Alcohol combustion is a key reaction in organic chemistry, demonstrating how molecular structure influences energy release and the identity of reaction products when burned in oxygen-containing environments.

Combustion of Alcohols

Alcohols undergo combustion when heated in the presence of sufficient oxygen, forming carbon dioxide and water as the primary products.

Ethanol burning on a shallow dish, producing a clean blue flame under conditions of complete combustion. The flame represents alcohol molecules reacting with oxygen to form carbon dioxide and water, releasing heat energy. Although this photograph does not show products directly, it visually reinforces the appearance of alcohol combustion discussed in the notes. Source

Complete Combustion

In complete combustion, an alcohol burns in an excess supply of oxygen.

Molecular-level diagram of complete combustion, where each carbon atom combines with sufficient oxygen to form carbon dioxide. The left side shows separate oxygen and carbon species, while the right side shows only CO₂ molecules after combustion. Although drawn for hydrocarbons in general, the same idea of full oxidation to CO₂ applies to alcohol fuels such as ethanol. Source

Products are predictable because the functional group and hydrocarbon chain oxidise fully to the highest possible oxidation states of carbon and hydrogen.

• Alcohol + excess O₂ → CO₂ + H₂O

• Produces a clean, blue flame (typical for small alcohols such as methanol and ethanol)

• Releases significant heat energy due to extensive bond formation

Complete combustion depends on adequate oxygen supply. Larger alcohols still combust cleanly when oxygen is abundant, though their flames may appear more orange due to incomplete vapour mixing.

Incomplete Combustion

Incomplete combustion occurs when oxygen availability is limited, preventing full oxidation.

Diagram showing how low oxygen levels lead to incomplete combustion, with carbon monoxide molecules forming instead of full conversion to carbon dioxide. The left side depicts separate oxygen and carbon species, while the right side shows CO molecules as the main carbon-containing product. Although drawn in a general way for hydrocarbon fuels, the same principle applies to alcohols when they burn in an oxygen-poor environment. Source

• Produces carbon monoxide (CO), carbon (soot), and water

• Produces a yellow, smoky flame

• Releases less energy than complete combustion

Incomplete combustion is common for longer-chain alcohols or when air supply is restricted (e.g., poorly ventilated burners). This reduced efficiency is important in evaluating alcohols as fuels.

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Comparing Alcohols with Alkanes

The OCR specification requires students to compare energy release and combustion products of alcohols to those of analogous alkanes. Both homologous series combust exothermically, but their molecular structures affect the overall energy profile.

Key comparison points:

• Alcohols contain an O–H group, which introduces polarity not present in alkanes.

• Alcohols typically exhibit lower volatility than alkanes of similar size because hydrogen bonding reduces evaporation rate.

• Despite this, both alcohols and alkanes produce CO₂ and H₂O under complete combustion.

• Alcohols generally release slightly less energy per mole than corresponding alkanes because the alcohol molecule is already partially oxidised due to its oxygen content.

Trends in Combustion Energy

The amount of heat released in combustion increases with carbon chain length. This applies to both alcohols and alkanes, but alcohol combustion must account for the presence of oxygen within the molecular structure.

Bullet-point trends:

• Heat released increases with molecular size (more C–H and C–C bonds available for oxidation).

• Alcohols produce less heat than alkanes with the same number of carbon atoms.

• Primary, secondary, and tertiary alcohols all undergo combustion; classification does not significantly change combustion pathways, as all are fully oxidised to CO₂ and H₂O under complete combustion conditions.

Physical Properties Relevant to Combustion

The OCR specification highlights comparisons to alkanes. Alcohols differ due to polarity and hydrogen bonding, both of which influence combustion behaviour.

Hydrogen bonding increases boiling points, making alcohols less volatile than alkanes, which can influence their burning characteristics—particularly ignition temperature and flame stability.

Practical Observations in Alcohol Combustion

Students should be familiar with typical laboratory observations when burning alcohols:

• Smaller alcohols (methanol, ethanol, propanol) ignite easily with a clear flame.

• Larger alcohols may burn with less consistent flames due to lower volatility.

• Alcohol burners rely on the wick drawing liquid fuel upwards, meaning vapour production affects combustion efficiency.

Combustion Products and Environmental Considerations

Although the OCR specification for this subsubtopic does not emphasise environmental chemistry, it remains essential to understand the implications of combustion products when comparing alcohols and alkanes.

Key points:

• Complete combustion → CO₂ + H₂O; contributes to greenhouse gas output.

• Incomplete combustion → CO + C (soot); CO is highly toxic and soot contributes to particulate pollution.

• Alcohols can be considered cleaner-burning than alkanes owing to a tendency to produce fewer particulates when fully combusted.

Overall, understanding alcohol combustion requires recalling core ideas from redox chemistry, energy changes, and intermolecular forces while focusing on the OCR specification requirement to compare combustion behaviour and energy release with analogous alkanes.