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Calculate the energy in kJ that is released as heat (70%) by the combustion of exactly...

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tdj4325 | (Level 1) Salutatorian

Posted June 28, 2012 at 9:06 PM via web

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Calculate the energy in kJ that is released as heat (70%) by the combustion of exactly 1 gallon of gasoline.

The combustion of gasoline releases and only about 30% of the energy from gasoline is used as work with the rest expelled as heat.

The chemical reaction of combustion is:

2 C8H18(l) + 25 O2 (g) → 16 CO2 (g) + 18 H2O (g) H = –10110 kJ

The density of octane is 0.702 g/mL. Assume gasoline is pure octane (1 gallon = 3.7854 L).

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chaobas | College Teacher | (Level 1) Valedictorian

Posted June 28, 2012 at 9:43 PM (Answer #1)

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here,

1 gallon = 3.7854 L = 3785.4 mL

 

so the volume of ocatne is 3785.4mL and the density is 0.702 g/mL

now 

density = mass/volume

mass= density * volume

Mass of octane = 0.702 g/mL * 3785.4 mL = 2657.35 g

 

now,

molar mass of octane = 114.23 g/mol and mass is 2657.35 g

now,

moles = mass/molar mass

 

moles of octane = 2657.35 g/(114.23 g/mol) = 23.26 mol

 

now the effiency is 70 % that is out of 100 moles, only 70 mol is combusted. so the number of moles combusted would be

23.26 mol * (70/100)

16.28 mol of octane.

 

now the reaction is 

2 C8H18 (l) + 25 O2 (g) → 16 CO2 (g) + 18 H2O (g) H = –10110 kJ

 

[dividing bothside by 2]

 C8H18 (l) + 25/2 O2 (g) → 8 CO2 (g) + 9 H2O (g) H = –5055 kJ/mol


so 1 mol of octane gives -5055 kj of heat, but we have 16.28 mol(70% efficient).

so the amount of heat released would be

16.28 mol * (-5055 kj/1 mol)

82925.4 Kj -- this is the amount of heat released.

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justaguide | College Teacher | (Level 2) Distinguished Educator

Posted July 3, 2012 at 8:19 AM (Answer #2)

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When gasoline, which is taken to be pure octane, undergoes combustion in an internal combustion engine 30% of the energy released is converted to mechanical work and the rest is expelled as heat. 1 gallon of octane is 3.7854 L and the density of octane is 0.702 kg/L

From the chemical equation:

2 C8H18(l) + 25 O2(g) → 16 CO2(g) + 18 H2O(g) `Delta H`=–10110 kJ

The molar mass of octane is 114 g/mole. The mass of 1 gallon of gasoline is 2.657 kg. This is approximately 2.657/114 = 23.31 moles of octane. Each mole of octane releases (10110/2)*0.7 = 5055*0.7 = 3538.5 kJ of heat.

23.31 moles releases 3538.5*23.31 = 82482.43 kJ of heat.

The amount of heat released when one gallon of gasoline is burnt is approximately 82482.43 kJ

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