Combustion of alkanes

Alkanes are the saturated hydrocarbons used as fuels, natural gas, petrol, diesel and liquefied petroleum gas are all alkane mixtures, so their combustion is central to the Form 5 Carbon Compounds chapter. The exam tests balanced equations, the difference between complete and incomplete combustion, and the observations at the flame.

Complete combustion

When an alkane burns in plenty of oxygen, every carbon atom ends up as carbon dioxide and every hydrogen atom as water. The reaction is strongly exothermic, which is why alkanes are fuels.

• Methane: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)
• Ethane: 2C₂H₆(g) + 7O₂(g) → 4CO₂(g) + 6H₂O(l)
• Propane: C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l)
• Butane: 2C₄H₁₀(g) + 13O₂(g) → 8CO₂(g) + 10H₂O(l)

The general equation for an alkane CnH2n+2 is:

CnH2n+2 + ((3n+1)/2) O₂ → n CO₂ + (n+1) H₂O

Balancing tip: fix the carbons and hydrogens first, then count the oxygen atoms needed and adjust the O₂ coefficient last. If the oxygen count is a fraction, double the whole equation, which is why the ethane and butane equations begin with a 2.

Incomplete combustion

When the oxygen supply is limited, the carbon is not fully oxidised, so poisonous carbon monoxide gas, or soot (carbon), forms instead of carbon dioxide, together with water. For example:

2CH₄(g) + 3O₂(g) → 2CO(g) + 4H₂O(l)

CH₄(g) + O₂(g) → C(s) + 2H₂O(l)

Incomplete combustion releases less heat than complete combustion and produces the carbon monoxide responsible for poisoning from faulty heaters, so it is an important safety and environmental point.

Conditions required

Combustion needs a supply of oxygen (from air) and a source of ignition, a spark or a flame to reach the ignition temperature. Whether combustion is complete or incomplete depends on how much oxygen is available.

Observations

• Complete combustion: a clean, non-luminous blue flame, a lot of heat, and no soot. The gas produced turns limewater milky (carbon dioxide) and water droplets condense on a cool surface.
• Incomplete combustion: a yellow, luminous, sooty flame, less heat, and a black deposit of soot on a cold surface such as a boiling tube.

The sootiness increases as the proportion of carbon in the molecule rises, so heavier alkanes and, even more, the unsaturated alkenes burn with a smokier flame than methane.

Why the products matter

Both products of complete combustion raise real issues that the syllabus links to combustion. Carbon dioxide is a greenhouse gas, and burning large quantities of alkane fuels adds to the carbon dioxide in the atmosphere. Carbon monoxide from incomplete combustion is a colourless, odourless and toxic gas that binds to haemoglobin and prevents it carrying oxygen, which is why fuel-burning appliances must have a good air supply and ventilation. Soot dirties surfaces and contributes to air pollution. Being able to name these products and explain their effects is often the final part of a combustion question, so link each product to its consequence when you revise.

Common mistakes to avoid

Do not forget to balance the oxygen last and to double the equation when the oxygen coefficient is a fraction. Do not write carbon dioxide as the product of incomplete combustion, that is carbon monoxide or carbon. Include state symbols; water is usually written (l) at room conditions.

How it appears in the SPM exam

In Paper 2 (4541/2) you may be asked to write and balance a combustion equation, to explain why a flame is sooty, or to compare the combustion of an alkane with that of an alkene of the same carbon number. In Paper 1 (4541/1), objective items test the products and the flame colour. Always relate the observation back to how much oxygen was available, and your combustion answers stay consistent and correct.