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Revision notes for AQA GCSE Chemistry Alcohols. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.

Alcohols

What you'll learn:

  • How to recognise and name the first four alcohols.
  • How alcohols react with oxygen, sodium, water, and oxidising agents.
  • The everyday uses of alcohols.
  • How ethanol is produced biologically through fermentation.

What are Alcohols?

You have probably heard of ethanol—it is the alcohol found in alcoholic drinks. But in chemistry, "alcohol" isn't just one substance; it is an entire family of organic compounds.

Alcohols form a homologous series. This means they are a family of molecules that all share the same general structure and similar chemical properties.

Definition

Functional group

A functional group is a specific atom or group of atoms in a molecule that determines how that molecule reacts. The functional group for all alcohols is the –OH group (a hydroxyl group).

The first four members of the alcohol family are:

  • Methanol
  • Ethanol
  • Propanol
  • Butanol

You can recognise an alcohol from its name because it always ends in -ol. They share the same prefix as the alkanes: meth- means one carbon, eth- means two, prop- means three, and but- means four.

Drawing Alcohols

You can represent alcohols using a structural formula (which lists the atoms as they appear in the chain) or a displayed formula (which draws out every single bond).

For example, the structural formula for ethanol is CH3CH2OH\text{CH}_3\text{CH}_2\text{OH}CH3​CH2​OH.

Below is the displayed formula for ethanol. Notice how the –OH functional group is attached to the end of the carbon chain.

Displayed formula of ethanol

Tip

Writing the formula

Always write the OH at the end of the structural formula rather than mixing the hydrogen in with the rest. Writing C2H5OH\text{C}_2\text{H}_5\text{OH}C2​H5​OH or CH3CH2OH\text{CH}_3\text{CH}_2\text{OH}CH3​CH2​OH clearly shows it is an alcohol. If you wrote C2H6O\text{C}_2\text{H}_6\text{O}C2​H6​O, the examiner wouldn't know if you meant an alcohol or a different type of molecule altogether!

Reactions of the Alcohols

The spec expects you to know how any of the first four alcohols behave in four specific situations: when they burn, when they are added to water, when they react with sodium, and when they react with an oxidising agent.

1. Combustion (Burning in air)

Alcohols are very flammable. They undergo complete combustion in a good supply of oxygen to produce carbon dioxide and water. They burn with a very clean blue flame.

The combustion of ethanol looks like this:

C2H5OH(l)+3O2(g)→2CO2(g)+3H2O(l) \text{C}_2\text{H}_5\text{OH(l)} + 3\text{O}_2\text{(g)} \to 2\text{CO}_2\text{(g)} + 3\text{H}_2\text{O(l)} C2​H5​OH(l)+3O2​(g)→2CO2​(g)+3H2​O(l)
Example

Balancing an alcohol combustion equation

Write the balanced symbol equation for the complete combustion of methanol (CH3OH\text{CH}_3\text{OH}CH3​OH).

  1. Write the unbalanced equation with the correct reactants and products:
CH3OH+O2→CO2+H2O \text{CH}_3\text{OH} + \text{O}_2 \to \text{CO}_2 + \text{H}_2\text{O} CH3​OH+O2​→CO2​+H2​O
  1. Count the atoms on both sides. Reactants: 1 Carbon, 4 Hydrogens (3+1), 3 Oxygens (1+2). Products: 1 Carbon, 2 Hydrogens, 3 Oxygens (2+1).
  2. Balance the Hydrogens first. We have 4 on the left and 2 on the right. Put a 2 in front of the water:
CH3OH+O2→CO2+2H2O \text{CH}_3\text{OH} + \text{O}_2 \to \text{CO}_2 + 2\text{H}_2\text{O} CH3​OH+O2​→CO2​+2H2​O
  1. Re-count the Oxygens. On the right: 2 (from CO2\text{CO}_2CO2​) + 2 (from 2H2O2\text{H}_2\text{O}2H2​O) = 4 Oxygens. On the left: we have 1 Oxygen in the methanol, leaving us needing 3 more from the O2\text{O}_2O2​. This means we need 1.5 molecules of O2\text{O}_2O2​.
CH3OH+1.5O2→CO2+2H2O \text{CH}_3\text{OH} + 1.5\text{O}_2 \to \text{CO}_2 + 2\text{H}_2\text{O} CH3​OH+1.5O2​→CO2​+2H2​O
  1. Multiply the whole equation by 2 to remove the decimal:
2CH3OH(l)+3O2(g)→2CO2(g)+4H2O(l) 2\text{CH}_3\text{OH(l)} + 3\text{O}_2\text{(g)} \to 2\text{CO}_2\text{(g)} + 4\text{H}_2\text{O(l)} 2CH3​OH(l)+3O2​(g)→2CO2​(g)+4H2​O(l)

2. Solubility in water

When you add the first four alcohols to water, they mix easily (they are soluble) and dissolve to form neutral solutions with a pH of 7.

Common Mistake

Confusing –OH with alkalis

Because alcohols have an –OH group, students often guess they are alkaline. They are not! Alkalis contain hydroxide ions (OH−\text{OH}^-OH−) that are free to move. In alcohols, the –OH group is covalently bonded to the carbon chain and does not split off into an ion. The solution stays perfectly neutral.

3. Reaction with sodium

If you drop a small piece of sodium metal into an alcohol, it reacts gently. You will see steady effervescence (fizzing) as hydrogen gas is produced. The other product is a salt called an alkoxide (for example, reacting sodium with ethanol produces sodium ethoxide and hydrogen). This reaction is similar to how sodium reacts with water, but much less vigorous.

4. Reaction with an oxidising agent

If an alcohol is gently heated with a chemical oxidising agent, the alcohol is naturally oxidised to form a completely different homologous series: the carboxylic acids. For example, oxidising ethanol produces ethanoic acid (the main acid in vinegar).

Uses of Alcohols

Because of their unique properties, alcohols are extremely useful in everyday life and industry. You need to recall their main uses:

  • As solvents: Alcohols can dissolve many things that water cannot, such as oils, fats, and certain plastics. This makes them excellent solvents for perfumes, mouthwashes, and marker pen inks.
  • As fuels: They burn very cleanly and release a lot of energy. Ethanol is often mixed with petrol to create a biofuel for cars. Methanol is used as a fuel in high-performance racing cars.
  • In alcoholic drinks: Ethanol is the specific alcohol found in beer, wine, and spirits.

Making Ethanol: Fermentation

While ethanol can be made from crude oil (by reacting ethene with steam), it is also widely produced biologically using plant sugars. This process is called fermentation.

Fermentation relies on single-celled fungi called yeast. The yeast contains enzymes that break down sugar solutions to produce ethanol and carbon dioxide gas.

Key Idea

The conditions for fermentation

For yeast to successfully convert sugar into ethanol, you must provide very specific conditions:

  • Sugars dissolved in water (an aqueous solution).
  • Yeast added to the mixture.
  • Warm temperatures of around 30°C. If it's too cold, the reaction is too slow; if it's too hot, the enzymes in the yeast denature and die.
  • Anaerobic conditions (no oxygen). If oxygen is present, the yeast will just respire aerobically and produce carbon dioxide and water, rather than ethanol.

In the lab, a fermentation setup uses a bung to keep oxygen out, whilst allowing the carbon dioxide gas to escape through a delivery tube into some limewater.

Fermentation setup

Exam technique

In the exam

  1. If asked to draw an alcohol, double-check that every carbon atom has exactly four bonds, every oxygen has two, and every hydrogen has one.
  2. Remember that you are only expected to write out balanced symbol equations for the combustion of alcohols. For reactions with sodium or oxidising agents, word descriptions (e.g. "hydrogen gas is produced") are enough.
  3. Be ready to list the conditions for fermentation. "Warm" isn't quite enough for full marks; state "around 30°C" and always mention "anaerobic / no oxygen".
Self review

Check yourself

  • What is the functional group present in all alcohols?
  • What gas is produced when sodium reacts with ethanol?
  • Why does a fermentation mixture need to be kept away from oxygen?

Reactions of alkenes and alcohols (chemistry only)

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