The enthalpies of combustion are different depending on the number of carbons and hydrogens in each substance. For example, the enthalpy combustion for methane is -890 kJ mol-1, however the enthalpy change of combustion for ethane or propane will be grea

Introduction on enthalpy of combustion

Standard enthalpy change of combustion is the energy change that occurs when 1 mol of fuel is completely burnt in oxygen under standard conditions. However, it is impossible for substances to be burnt under standard conditions.

The enthalpies of combustion are different depending on the number of carbons and hydrogens in each substance. For example, the enthalpy combustion for methane is

-890 kJ mol-1, however the enthalpy change of combustion for ethane or propane will be greater because more bonds are involved and therefore more bonds are broken and made.

All combustion reactions are exothermic because energy is given out to make bonds.

Figure 1: Exothermic reaction

Before you start doing calculations for enthalpy change of combustion you firstly need to balance the equation.

For example:

Methane CH4 (g) + 2O2 (g) 2H2O (g) + CO2 (g)

Also it is important to note when making a combustion reaction balanced equation that the state symbols for all the substances are gaseous even water.

Moreover, it is only in a combustion reaction that you use halves in an oxygen molecule. An example for octane is: C8H12 (g) + 12.5O2 (g) 8CO2 (g) + 9H2O (g)