Combustion of alcohols

Alcohols are a homologous series with the -OH functional group, and they are important fuels, ethanol is blended into petrol and burned in spirit lamps. Because an alcohol molecule already contains one oxygen atom, it burns readily and cleanly, which is why the combustion of alcohols is a standard part of the Form 5 Carbon Compounds chapter. The exam tests the balanced equation, the conditions, the flame observations and the trend in heat released down the series.

Complete combustion

When an alcohol burns in plenty of oxygen, every carbon atom becomes carbon dioxide and every hydrogen atom becomes water. The reaction is strongly exothermic. With correct formulae and state symbols the common equations are:

• Methanol: 2CH₃OH(l) + 3O₂(g) → 2CO₂(g) + 4H₂O(l)
• Ethanol: C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l)
• Propan-1-ol: 2C₃H₇OH(l) + 9O₂(g) → 6CO₂(g) + 8H₂O(l)

The general equation for an alcohol CnH2n+1OH is:

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

Balancing tip: fix the carbon atoms (n CO₂) and the hydrogen atoms ((n+1) H₂O) first, then count the oxygen atoms on the right, subtract the one oxygen the alcohol already provides, and adjust the O₂ coefficient last. If that coefficient comes out as a fraction, double the whole equation, this is why the methanol and propan-1-ol 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:

2C₂H₅OH(l) + 5O₂(g) → 4CO(g) + 6H₂O(l)

Incomplete combustion releases less heat than complete combustion, so a fuel that burns incompletely wastes energy and produces toxic carbon monoxide.

Conditions required

Combustion needs a supply of oxygen from the air and a source of ignition, a spark or flame that raises the alcohol vapour to its ignition temperature. Whether combustion is complete or incomplete depends only on how much oxygen is available. No catalyst is used.

Observations

• Complete combustion: a clean, non-luminous blue flame with very little soot, and a large amount of heat is given out. The gas produced turns limewater milky (carbon dioxide) and colourless liquid droplets condense on a cool surface (water).
• Incomplete combustion: a yellow, luminous, slightly sooty flame, less heat, and a black deposit of soot may form on a cold surface held in the flame.

Because an alcohol carries its own oxygen atom, it burns more cleanly than an alkene of the same carbon number, which has a higher proportion of carbon and burns with a smokier, more luminous flame.

The trend in heat of combustion

Going down the homologous series, each successive alcohol has one more -CH₂⁻ unit, so more carbon and hydrogen atoms are oxidised and more bonds form in the products. The heat given out per mole therefore increases from methanol to ethanol to propanol. This trend is a favourite comparison question and links straight to the heat-of-combustion calculation.

Common mistakes to avoid

Remember that the alcohol already supplies one oxygen atom, so do not treat it like an alkane when you count oxygen. Balance the O₂ last and double the equation when the coefficient is a fraction. Do not write carbon dioxide as the product of incomplete combustion, that is carbon monoxide or carbon. Always include state symbols, writing water as (l) at room conditions.

How it appears in the SPM exam

In Paper 2 (4541/2) you may be asked to write and balance the combustion equation of a named alcohol, to explain why the flame is clean, or to compare the heat released by successive members of the series. In Paper 1 (4541/1), objective items test the products and the flame colour. Relate every observation back to how much oxygen was available, and your combustion answers stay accurate and consistent.