Alkanes, also known as saturated hydrocarbons (this experiment deals with aliphatic hydrocarbons with general formula-CnH2n+2), generally burn in air-oxygen well and when there is enough oxygen, the reaction products will be carbon dioxide and water (as vapor):
CnH2n+2 + (3n+1)/2O2(g) → nCO2(g) + (n+1)H2O(g)
In everyday life you can see burning alkanes using gas stove or a lighter. Propane (C3H8) is used as a bottled gas (so called liquefied gas), main component of natural gas (gas network) is generally methane (CH4). In gasoline engine gasoline is burned, containing a mixture of different hydrocarbons. In the following experiments combustion reactions of several well known and widely used hydrocarbons are shown.
1) In laboratoryies in the gas burners, natural gas is generally burned. Methane and also other gaseous saturated hydrocarbons are burning in the air with an almost colorless flame. The following equation describes the combustion of methane:
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
2) In the refillable gas burners, used in several experiments, the burning substance is butane (C4H10) (or a mixture of butane and propane-C3H8):
2C4H10(g) + 13O2(g) → 8CO2(g) + 10H2O(g)
C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g)
3) Common candle wax (paraffin) is also a mixture of hydrocarbons, with the following approximate formula C23-34H48-70. Combustion of paraffin emits carbon dioxide and water vapor.
4) Main components of motor fuel-petrol (gasoline) are C5-C12 branched and straight chain alkanes as well as aromatic hydrocarbons. Combustion of petrol gasoline was demonstrated in an open vessel. The same combustion reaction also powers gasoline engines. In gasoline car’s engine cylinders, the mixture of petrol gasoline vapor and air (air oxygen) is ignited with a spark.
5) Compared to petrol (gasoline), diesel fuel contains higher- molecular-weight hydrocarbons. The averaged chemical formula for diesel fuel is about C12H23.
6) To demonstrate the combustion of unsaturated hydrocarbons, benzene (C6H6) is ignited in an open vessel.
It was seen that benzene, which has higher percentage of carbon, burned with a sooty flame. Besides carbon dioxide and water, also some carbon particles emerged during the combustion.
Combustion of benzene can be described with the following simplified equation:
2C6H6(l) + 13O2(g) → 10CO2(g) + 6H2O(g) + 2C(s)