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Question

Consider the following reactions,


C(s)+O2(g)CO2(g),ΔH=94 kcal
2CO(g)+O22CO2(g),ΔH=135.2 kcal

Then, the heat of formation of CO(g) is:

A
26.4 kcal
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B
26.4 kcal
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C
41.2 kcal
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D
41.2 kcal
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Solution

The correct option is A 26.4 kcal
According to Hess's law, the total heat changes occuring during a chemical reaction are independent of path.
2CO(g)+O2(g)2CO2(g)N ,
ΔH=135.2 kcal

The above reaction is reversed and halved, we get-
CO2(g)CO+12O2(g)
ΔH=135.22 kcal ....(i)

C+O2(g)CO2(g)
ΔH= -94 kcal ...(ii)

Required equation
C(s)+12O2(g)CO(g);

Adding Eqs. (i) and (ii), we get

C(s)+12O2(g)CO(g),

ΔH =135.2294=26.4 kcal

Option B is correct.

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