Bond Energy or Bond enthalpy



Bond energy of a bond is amount of energy required to break 1 mole of a bond in gaseous state or energy released when one mole of a bond is formed in gaseous state.

In terms of bond energy,

            Hreaction = Heat absorbed – Heat released = bond energies of reactants – bond energies of products

 

Estimate the enthalpy change for the reaction

H2 (g) + Cl2 (g)              →          2HCl (g)

Given bond energy of H – H   = 435 KJ mole-1

            Bond energy of Cl-Cl = 243 KJmole-1

            Bond energy of H-Cl  = 430 KJmole-1

Sol:

The amount of energy released during formation of 2moles of H-Cl bond = 2 × 430 = 860 KJ

H reaction = Energy absorbed – Energy released

                        = 618 – 860

                        = -182 KJ

Calculate the enthalpy change for the hydrogenation of ethane gas.

            CH2 = CH2 (g) + H2 (g)          →            CH3 – CH3 (g)

Given, bond energy of H -H bond = 435 KJ mole-1

            Bond energy of C – H bond = 413 KJ mole-1

            Bond energy of C -C bond     = 347 KJ mole-1

                Bond energy of C=C bond     = 615 KJ mole-1

 The amount of energy absorbed during reaction

            = 413 × 4 + 615 + 435

            = 2702 KJ

Energy released during reaction

            = 6 × 413 + 347

            = 2825 KJ

 

H reaction = 2825 – 2702 KJ  = -123 KJ