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 $
