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Question: The energy released when \( 6 \) moles of octane is burnt in air will be : [Given , \( \Delta {H_f} ...

The energy released when 66 moles of octane is burnt in air will be : [Given , ΔHf\Delta {H_f} for CO2(g)C{O_2}(g) , H2O(g){H_2}O(g) and C8H18(l){C_8}{H_{18}}(l) , respectively are −490,−240- 490, - 240 and +160J/mol+ 160J/mol ]
A. −37.4kJ- 37.4kJ
B. −20kJ- 20kJ
C. −6.2kJ- 6.2kJ
D. −35.5kJ- 35.5kJ

Explanation

Solution

Octane is a hydrocarbon and an alkane with the chemical formula C8H18{C_8}{H_{18}} and the structural formula CH3(CH2)6CH3C{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. 2,2,4−2,2,4 - 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+O2→CO2,Δf∘=−490kJ/mol−−−−(1)C + {O_2} \to C{O_2},\Delta _f^ \circ = - 490kJ/mol - - - - (1)
H2+12O2→H2O,ΔHf∘=−240kJ/mol−−−−(2){H_2} + \dfrac{1}{2}{O_2} \to {H_2}O,\Delta H_f^ \circ = - 240kJ/mol - - - - (2)
8C+18H→C8H18,ΔHf∘=+160kJ/mol−−−−(3)8C + 18H \to {C_8}{H_{18}},\Delta H_f^ \circ = + 160kJ/mol - - - - (3)
To find: Energy released when 66 moles of octane is burnt in air
Multiplying equation 11 by 88 , equation 22 by 99 and then adding all the equations 1,21,2 and 33 .
C8H18+252O2→8CO2+9H2O{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.
ΔH∘=−3920−2160−160=6240kJ/mol\Delta {H^ \circ } = - 3920 - 2160 - 160 = 6240kJ/mol
Therefore, ΔH∘\Delta {H^ \circ } for 66 moles of octane =6240×6= 6240 \times 6
ΔH∘=37440kJ/mol\Delta {H^ \circ } = 37440kJ/mol
ΔH∘=−37.4kJ\Delta {H^ \circ } = - 37.4kJ
Therefore, −37.4kJ- 37.4kJ is energy released when 66 moles of octane are burnt in air.
Hence, the correct option is A. −37.4kJ- 37.4kJ .

Additional Information:
Octane is present in the gasoline you use in your car. When octane is burned, CO2C{O_2} and H2O{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.