Bond Energy and Bonds dissociation Energy

Bond energy and bonds dissociation energies for the monoatomic molecule define the same value but polyatomic molecules we use average energies for calculation of bond energy. When two or more atoms combine to form a chemical bond there is the liberation of heat or energy and this negative value is called bond energy. But when the same magnitude of energies supplied for breaking different type of bonds is called bond dissociation energy.

Thus there are two types define separately

  1. Dissociation energies (define by D)
  2. Bonds energies ( define by E)

The definition of bonds dissociation energy is the energy required to break a particular bond in polyatomic molecules in the gaseous phase to neutral fragments or free radicals.

Y – Z (g) + D → Y• (g) + Z• (g)

Calculation of bonds dissociation energies bond energy of the molecule

Average Bond Energy of Molecules

In polyatomic molecules like methane four bonds are equivalent but the energy required to break the first bond is not the same as the second bond. Therefore the energy required for breaking the bonds depends on the nature of the molecule.

Hence we can take the average of all the different values and this average value termed as bonding energies. Thus for diatomic molecule bond energies and dissociation energy identical.

Chemical Bonds Energies
kJ Kcal
CH3 – H 426.8 102
MeCH2 – H 405.8 97
Me2CH – H 393.3 94
Me3C – H 374.5 89.5
C – C 347.3 83
C = C 606.7 145
N – N 163.2 39
N = N 418.4 100

Ionic Resonance Energy

Greater the polarity of a bond greater will be bond dissociation energy. This extra energy appearing in A – B molecules arise by electrostatic attraction between A and B.

Hence this extra energy possessed by molecules with partial ionic character is called ionic resonance energy. These values used by pulling for determination of electronegativity difference of atom in the molecules.

This empirical formula for A – B molecule is

XA – XB = 0.208 √Δ + α

Where XA and XB = electronegativity of A and B and α = arbitrary constant which nearly zero for elements other than C, N, O, and F. He used fluorine the most electronegative elements as the reference point with the value of 4.0.

Bonds Energies and Resonance

The various factor is responsible for the difference in energies of given bonds in different compounds. These are resonance and steric effects.

The property of the molecules in the ground state describes by resonance energy. Most of the resonance energies obtained from the different type of thermodynamics heat of combustion or hydrogenation of bond.

Resonance can occur only when all the atoms involved lie in the same plane. But any change in structure which prevents the planarity diminishes or inhibit resonance. Thus following points useful to describe the links between resonance type and different bonding energies.

  • The more stable structure is the larger contribution in resonance. Generally the structure with the largest number of bonds more stable. This is because the sum of these energies in the molecules gives its stability.
  • If different resonating structures have the same number of bonds, but some are charged, then the molecule will less stable than the uncharged structure.

Calculation of the Value of Bond Energy

Enthalpy value is very useful for the calculation of bond dissociation energies. A chemical reaction is essential for breaking and making bonds.

Bonds breaking is an endothermic and making is an exothermic process. When the bonds formed in a chemical reaction are more stable than the broken bonds, the extra energies obtained in the forms of heat. This constitutes the heat of the reaction of the broken bond.

H2 → 2H +435.9 kJ
Cl2 → 2Cl +241.8 kJ
Bond Making 2H + 2Cl → 2HCl -2 × 431 kJ

Therefore the overall reaction is exothermic to the extent of ( -2 × 431 + 435.9 + 241.8) kJ = -184.3 kJ.

H2 + Cl2 → 2HCl ΔHf = -92.15 kJ

Therefore, enthalpy data for known bonds and known compounds may be used to calculate unknown energies.