We will approximate gasoline as being composed of the hydro-carbon, octane, C8H1

Question

We will approximate gasoline as being composed of the hydro-carbon, octane, C8H18. Another fuel that is common in racing applications is nitromethane, CH3NO2, used in top fuel dragsters. Let's consider the combustion of each fuel with oxygen in an internal combustion engine. Let the 8-cylinder engine displacement be 351 in 3 total.

GOAL: Determine and compare the maximum enthalpy of reaction, Hrxn, for the combustion of both octane and nitromethane in a single cylinder of the engine. This means determine the maximum amount of fuel that can burn in the cylinderlled with 1 atm of air and the amount of heat that reaction would generate.

Here are some suggested steps to guide you along.

1. Write a balanced chemical equation for the reaction of octane and nitromethane with O2 gas, assuming complete combustion. For octane, the products would be CO2 and H2O. For nitromethane, the products would be CO2, H2O; and N2.

2. Determine Hrxn, at 298 K, for both in kJ mol fuel . Which is greater?

3. Determine the amount of O2 gas available for the reactions in a single cylinder. Assume T = 298 K.

4. Determine how much fuel can be reacted with the amount of O2 in the cylinder.

5. Determine the heat released, H at 298 K, for burning that much fuel for both octane and nitromethane.

Ultimately, Which fuel gives the highest total heat output and therefore greatest engine perfomance?

Explanation / Answer

1. Rqn for octane; 2 C8H18 + 5 O2-------> 16 CO2+ 18 H2O

For nitromethane, 4CH3NO2 + 3O2 ------> 4CO2 + 6H2O + 2N2

2. Heat of formation of octane= -426.6 KJ/mol, heat of formation of CO2= 393.5 KJ/mol, heat of formation of H2O= 285.83 Kj/mol

using balanced eqn, C8H18 + 2.5 O2-------> 8 CO2+  9 H2O

Using Hess Law, heat of reaction= 8(CO2)+9(H2O)-(C8H18)= 8(-393.5)+9(-285.83)-(-426.6)= -3148.0-2572.47+426.6 = -5293.9 Kj

Heat of formation of nitromethane= -113.0 kj/mol, heat of formation of N2= 0

Heat of reaction= 4(-393.5)+ 6(-285.83)+ 2(0)-4(-113.0)= -1574-1714.98+452 = -2836.98 kj

Hrxn for octane is greater than nitromethane.

3. Molar mass of octane= 114 gm, nitromethane= 61 gm

114 gm octane require oxygen=2.5*16= 40 gm O2

61 gm nitromethane require oxygen= 3/4*16= 12 gm O2

Oxygen to fuel ratio=114+61/40+12= 3.36

So, to burn 1 kg of fuel, 3.36 kg og O2 is required.

5. Heat release will be same as the heat of reaction

Octane is better fuel than nitromethane.

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