Given: C(s)+O2(g)→CO2(g)+94.2kcal H2(g)+12O2(g)→H2O(l)+68.3kcal CH4+2O2(g)→CO2(g)+2H2O(l)+210.8kcal The heat of formation of methane in kcal will be:
A
45.9
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B
47.8
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C
20.0
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D
47.3
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Solution
The correct option is B20.0 First reaction is formation reaction of carbon dioxide. Hence, heat of formation of carbon dioxide = 94.2Kcal. Second reaction is formation reaction of water. Hence, heat of formation of water = 68.3kCal. Since, ΔH for third reaction = 210.8Kcal Also, ΔH = ΔHf(CO2)+2×ΔHf(H2O)−ΔHf(CH4)
=> 210.8=94.2+2×68.3−ΔHf(CH4) => ΔHf(CH4)=94.2+136.6−210.8=20Kcal. Hence, answer is option C.