Bond energy is the amount of energy released when 1 mole of bonds are formed from the isolated atoms in the gaseous state or the amount of energy required to dissociate one mole of bonds present between the atoms in the gaseous molecules.
It is represented by Δ b H or Δbond H.
For diatomic molecules like H2, O2, N2, Cl2, HCl, HF etc. the bond energies are equal to their dissociation energies. These may also be called as their enthalpy of atomisation.
For polyatomic molecules ,the bond energy of a particular bond is not the same when present in different types of compounds.
The bond energy of a particular type of bond is not same even in the same compound.
For Ex: In CH4 the bond energy for first, second ,third and fourth C-H bonds are not equal: their values being 427, 439, 452 and 347 KJ mol-1
ΔaH° = 427 + 439 + 452 + 347
ΔaH° = 1665 KJ mol-1
To calculate bond energy, an average is taken.
ΔC-H = 1665 / 4 = 416 KJ mol-1
Bond energy usually means bond dissociation energy. It is always taken as positive because it is the energy required and not the energy released. It is the energy for one mole of a particular type of bonds and not for 1 mole of the substance.
Bond dissociation energies are obtained at 0 K by spectroscopic method whereas bond enthalpy are calculated by considering combustions from heat capacities and pΔV terms.
Bond enthalpy helps to calculate the enthalpy of formation of atoms.
For polyatomic molecules ,the bond enthalpy of a particular bond is found from the values of the enthalpies of formation.
Bond enthalpy of heteronuclear diatomic molecules can be obtained directly from the experiments or may be calculated from the bond enthalpies of homonuclear diatomic molecules.