Combustion of organic compounds

Complete combustion

Like any organic compound made of carbon and hydrogen (and sometimes oxygen), ethanol burns in an excess of dioxygen to give carbon dioxide and water: this is complete combustion.

C2H6O + 3 O2 -> 2 CO2 + 3 H2O

Check: carbon 2=2; hydrogen 6=3×2=6; oxygen, on the left 1 (in C2H6O) + 3×2 (in 3 O2) = 7, on the right 2×2 (in 2 CO2) + 3×1 (in 3 H2O) = 4+3=7. The equation is indeed balanced.

Incomplete combustion

When dioxygen is lacking, combustion is incomplete: it then produces carbon monoxide CO (a toxic, odorless gas) or even soot (solid carbon), in addition to or instead of carbon dioxide. This is why a poorly adjusted or poorly ventilated gas appliance is dangerous.

Why these reactions are exothermic

Like all combustion, that of organic compounds is strongly exothermic: it releases the chemical energy stored in carbon-hydrogen and carbon-carbon bonds, which explains the use of hydrocarbons and alcohols as fuels.

Generalizing to another alcohol

The same principle applies to propan-1-ol, C3H8O:

C3H8O + 4.5 O2 -> 3 CO2 + 4 H2O

Check: carbon 3=3; hydrogen 8=4×2=8; oxygen, on the left 1+4.5×2=1+9=10, on the right 3×2+4×1=6+4=10. Balanced (the non-integer coefficient in front of O2 is common; it can be doubled to obtain whole numbers if needed: 2 C3H8O + 9 O2 -> 6 CO2 + 8 H2O).

Common mistake

Do not forget to check oxygen last: it is the element that is easiest to miscount, since it comes both from dioxygen and sometimes from the organic compound itself (if it already contains an oxygen atom, like an alcohol).