Figure 1. Space filling model of octane, the grey spheres represent hydrogen atoms and the black spheres represents carbon atoms.[1]

Octane is an alkane with the chemical formula C8H18. As a hydrocarbon, it can be burned as a fuel. This organic molecule is a colourless liquid at room temperature with a characteristic "gasoline" odour.[2] Its principle use is as a component of gasoline, and the performance of this gasoline depends on its octane rating which gets its name from this molecule. Note however, that the concept of octane rating is a little more complicated than just 'how much octane there is in the fuel'. The balanced chemical equation for the complete combustion of octane is:

[math]\ce{2C_8H_{18} + 25(O_2)\leftrightarrow 16(CO_2) + 18(H_2O)}[/math]


Below is a table of some of the basic properties of octane.

Formula C8H18
Molar mass 114.23 grams/mole
Energy density 47.9 MJ/kg [3]
Melting Point -57oC[4]
Boiling Point 125oC[4]

Combustion Reaction

As is the case with other hydrocarbons, octane undergoes hydrocarbon combustion, combining with oxygen to form carbon dioxide. The balanced chemical equation for the complete combustion of octane is:

2C8H18 + 25O2 → 16CO2 + 18H2O + Heat Energy (Enthalpy)

The hydrocarbon combustion reaction releases heat energy and is an example of an exothermic reaction. The reaction also has a negative enthalpy change (ΔH) value.

For Further Reading


  1. "N-octane-spaceFilling" Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:N-octane-spaceFilling.png#mediaviewer/File:N-octane-spaceFilling.png
  2. National Center for Biotechnology Information. (2015). Properties of Octane [Online]. Available: http://pubchem.ncbi.nlm.nih.gov/compound/octane#section=Top [February 16, 2015].
  3. Glenn Elert. (2015). The Physics Hypertextbook - Chemical Potential Energy [Online]. Available: http://physics.info/energy-chemical/ [February 16, 2015].
  4. 4.0 4.1 Charles E. Ophardt. (2003). Virtual Chembook - Hydrocarbon Boiling Points [Online]. Available: http://www.elmhurst.edu/~chm/vchembook/501hcboilingpts.html [February 16,2015].