What is Bond Energy with Examples -Bond energy Calculations

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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

Trick to find Bond Order, Bond Lengths, Bond Strengths and Bond Energy
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