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Question

The bond dissociation energy of gaseous H2,Cl2 and HCl are 104, 58 and 103 kcal mol1 respectively. The enthalpy of formation for HCl gas will be

A
44.0 kcal
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B
22.0 kcal
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C
22.0 kcal
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D
44.0 kcal
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Solution

The correct option is B 22.0 kcal
H2 (g)2H (g);H=104 kcal ....(1)Cl2 (g)2Cl (g)H=58 kcal ....(2)HCl (g)H (g)+Cl (g)H=103 kcal ....(3)
Heat of formation for HCl
12H2 (g)+12Cl2 (g)HCl (g) H=?
Divide equations (1) and (2) by 2, and then add
12H2(g)+12Cl2(g)H(g)+Cl(g); H=81 kcal....(4)

HCl (g)H (g)+Cl (g); H=103 kcal
Subtracting equation (3) from equation (4) gives enthalpy of formation of HCl
Enthalpy of formation of HCl gas = 22.0 kcal

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