10.3 Reactions and Uses of Alcohols

A free GCSE Chemistry lesson for AQA (8462) from OBA School: how alcohols react with sodium, oxygen and oxidising agents, how ethanol is made by fermentation, and the uses of alcohols. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe what happens when alcohols are added to water and react with sodium
• Write balanced equations for the complete combustion of alcohols
• Describe the oxidation of alcohols to carboxylic acids
• Describe the conditions for making ethanol by fermentation and calculate masses of product
• Compare fermentation with the hydration of ethene, give uses of alcohols, and compare alcohols as fuels

Key words

• Fermentation: Making ethanol (and carbon dioxide) from sugar solution using enzymes in yeast, without oxygen.
• Anaerobic: Without oxygen (air).
• Oxidising agent: A substance that oxidises another, for example by giving it oxygen. Acidified potassium dichromate is one.
• Oxidation: Gain of oxygen (or loss of electrons). Alcohols are oxidised to carboxylic acids.
• Solvent: A liquid that dissolves other substances.
• Biofuel: A fuel made from plant or animal material, e.g. ethanol from fermented sugar cane.
• Atom economy: The percentage of the mass of reactants that ends up in the useful product.

Alcohols in water and with sodium

Methanol, ethanol and propanol mix completely with water. The solution is neutral (pH 7), because the –OH group in an alcohol is covalently bonded and does not release hydroxide ions. Butanol and longer alcohols are less soluble.

A small piece of sodium reacts with an alcohol. It fizzes as hydrogen gas is given off and slowly disappears. The reaction is much gentler than sodium with water.

ethanol + sodium → sodium ethoxide + hydrogen

2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂

Because they share the –OH functional group, the first four alcohols all react in the same ways. Only the number of carbon atoms changes in the products:

• methanol; With sodium sodium methoxide + hydrogen; Oxidised to methanoic acid; Complete combustion 2CH₃OH + 3O₂ → 2CO₂ + 4H₂O
• ethanol; With sodium sodium ethoxide + hydrogen; Oxidised to ethanoic acid; Complete combustion C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
• propanol; With sodium sodium propoxide + hydrogen; Oxidised to propanoic acid; Complete combustion 2C₃H₇OH + 9O₂ → 6CO₂ + 8H₂O
• butanol; With sodium sodium butoxide + hydrogen; Oxidised to butanoic acid; Complete combustion C₄H₉OH + 6O₂ → 4CO₂ + 5H₂O

Combustion and oxidation

Alcohols burn in air with a clean, almost invisible blue flame, which is why they are good fuels. Complete combustion makes carbon dioxide and water.

Worked example: balancing the combustion of ethanol

C₂H₅OH contains 2 C, 6 H and 1 O. C: 2CO₂. H: 3H₂O.
O on the right = 4 + 3 = 7. Ethanol already supplies 1, so 6 O atoms must come from O₂ → 3O₂.

Answer: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

Alcohols can be oxidised by an oxidising agent to form carboxylic acids. In the lab, ethanol is heated with acidified potassium dichromate(VI) solution, which turns from orange to green. Ethanol is oxidised to ethanoic acid.

Key fact: Microbes in the air can also oxidise ethanol. This is why an opened bottle of wine slowly turns sour: ethanol becomes ethanoic acid (vinegar).

Making ethanol by fermentation

Ethanol for drinks, and some ethanol for fuel, is made by fermentation of sugar solution using yeast. Enzymes in the yeast catalyse the reaction.

glucose → ethanol + carbon dioxide

C₆H₁₂O₆(aq) → 2C₂H₅OH(aq) + 2CO₂(g)

• Yeast added; Reason yeast contains the enzymes that catalyse fermentation
• Warm, about 30 °C (25–35 °C); Reason fast enough; above about 40 °C the enzymes are denatured, and below it the reaction is slow
• No air (anaerobic); Reason with oxygen, yeast respires aerobically and microbes oxidise the ethanol to ethanoic acid

Worked example: mass of ethanol from glucose (C = 12, H = 1, O = 16)

What mass of ethanol can be made from 90 g of glucose?
Mr(C₆H₁₂O₆) = 180, so moles = 90 ÷ 180 = 0.5 mol.
Ratio glucose : ethanol = 1 : 2, so 1.0 mol ethanol. Mr(C₂H₅OH) = 46, so mass = 46 g.

Answer: 46 g of ethanol

When the ethanol reaches about 15%, the yeast dies and fermentation stops. A more concentrated solution can be made by fractional distillation.

Fermentation or hydration of ethene?

• Raw material; Fermentation sugar from crops (sugar cane, sugar beet): renewable; Hydration of ethene ethene from crude oil: finite
• Conditions; Fermentation about 30 °C, normal pressure: little energy needed; Hydration of ethene 300 °C, 60–70 atm: lots of energy needed
• Process; Fermentation batch: slow; Hydration of ethene continuous: fast
• Product; Fermentation dilute ethanol; needs fractional distillation; Hydration of ethene pure ethanol
• Atom economy; Fermentation 2 × 46 ÷ 180 × 100 = 51%; Hydration of ethene 100% (only one product)

Exam tip: In “evaluate” questions, give points for both methods, then a conclusion. For example, a country with lots of land for crops and no oil may choose fermentation.

Uses of alcohols

• Fuels: ethanol is mixed with petrol for cars; methanol and ethanol are used in spirit burners. Ethanol from fermented crops is a biofuel: the carbon dioxide it releases was taken in by the plants as they grew, and new crops can be grown.
• Solvents: small alcohols mix with water and also dissolve many oily substances that water cannot, so they are used in perfumes, aftershaves and mouthwashes. Methylated spirits is mostly ethanol with poisonous methanol and a purple dye added so that no one drinks it; it removes marker-pen ink and grease.
• Drinks: ethanol is the alcohol in beer, wine and spirits.

Comparing alcohols as fuels

You can compare how much energy different alcohols release by using each one to heat the same volume of water. Weigh the spirit burner before and after, and measure the temperature rise of the water.

• Independent variable: the alcohol. Dependent variable: temperature rise of the water.
• Control variables: same volume of water, same can, same distance from flame to can, same starting temperature, draught shield.
• Safety: alcohols are highly flammable; keep the stock bottles away from flames and wear eye protection.
• Errors: much energy is lost to the air and the can, and some alcohol may burn incompletely (soot on the can). Keep the cap on the burner when weighing, because alcohols evaporate.

Worked example: temperature rise per gram

A burner of butanol has a mass of 85.46 g before and 84.86 g after. The water warms from 20.5 °C to 39.1 °C.
Mass burned = 85.46 − 84.86 = 0.60 g. Temperature rise = 39.1 − 20.5 = 18.6 °C.
Temperature rise per gram = 18.6 ÷ 0.60 = 31 °C per gram.

Answer: 31 °C/g; repeat for each alcohol and compare

Common mistakes

• Saying alcohols are alkaline because they contain “OH”. Solutions of alcohols are neutral, pH 7.
• Saying fermentation needs oxygen. It must be anaerobic.
• Saying a higher temperature is always better for fermentation. Above about 40 °C the yeast enzymes are denatured.
• Forgetting the oxygen atom already in ethanol when balancing its combustion.
• Saying oxidation of ethanol gives an ester. It gives a carboxylic acid (ethanoic acid).
• Comparing fuels by temperature rise alone. Divide by the mass burned to compare per gram.

Revision questions

Try these, then check your answers below each one.

Question 1. Which gas is given off when sodium is added to ethanol? (Choose from: Oxygen, Hydrogen, Carbon dioxide, Chlorine.)

Answer: Hydrogen

Working: Hydrogen: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂.

Question 2. Balance: C₂H₅OH + xO₂ → 2CO₂ + 3H₂O. What is x?

Answer: 3

Working: O on the right = 4 + 3 = 7. Ethanol gives 1, so 6 O atoms come from O₂: x = 3.

Question 3. True or false? A solution of ethanol in water has a pH of about 13 because ethanol contains an –OH group.

Answer: False

Working: False. The –OH in an alcohol is covalently bonded and does not release OH− ions. The solution is neutral, pH 7.

Related lessons

• Previous lesson: 10.2 Structures of Alcohols, Carboxylic Acids and Esters
• Next lesson: 10.4 Carboxylic Acids and Esters
• In this chapter: 10.1 Reactions of the Alkenes

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This lesson is section 10.3 (AGCCH1003) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.