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 = H_FINAL - H_INITIAL or ∆H = H_PRODUCTS - H_REAGENTS

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

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

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.

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, CH_4(g)

CH_4(g) + ½ the₂ → 1 CO_2(g) + 2 H₂O_(ℓ) ∆H⁰_combustion = -890.4 kJ/mol

• Complete combustion of ethanol, C₂H₆O₍₁₎

1C₂H₆O_(ℓ)+30₂ → 2 CO_2(g) + 3 H₂O_(ℓ) ∆H⁰_combustion = -1366.1 kJ/mol

• Complete combustion of butane, C₄H_10(g)

2C₄H_10(g)+13O₂ → 8 CO_2(g) + 10 H₂O_(ℓ) ∆H⁰_combustion = -2878.6 kJ/mol

Note that in all cases the values ​​of∆H⁰_combustion are negative (∆H⁰_combustion = < 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

Source: Brazil School - https://brasilescola.uol.com.br/quimica/entalpia-combustao.htm