$$

since we use all the individual $\Delta H$ of the bonds to calculate the enthalpy, e.g.

$$\Delta H_\ce{C2H6} = 2\Delta_\mathrm{bond}H_\ce{C-C} + 6\Delta_\mathrm{bond}H_\ce{C-H}$$

But if a molecule is in resonance, how is the bond enthalpy taken? For example,

putative resonance structures

resonance structures

Is bond enthalpy the sum of the bond energies of that canonical structure which is the most stable or the average of all canonical structures or something else?

  • $\begingroup$ To every resonance form you can assign those value. But none is there. The hybrid structure has less energy. This question suggests 1) you took values as physical constants, and they are not; 2) the concept of resonance isn't clear to you. $\endgroup$ – Alchimista Jun 20 '19 at 10:02

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