If Δf H° (C2H4) & Δf H°(C2H6) are X1 & X2 Kcal/mol, then heat of hydrogenation of C2H4 is

The heat of hydrogenation of ethene (C2H4) refers to the enthalpy change when one mole of ethene undergoes hydrogenation to form ethane (C2H6).

Given:
Standard enthalpy of formation of ethene (C2H4) = Δf H°(C2H4) = X1 kcal/mol
Standard enthalpy of formation of ethane (C2H6) = Δf H°(C2H6) = X2 kcal/mol
Reaction:
The hydrogenation reaction of ethene is:

C2H4 (g) + H2 (g) → C2H6 (g)

By definition, the heat of reaction (ΔH) is calculated as:

ΔH = Σ(Δf H° of products) - Σ(Δf H° of reactants)

Since the enthalpy of formation of an elemental substance in its standard state is zero, the standard enthalpy of formation of H2(g) is 0.

Thus, the heat of hydrogenation (ΔH) is:

ΔH = [Δf H°(C2H6)] - [Δf H°(C2H4) + Δf H°(H2)]
ΔH = X2 - (X1 + 0)
ΔH = X2 - X1

Conclusion:
The heat of hydrogenation of C2H4 is (X2 - X1) kcal/mol.