First slide

Enthalpy of formation

Question

If $Δ_{f} H^{\circ} (C_{2} H_{4}) and Δ_{f} H^{\circ} (C_{2} H_{6})$ areX₁, and X₂, kcalmol^-1, then heat of hydrogenation of C₂H₄ is

Easy

A

x₁ + x₂
B

X₁-X₂
C

X₂- X₁,
D

X₁+2X₂

Solution

2C+2H₂⟶C₂H₄ , ΔH_0f=x1kcal/mol
C₂H₄⟶2C+2H₂ , ΔH_0f=−x₁kcal/mol.....(1)
2C+3H₂⟶C₂H_{6 ,}ΔH_0f=x₂kcal/mol.....(2)
adding eqn(1)&(2), we have
C₂H₄+2C+3H₂⟶C₂H₆+2C+2H₂ΔH_0R=−x₁+x₂
C₂H₄+H₂⟶C₂H₆ , ΔH_0R=(x₂−x₁)kcal/mol
Hence the heat of hydrogenation of C₂H₄ is (x₂−x₁)kcal/mol.

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