The data given below are for vapour phase reactions at constant pressure.
C2H6→∙C2H5+∙H; ΔH=420 kJ mol−1
∙C2H5→C2H4+∙H; ΔH=168 kJ mol−1
The enthalpy change for the reaction for the given reaction is :
2∙C2H5→C2H6+C2H4