Given, 21H2+21Cl2⟶HCl,ΔHf=−93kJ/mol ∵ In this reaction, one mole of HCl is formed. ∴ΔHf of the reaction will remain same, i.e. −93kJ/mol.
Let the bond dissociation energy of H−Cl bond be x.
For the given reaction, ΔHf=ΣB⋅E(reactants )−ΣB⋅E(products) −93=21(434)+21(242)−x −93=+338−x ⇒x=431kJ/mol