Core practical: combustion of alcohols Revision Notes | GCSE Edexcel Higher Chemistry

1. What Makes Alcohols Special

Picture a hydrocarbon chain with one crucial difference - somewhere along that chain, there's an -OH group attached. That single addition transforms the entire molecule's behaviour. This is what makes alcohols so useful in everything from hand sanitiser to car fuel.

The -OH group is called the functional group because it controls how the molecule behaves. Think of it like the steering wheel of a car - the rest of the molecule is the engine, but the -OH group decides where it goes chemically.

2. The Four Key Alcohol Reactions

Alcohols are reactive molecules. The -OH group makes them behave predictably in four main ways. Each reaction tells us something different about alcohol chemistry and has real-world applications.

Reaction 1: Alcohols + SodiumWhen you drop sodium metal into an alcohol, it fizzes and produces hydrogen gas. The alcohol becomes a sodium alkoxide. This reaction is slower than sodium with water, but follows the same pattern.

Reaction 2: Combustion in AirAlcohols burn completely in oxygen to produce carbon dioxide and water. This releases lots of energy, which is why ethanol is used as a biofuel. The general pattern is: alcohol + oxygen → carbon dioxide + water.

Worked example: Balancing Alcohol Combustion

Write the balanced equation for ethanol burning in oxygen.

1. ethanol + oxygen → carbon dioxide + water — Start with what you know happens
2. $C_2H_5OH + O_2 \rightarrow CO_2 + H_2O$ — Ethanol is $C_2H_5OH$
3. $C_2H_5OH + O_2 \rightarrow 2CO_2 + H_2O$ — 2 carbons on left, need 2 on right
4. $C_2H_5OH + O_2 \rightarrow 2CO_2 + 3H_2O$ — 6 hydrogens on left (5+1), need 6 on right
5. $C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O$ — 7 oxygens on right, need 3 $O_2$ molecules

Answer: $C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O$

Reaction 3: Dissolving in WaterThe first three alcohols (methanol, ethanol, propanol) dissolve completely in water. This happens because the -OH group can form hydrogen bonds with water molecules. Butanol is less soluble because its longer carbon chain gets in the way.

Reaction 4: OxidationWhen alcohols react with oxidising agents like potassium dichromate, they lose hydrogen atoms. Primary alcohols form aldehydes then carboxylic acids. Secondary alcohols form ketones. This reaction is used to make vinegar from wine.

3. Alcohol Properties and Solubility Patterns

As the carbon chain gets longer, alcohol properties change in predictable ways. This pattern explains why different alcohols have different uses in industry and everyday life.

Methanol, ethanol and propanol all dissolve completely in water. The -OH group acts like a 'water-loving' anchor, pulling the molecule into solution through hydrogen bonding. But butanol is different - its four-carbon chain is too long and 'water-hating' for complete mixing.

This solubility pattern matters for real applications. Ethanol dissolves in water to make alcoholic drinks, but longer alcohols like octanol are used as solvents for oil-based substances because they don't mix with water.

4. Real-World Uses of the First Four Alcohols

Each alcohol has specific industrial and domestic uses based on its chemical properties. Understanding these uses helps explain why we study alcohol reactions in the first place.

Methanol is used as a solvent in industry and as a fuel additive. It's highly toxic - drinking even small amounts causes blindness or death. Ethanol is the alcohol in drinks, but it's also used as a biofuel and antiseptic. Propanol appears in cosmetics and cleaning products. Butanol is used as a solvent for paints and plastics.

The choice of alcohol depends on the job. Need something that mixes with water? Use ethanol or propanol. Need something that dissolves oils and doesn't mix with water? Use butanol. Need a fuel that burns cleanly? Methanol or ethanol work well.

5. Combustion Reactions in Detail

When alcohols burn, they follow the same pattern as hydrocarbons but with one key difference - they already contain oxygen in the -OH group. This affects the balancing of combustion equations.

Complete combustion always produces carbon dioxide and water. The energy released makes alcohols useful as fuels. Ethanol is added to petrol in many countries, and methanol powers some racing cars.

6. Exam Technique and Common Mistakes

Alcohol questions appear regularly in GCSE chemistry papers. The examiners have favourite tricks, but once you know them, the marks are straightforward to collect.

The biggest trap is mixing up the different types of alcohol reactions. Combustion always gives carbon dioxide and water. Oxidation gives different products depending on the alcohol type. Reaction with sodium gives hydrogen gas and a sodium alkoxide.