Enthalpy change MC~

The standard enthalpy change of combustion of but-1-ene, C4H8(g) is x kJ mol⁻¹. The standard enthalpy change of the reaction 2C2H4(g) → C4H8(g) is y kJ mol⁻¹. The standard enthalpy change of combustion of ethene, C2H4(g) is...

A. (x/2 + y) kJ mol⁻¹
B. [(x+y)/2] kJ mol⁻¹
C. [(x-y)/2] kJ mol⁻¹
D. (x + y/2) kJ mol⁻¹


It is given that:
(1/2)C4H8(g) + 3O2(g) → 2CO2­(g) + 2H2O(l) .. ΔH1° = x/2 kJ mol⁻¹
C2H4(g) → (1/2)C4H8(g) .. ΔH2­° = y/2 kJ mol⁻¹

Add the above two equations together, and cancel (1/2)C­4H8 on the both sides:
C2H4(g) + 3O2(g) → 2CO2­(g) + 2H2O(l) .. ΔHc°

Standard enthalpy change of combustion of C2H4(g) = ΔHc°
=ΔH1° + ΔH2°
= [(x/2) + (y/2)] kJ mol⁻¹
= [(x+y)/2] kJ mol⁻¹ ...... (option B)