BOND ENERGY (BOND ENTHALPY)

 BOND ENERGY (BOND ENTHALPY):

•Definition:

"The energy required to break all the bonds present in one mole of a substance is called bond energy or bond enthalpy".

"It is the energy required to break up an Avogadro number of bonds i.e one mole of bond".

• Unit:

It has unit kJmol-1.

• Information:

The strength of a bond is measured by its bond energy. The larger the bond energy, the stronger is the bond.

• Measurement:

It is determined experimentally by measuring the heat involved in a chemical reaction. It is also called bond enthalpy, as it is measure of enthalpy change at 298K.

•FACTORS:

• Electronegativity difference:

Greater the electronegativity difference,  greater will be the bond energy because of more attraction. So a polar covalent bond is stronger than a non polar covalent bond.

• Atomic character:

Greater the electronegativity difference, greater will be the ionic character and greater will be the bond energy because of more attraction. For example, H-F bond energy is 567 kJmol-1 but H-Cl bond energy is 431 kJmol-1.

• Size of atoms:

Greater size of atoms, smaller Will be the bond energy because of the weak intermolecular forces. For example, the bond energy of H-H bond is 435 kJmol-1 and that Cl-Cl is 243 kJmol-1. It is due to the shorter bond length in H2 molecule and greater bond length in Cl2 molecule.

• Bond length: 

Greater the size of atoms, greater will be bond length and smaller Will be the bond energy because of weak intermolecular forces. For example, bond energy of Cl-Cl is 242 kJmol-1 and that of I-I is 151 kJmol-1.

• Bond order:

It is found that energies of multiple bonds are greater than those of single bonds. But a double bond is not twice as strong as a single bond or a triple bond is not thrice as strong as a single bond. It means that a sigma bond is greater than a pi bond.