Question

What is the bond energy of covalent $\mathrm{O}-\mathrm{H}$ bond in water?

Answer 1

Bond energy:

• Bond energy or bond enthalpy is the amount of energy that is required to break one mole of bond in two atoms.
• Bond enthalpy is given in unit kJ mol^-1
• It depends on the size of the atoms and the multiplicity of bonds
• The larger the size of atoms, the lower the overlapping and thus lesser bond energy. In the case of bonds, the energy required to break double and triple is higher than that required to break a single bond.

The bond energy of $\mathrm{O}-\mathrm{H}$ covalent bond in ${\mathrm{H}}_2\mathrm{O}$:

• Water is a polyatomic molecule, the energy required to break the two $\mathrm{O}-\mathrm{H}$ is different

Thus, the average of the two energies is taken

• $△{\mathrm{E}}_1=502\mathrm{kJ}{\mathrm{mol}}^{-1}$, energy required to break $\mathrm{H}-\mathrm{OH}$ bond
• $∆{\mathrm{E}}_2=427\mathrm{kJ}{\mathrm{mol}}^{-1}$ energy required to break $\mathrm{O}-\mathrm{H}$ bond.
• $∆\mathrm{E}=(∆{\mathrm{E}}_1+∆{\mathrm{E}}_2)/2=(502+427)/2=464.5\mathrm{kJ}{\mathrm{mol}}^{-1}$

Thus, the bond energy of the covalent $\mathrm{O}-\mathrm{H}$ bond in ${\mathrm{H}}_2\mathrm{O}$ is $464.5\mathrm{kJ}{\mathrm{mol}}^{-1}$