Enthalpy of Combustion. Combustion Enthalpy Variation

THE enthalpy (H) corresponds to the energy content of a substance. But so far it is only possible to determine the value of the enthalpy variation (∆H) of chemical processes. This is done by decreasing the final enthalpy (of the products) by the initial enthalpy (of the reactants).

∆H = HFINAL - HINITIAL or ∆H = HPRODUCTS - HREAGENTS

It is customary to relate the type of enthalpy variation with the chemical phenomenon involved in the process. For example, when a compost burns, we calculate the variation of enthalpy of combustion (Hcombustion).

When the enthalpy is measured for 1 mole of the substance under standard conditions (1 atm and 25°C) we say that we have a standard enthalpy, symbolized by HO. So we have:

Standard enthalpy of combustion concept

Combustion reactions always involve a fuel, which are flammable substances that can be in any of the three physical states, and also involve a oxidizer, which is the oxygen gas.

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Combustion can occur completely or incompletely.

The standard enthalpy range is determined only for complete combustions. When the fuel is a substance made up only of carbon, hydrogen and/or oxygen, complete combustion will only produce carbon dioxide and water, as shown in the examples below:

  • Complete combustion of methane, CH4(g)

CH4(g) + ½ the2 → 1 CO2(g) + 2 H2O(ℓ) ∆H0combustion = -890.4 kJ/mol

  • Complete combustion of ethanol, C2H6O(1)

1C2H6O(ℓ)+ 3O2 → 2 CO2(g) + 3 H2O(ℓ) ∆H0combustion = -1366.1 kJ/mol

  • Complete combustion of butane, C4H10(g)

2C4H10(g)+13O2 → 8 CO2(g) + 10 H2O(ℓ) ∆H0combustion = -2878.6 kJ/mol

Butane combustion

Note that in all cases the values ​​of∆H0combustion are negative (∆H0combustion = < 0), this is because combustion reactions release energy (they are exothermic), so the final enthalpy value will be lower than the initial one.


By Jennifer Fogaça
Graduated in Chemistry

Would you like to reference this text in a school or academic work? Look:

FOGAÇA, Jennifer Rocha Vargas. "Enthalpy of Combustion"; Brazil School. Available in: https://brasilescola.uol.com.br/quimica/entalpia-combustao.htm. Accessed on June 27, 2021.

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