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Bond Enthalpy

Bond enthalpy of a given bond is defined as the average of enthalpies required to dissociate the said bond present in different gaseous compounds into free atoms or radicals in the gaseous states. The bond enthalpy may be distinguished from the term bond dissociation enthalpy that is defined as the energy required to dissociate a given bond of some specific compound.

Bond enthalpies can be obtained from data on enthalpies of combustion and enthalpies of dissociation. Taking an example of the bond enthalpy of C-H, we have
CH4(g)  C(g) + 4H(g)  C-H =

Data on the bond enthalpies can be employed to calculate the approximate enthalpy of formation of a substance of known structure by adding the appropriate bond enthalpies. Similarly, approximate estimate of enthalpy of a reaction can be determined from the bond enthalpy data. Taking an example of the reaction
H2(g) + O2(g) H2O(g)

We have,
Breaking of H - H bond, ΔH = 433 KJ mol-1
Breaking of O-O bond, Δ H = (492 KJ mol-1)
Making of two O-H bonds, Δ H = - 2(464 KJ mol-1)
Adding these, we getΔ H = 433 KJ + (492 KJ) - 2(464 KJ mol-1)
                          = - 294 KJ mol-1

The above changes in enthalpies are shown in Fig. 8.6. For the reaction
C2H4(g) + H2(g) C2H6(g)
We have,
Breaking of 1 C=C bond Δ H = 606.68 KJ mol-1
Breaking of 4 C-H bonds Δ H = 4 410.87 KJ mol-1
Breaking of 1 H-H bond Δ H = 431.79 KJ mol-1
Making of 1 C-C bond Δ H = - 336.81 KJ mol-1
Making of 6 C-H bonds Δ H = - 6 410.87 KJ mol-1
Adding these, we getΔ H = - 120.08 KJ

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