If ΔfH∘C2H4 and ΔfH∘C2H6 areX1, and X2, kcalmol-1, then heat of hydrogenation of C2H4  is

2C+2H2​⟶C2​H4​ , ΔH0f​=x1​kcal/molC2​H4​⟶2C+2H2​ , ΔH0f=−x1​kcal/mol.....(1)2C+3H2​⟶C2​H6 , ​ΔH0f​=x2​kcal/mol.....(2)adding eqn(1)&(2), we haveC2​H4​+2C+3H2​⟶C2​H6​+2C+2H2​  ΔH0R​=−x1​+x2​C2​H4​+H2​⟶C2​H6​ , ΔH0R​=(x2​−x1​)kcal/molHence the heat of hydrogenation of C2​H4​ is (x2​−x1​)kcal/mol.