Question

How much energy is released when 6 moles of octane is burnt in air ? Given $\Delta H_{f}{ }^{\circ}$ for $CO _{2}(g), H _{2} O (g)$ and $C _{8} H _{18}(l)$ respectively are $-490,-240$ and $+160\, J / mol :$

• A. - 6.2 kJ
• B. - 37.4 kJ
• C. - 35.5 kJ
• D. - 20.0 kJ

Answer 1

Answer: (B)

$ C + O_2 \longrightarrow CO_2; \Delta H_f^{\circ} = -490 \, kJ /mol \times 8 $
$ H_2 + \frac{1}{2} O_2\longrightarrow H_2 O ; \Delta H_f^{\circ} = -240\, kJ /mol \times 9 $
$ 8C + 18H \longrightarrow C_8 H_{18} ; \Delta H_f^{\circ} = +160 \, kJ /mol $
$8C + 8O_2 + 9H_2 + \frac{9}{2} O_2 - 8C - 18 H \longrightarrow 8CO_2 + 9 H_2 O - C_8 H_{18} ; $
$\Delta H_f^{\circ} = -3920 - 2160 - 160 $
$ C_8 H_{18} + \frac{25}{2} O_2 \longrightarrow 8CO_2 + 9H_2O $;
$ \Delta H_f^{\circ} = 6240 \, kJ /mol $
$\Delta H^{\circ} $ for $6$ moles of octane = $ 6240 \times 6 = 37440 \, kJ / mol$.