The standard heats of formation of NO2 (g) and N2O4 (g) are 8.0 and 2.0kcalmol−1 respectively, the heat of dimerization of NO2 in kcal is
A
10.0
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B
−6.0
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C
−12.0
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D
−14.0
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Solution
The correct option is D−14.0 (i)12N2+O2→NO2;△H=8.0kcal(ii)N2+2O2→N2O4;△H=2.0kcal2NO2→N2O4;△H=? Multiply (i) by 2 and subtract (i) from (ii) we get: 2NO2→N2O4 △H=2−(8×2)=−14kcal