First, we write the reaction equation for the formation of HCl from hydrogen and chlorine:
H₂ + Cl₂ → 2HCl
Now, we substitute the values of the enthalpies, denoting that of the Cl-Cl bond by x:
435 + x = -92 + 2(431)
x = 335
The bond dissociation enthalpy for the Cl-Cl bond is 335 kJ/mol