Reactions with oxygen (combustion reaction)

بِسْــــــــــــــــمِ اﷲِالرَّحْمَنِ اارَّحِيم

All alkanes react with oxygen in a combustion reaction, although they become increasingly difficult to ignite as the number of carbon atoms increases. The general equation for complete combustion is:

C_nH_2n+2 + (1.5n+0.5)O₂ → (n+1)H₂O + nCO₂

In the absence of sufficient oxygen, carbon monoxide or even soot can be formed, as shown below:

C_nH_(2n+2) + (n+0.5)O₂ → (n+1)H₂O + nCO

C_nH_(2n+2) + (0.5n+0.5)O₂ → (n+1)H₂O + nC

For example methane:

2CH₄ + 3O₂ → 2CO + 4H₂O

CH₄ + 1.5O₂ → CO + 2H₂O

See the alkane heat of formation table for detailed data. The standard enthalpy change of combustion, Δ_cH^o, for alkanes increases by about 650 kJ/mol per CH₂ group. Branched-chain alkanes have lower values of Δ_cH^o than straight-chain alkanes of the same number of carbon atoms, and so can be seen to be somewhat more stable.