Engineering
Chemistry
Kirchoffs Equation
Question

The bond dissociation energy of gaseous H2, Cl2 and HCl are 104, 58 and 103 kcal mol–1 respectively. The enthalpy of formation for HCl gas will be

44.0 kcal

22.0 kcal

– 22.0 kcal

– 44.0 kcal

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Solution
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Given

H2 (g) → 2H (g);                     ΔH = 104 kcal               ...(1)

Cl2 (g) → 2Cl(g) ;                   ΔH = 58 kcal                 ...(2)

HCl (g) → H(g) + Cl(g) ;        ΔH = 103 kcal               ...(3)

Heat of formation for HCl

H2 (g) + Cl2 (g) → HCl (g) ;      ΔH = ?

Divide equation (1) and (2) by 2, and then add

H2 (g) + Cl2 (g) → H(g) + Cl(g) ; ΔH = 81 kcal                 ...(4)

Subtracting equation (3) from equation (4)

HCl (g) → H(g) + Cl(g) ;          ΔH = 103 kcal                  ...(3)

  -               –         –                     –

__________________________________________________

12 H2 (g) + 12 Cl2 (g) → HCl(g) ;       ΔH = – 22.0 kcal

∴  Enthalpy of formation of HCl gas = – 22.0 kcal

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