Petrol or gasoline is a fuel, so energy will be released on its combustion.
The energy content of petrol is about 120 MJ/gal. Since 1 gallon contains 3.78 lts, so the given case:
120/3.78 MJ = 31.75 MJ energy will be released on complete combustion of 1 lt of petrol.
Kindly note that petrol of different octane ratings is available in the market and depending on the octane rating the energy released will be different. Also, for the calculations 100% or complete combustion has been assumed. This may not be true for practical setting (a petrol based automobile engine, etc.) and hence the value may be less. In some countries, ethanol is mixed with petrol (up to a certain fraction) and will also cause different energy release.
Hope this helps.
Heat of Combustion of a substance is the heat liberated when 1 mole of the substance undergoes complete combustion with oxygen at constant pressure. Chemical reactions take in energy to break bonds and give off energy when they make bonds.
The balanced reaction of combustion of Gasoline ( also known as Petrol ) is
2C₈H₁₈ + 25O₂ `->` 16CO₂ + 18H₂O
Now the Heat of combustion will be = Energy used in breaking of bonds of reactants i.e. Octane and Oxygen - Energy released in formation of bonds of CO₂ and Water
Octane has 18 numbers of C-H (Bond energy 411 kJ/mol ) and 7 numbers of C-C bonds ( Bond energy 346 kJ/mol)
Oxygen has 1 number of O=O bond ( Bond energy 494 kJ/mol )
CO₂ has 2 numbers of C=O bond ( Bond energy 799 kJ/mol)
Water has 2 numbers of O-H bonds ( Bond energy 459 kJ/mol )
Therefore Energy used in breaking bonds of reactants = 36*411 + 14*346 + 25*494 = 31990 kJ/mol
Energy released in formation of bonds of products = 32*799 + 36*459
= 42092 kJ/mol
Heat of combustion = 31990 – 42092 = - 10102 kJ ( Exothermic reaction )
This is the energy released by 2 moles of Octane
Therefore energy released by one mole of Octane will be = 10102/2 = 5051 kJ/mol
Now we have to convert this energy for Litre of Gasoline
Molar mass of Octane = 114.23 g/mol
Density = 0.71 Kg/L
Energy released = (5051*1000*0.71)/114.23 kJ/L =31.4 MJ/L
Actual energy released may vary due to the difference in density and octane rating of gasoline.