Question

Write the equation for the complete combustion of octane. If the heat of combustion of octane is -5294 kJ/mol, determine how many kJ of energy could be obtained from the combustion of one gallon of gasoline (assume the density is 703 kg/m3).​

Answer 1

Alkanes are represented by CnH2n+2, n is no of carbon atoms.

when n=8, it is C₈H_8*2+2= C8H18, it is called octane.

the combustion of octane is C8H18 + O2 --------->8CO2 + 9H2O ( since there are 8 carbon atoms and 18 Hydrogen atoms, 8 has to be added to CO2 and 9 has to be added to H2O),

this gives rise to 8*2+9*1= 25 oxygen atom or 25/2= 12.5 oxygen molecules

so the balanced reaction of combustion of octane is C8H18+12.5O2 ------->8CO2 + 9H2O

Assuming gasoline is only octane, moles of octane= volume of octane* density/ molar mass

volume of octane = 1 gallon=3.78 liters= 3.78*10-3 m3, ( 1000L= 1m3)

mass of octane = volume of octane* density = 3.78*10^-3* 703 kg =2.65 kg= 2.65*1000 gm ( 1 kg= 1000gm)

moles of octane = mass/molar mass, molar mass of octane (C8H18)= 8*12+18*1=114 g/mole

moles of octane = 2.65*1000/114 =23.24

1 mole of octane gives 5294 Kj of heat energy

23.24 moles of octane gives 23.24*5294 Kj/mole =123062.3 Kj of heat energy is obtained.