O2→H2O;‌‌∆H=−286.2‌kJ (III) C2H4+3O2→2CO2+2H2O; ∆H=−1410.8‌kJ Multiply the equations (I) and (II) by 2 (IV) 2C+2O2→2CO2;∆H=−787.0‌kJ (V) 2H2+O2→2H2O;∆H=−572.4‌kJ Add the equations (IV) and (V) and then subtract (III). 2C+2H2→C2H4;‌‌∆H=51.4‌kJ