Question

The energy released when $ 6 $ moles of octane is burnt in air will be : [Given , $ \Delta {H_f} $ for $ C{O_2}(g) $ , $ {H_2}O(g) $ and $ {C_8}{H_{18}}(l) $ , respectively are $ - 490, - 240 $ and $ + 160J/mol $ ]
A. $ - 37.4kJ $
B. $ - 20kJ $
C. $ - 6.2kJ $
D. $ - 35.5kJ $

Answer 1

Hint: Octane is a hydrocarbon and an alkane with the chemical formula $ {C_8}{H_{18}} $ and the structural formula $ C{H_3}{(C{H_2})_6}C{H_3} $ in its condensed form. The amount and location of branching in the carbon chain change amongst structural isomers of octane. $ 2,2,4 - $ trimethylpentane (also known as iso-octane) is one of these isomers and is used as one of the standard values in the octane rating scale.

Complete answer:
Given:
 $ C + {O_2} \to C{O_2},\Delta _f^ \circ = - 490kJ/mol - - - - (1) $
 $ {H_2} + \dfrac{1}{2}{O_2} \to {H_2}O,\Delta H_f^ \circ = - 240kJ/mol - - - - (2) $
 $ 8C + 18H \to {C_8}{H_{18}},\Delta H_f^ \circ = + 160kJ/mol - - - - (3) $
To find: Energy released when $ 6 $ moles of octane is burnt in air
Multiplying equation $ 1 $ by $ 8 $ , equation $ 2 $ by $ 9 $ and then adding all the equations $ 1,2 $ and $ 3 $ .
 $ {C_8}{H_{18}} + \dfrac{{25}}{2}{O_2} \to 8C{O_2} + 9{H_2}O $
When octane is burned, carbon dioxide gas is generated, as shown in the reaction equation. This is typical of hydrocarbon combustion reactions, such as octane and propane.
 $ \Delta {H^ \circ } = - 3920 - 2160 - 160 = 6240kJ/mol $
Therefore, $ \Delta {H^ \circ } $ for $ 6 $ moles of octane $ = 6240 \times 6 $
 $ \Delta {H^ \circ } = 37440kJ/mol $
 $ \Delta {H^ \circ } = - 37.4kJ $
Therefore, $ - 37.4kJ $ is energy released when $ 6 $ moles of octane are burnt in air.
Hence, the correct option is A. $ - 37.4kJ $ .

Additional Information:
Octane is present in the gasoline you use in your car. When octane is burned, $ C{O_2} $ and $ {H_2}O $ are produced.
The availability of oxygen is a critical component in the process. Combustion is impossible in an oxygen-deficient environment.

Note:
If a hydrocarbon is completely ignited, it will burn with a blue flame. As the molecular mass of a hydrocarbon grows, it begins to burn with a yellow flame that indicates incomplete combustion. With rising molecular weights, burning hydrocarbons becomes more challenging.