Chemical Thermodynamics Question 311

Question: How much energy is released when 6 mole of octane is burnt in air - Given $ \Delta H_{f}^{o} $ for $ CO_2(g),,H_2O(g) $ and $ C_8H_{18}(l) $ respectively are - 490,- 240 and + 160 kJ/mol [AIIMS 2004]

Options:

A) - 6.2 kJ

B) - 37.4 kJ

C) - 35.5 kJ

D) - 20.0 kJ

Show Answer

Answer:

Correct Answer: B

Solution:

$ C+O_2\to CO_2,;,\Delta H_{F}=-490,KJ/mol $ - (I) $ H_2+1/2O_2\to H_2O;\Delta H_{F}=-240,KJ/mol $ - (II) $ 8C+18H\to C_8H_{18};\Delta H_{F}=+160KJ/mol $ - (III) applying (I) $ \times ,8+ $ (II) $ \times 9+ $ (III) $ C_8H_{18}+\frac{25}{2}O_2\to 8CO_2+9H_2O $

$ \Delta H^{o}=-3920-2160-160=6240,KJ/mol $

$ \Delta H^{o}= $ for 6 moles of octane $ =6240\times 6 $

$ =37440,KJ/mol=-37.4,KJ $



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