Skip to content
qorganica.es qorganica.comOrganic Chemistry made transparent Course map Español

Combustion

The combustion of alkanes is a very important reaction that directly affects our everyday life. It is one of the most important sources of energy in our contemporary society.

Blue flame of a burning gas
General equation for the combustion of an alkane: CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → n CO₂ + (n+1) H₂O

Burning an alkane releases on average about 165 kcal/mol per carbon or, in general, an organic compound.

Table of heats of combustion (ΔH°comb, kcal/mol) of methane, ethane, propane, butane, 2-methylpropane, hexane, cyclohexane, ethanol and sucrose
Comparison of the heats of combustion of butane (−687.4 kcal/mol) and 2-methylpropane (−685.4 kcal/mol)

Heats of combustion are experimental numbers that allow us to estimate the relative stability of alkanes bearing the same number of carbons.

Measured heats of combustion allow us to obtain heats of formation, using the heats of combustion of the two main elements (C and H2) and taking into account the heat of vaporization of water.

Heats of combustion of graphite (−94.1 kcal/mol) and hydrogen (−57.8 kcal/mol, to gaseous water) and heat of condensation of water (−10.52 kcal/mol)

The difference between the 'real' heat of combustion and the 'theoretical' one calculated from the elements is the heat of formation. If the latter ends up as a negative value, it means that the compound is more stable than its isolated elements. This is the usual case, but it does not always happen.

Calculation of the heat of formation of butane: −718.0 − (−687.4) = −30.6 kcal/mol