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Bond energy is the measure of strength of bond. Polar bonds are stronger than non-polar bonds.
Bond Energy Definition:
“The energy required for breaking one mole of bonds or energy released during formation of one mole bonds is called bond energy”
Bond Energy Unit:
S.I unit for bond energy is kJ/mol.
Bond Energy Calculation:
e.g bond energy of HCl:
Energy required to break one mole H – H bond = 436 kJ/mol bond
Energy required to break one H – H bond = 436/6.02×1023 = 7.242×10-22 kJ/bond
Contribution of 1 H-atom to bond energy = 7.242×10-22/2 = 3.621×10-22 kJ/H-atom
Energy required during breaking of one mole Cl – Cl bond = 243 kJ/mol bond
Energy required during breaking of one Cl – Cl bond= 243/6.02×1023 = 4.037×10-22 kJ/bond
Contribution of 1 Cl-atom to bond energy = 4.037×10-22/2 = 2.0185×10-22 kJ/Cl-atom
By considering H – Cl bond to be non-polar with equal sharing of electron pair, we can calculate bond energy of H – Cl by adding the contribution of Cl-atom and H-atom to the bond.
Bond energy per H – Cl bond = 3.621×10-22 + 2.0185×10-22 = 5.6395×10-22 kJ/bond
But experimentally measured value = 7.26 x 10-22 kJ/bond
So actual bond energy is higher than calculated value, this is due to polar nature of the bond.
Factors Affecting Bond Energy:
Different bonds have different bond energies which depends upon:
Electronegativity difference between bonded atoms. More is electronegativity difference between atoms more will be the polarity of a bond, stronger will be the bond and higher will be its bond energy. Bond polarity and bond energy order in halogen acids is
HF > HCl > HBr > HI
Electronegativity difference 1.9 0.9 0.7 0.4
Bond energy (kJ mol-1) 562 431 366 299
HF being highly polar has higher bond energy so it is difficult to break its bond and hence it is a weak acid.
2.Size of Bonded atoms :
Larger is the size of atoms weaker will be the bond and low will be bond energy and vice versa e.g.
Cl2 > Br2 > I2
Bond energy (kJ mol-1) 243 193 151
F Bond energy of F2 is exceptionally low i.e. 159 kJ mol-1.
3. Bond length:
Shorter is the bond high will be bond energy and vice versa.
HF > HCl > HBr > HI
Bond length (m) 9.17×10-11 1.27×10-10 1.41×10-10 1.61×10-10
Bond energy (kJ mol-1) 567 436 366 299