Bond Enthalpy values of H-H, Cl-Cl are 436kJ/mole and 243kJ/mole respectively. Bond Enthalpy of H-Cl  will be ∆f0HCl=-93kJ/mole

12H2g+12Cl2g →HClg; ∆H=-93kJ/mole H2g+Cl2g →2HClg;        ∆H=-186kJ/mole    ∆H=HR-HP -186=HH-H+HCl-Cl-2X -186=+436+243-2X       2X= (436+243+186)kJ/mole         X=432.5 kJ/mole