C20: Alcohols, Carboxylic Acids and Polymers
Alcohols, carboxylic acids, esters and polymerisation
📋 Alcohols
Alcohols are a homologous series with the functional group –OH (hydroxyl group). Their general formula is CₙH₂ₙ₊₁OH. They are used as solvents, fuels and in alcoholic drinks.
| Alcohol | Formula | Uses |
|---|---|---|
| Methanol | CH₃OH | Industrial solvent, chemical feedstock |
| Ethanol | C₂H₅OH | Alcoholic drinks, fuel, solvent |
| Propanol | C₃H₇OH | Solvent, cleaning agent |
📝 Reactions of Alcohols
Alcohols undergo three key reactions: combustion, oxidation and reaction with sodium.
1. Combustion
Alcohol + Oxygen → Carbon dioxide + Water
e.g. C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
e.g. C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
Alcohols burn cleanly, making them useful fuels.
2. Oxidation
Alcohols are oxidised to form carboxylic acids. Oxidising agents such as acidified potassium dichromate(VI) are used. Ethanol oxidises to ethanoic acid.
C₂H₅OH + [O] → CH₃COOH + H₂O
ethanol → ethanoic acid + water
ethanol → ethanoic acid + water
3. Reaction with Sodium
2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂
ethanol + sodium → sodium ethoxide + hydrogen
ethanol + sodium → sodium ethoxide + hydrogen
Sodium sinks in ethanol, fizzes steadily (hydrogen gas), and dissolves. This is less vigorous than sodium with water.
📝 Carboxylic Acids
Carboxylic acids have the functional group –COOH. They dissolve in water to give acidic solutions (pH below 7). They are weak acids because they only partially ionise in water.
| Carboxylic Acid | Formula | Common Name |
|---|---|---|
| Methanoic acid | HCOOH | Found in ant stings |
| Ethanoic acid | CH₃COOH | Vinegar (dilute solution) |
| Propanoic acid | C₂H₅COOH | Food preservative |
Carboxylic acids react like typical acids:
- With metals: carboxylic acid + metal → salt + hydrogen
e.g. 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂ - With bases: carboxylic acid + base → salt + water
e.g. CH₃COOH + NaOH → CH₃COONa + H₂O - With carbonates: carboxylic acid + carbonate → salt + water + CO₂
e.g. 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
Carboxylic acids are weak acids because they partially ionise in water. This means fewer H⁺ ions are released compared to strong acids like HCl, so their reactions are less vigorous.
📝 Esters
An ester is formed when an alcohol reacts with a carboxylic acid. Water is also produced. Esters have pleasant fruity smells and are used in perfumes and flavourings.
Alcohol + Carboxylic Acid → Ester + Water
e.g. C₂H₅OH + CH₃COOH → CH₃COOC₂H₅ + H₂O
ethanol + ethanoic acid → ethyl ethanoate + water
e.g. C₂H₅OH + CH₃COOH → CH₃COOC₂H₅ + H₂O
ethanol + ethanoic acid → ethyl ethanoate + water
An acid catalyst (e.g. concentrated sulfuric acid) is usually needed for this reaction.
📝 Addition Polymers
Addition polymers are made from alkene monomers. The C=C double bond opens and the monomers join together in a long chain. No other substance is produced.
| Monomer | Polymer | Uses |
|---|---|---|
| Ethene (C₂H₄) | Polyethene (polythene) | Carrier bags, bottles |
| Propene (C₃H₆) | Polypropene (polypropylene) | Rope, crates, car parts |
| Chloroethene | PVC (polychloroethene) | Window frames, pipes |
n C₂H₄ → (—CH₂—CH₂—)ₙ
n ethene → polyethene
n ethene → polyethene
📝 Condensation Polymers
Condensation polymers are made from two different monomers that join together with the loss of a small molecule (usually water). A common example is polyesters.
- One monomer has a diol (two –OH groups)
- The other monomer has a dicarboxylic acid (two –COOH groups)
- Each time they join, a molecule of water is lost
| Feature | Addition Polymers | Condensation Polymers |
|---|---|---|
| Monomers | One type (alkene) | Two different types |
| By-product | None | Water (or other small molecule) |
| Bond formed | C—C from opened C=C | Ester linkage (—COO—) |
| Example | Polyethene | Polyester (e.g. Terylene) |
❓ Practice Questions
Q1: Name the functional group present in alcohols and give one use of ethanol.
Q2: Write a balanced symbol equation for the complete combustion of ethanol.
Q3: Explain why carboxylic acids are described as weak acids.
Q4: Higher Write a word equation for the formation of an ester from propanol and ethanoic acid. State the conditions needed.
Q5: Describe how addition polymers are formed from alkene monomers. Use ethene and polyethene as an example.
Q6: Higher Compare addition polymerisation and condensation polymerisation.
✅ Answers
- The functional group in alcohols is the hydroxyl group (–OH). Ethanol is used as a fuel, a solvent, or in alcoholic drinks.
- C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
- Carboxylic acids are weak acids because they partially ionise in water. Only a small proportion of acid molecules dissociate to release H⁺ ions, so their reactions are less vigorous than strong acids.
- Propanol + Ethanoic acid → Propyl ethanoate + Water. An acid catalyst (e.g. concentrated sulfuric acid) is needed, and the mixture is heated.
- Addition polymerisation involves opening the C=C double bond in each alkene monomer. The monomers join together in a long chain. For ethene: the C=C double bond opens, and the molecules link to form polyethene, (—CH₂—CH₂—)ₙ. No other substance is produced.
- Addition polymerisation uses one type of monomer (alkenes); condensation uses two different monomers. Addition produces no by-product; condensation produces water. Addition polymers form C—C bonds from opened C=C; condensation polymers form ester linkages.
🎯 Exam Tips
- When writing equations for alcohol combustion, always balance C first, then H, then O
- For polymerisation questions: addition polymers come from alkenes (double bond opens), condensation polymers come from two different monomers (water lost)
- Carboxylic acids are WEAK acids — they partially ionise, unlike HCl which is a strong acid that fully ionises
- When drawing polymer repeating units, open the C=C double bond and show the bonds going outside the brackets
- Remember: alcohols + sodium → less vigorous than water + sodium (ethanol is less acidic than water)
🔬 Required Practical
Required Practical: Combustion of Alcohols
Aim: Investigate how the length of the carbon chain in alcohols affects the energy released per mole during combustion.
Method: 1. Measure 100 cm³ of water into a copper calorimeter. 2. Record the initial temperature of the water. 3. Weigh a spirit burner containing an alcohol (e.g. methanol). 4. Place the burner under the calorimeter and light it. 5. Stir the water constantly and heat until the temperature rises by 20°C. 6. Extinguish the flame and record the final temperature. 7. Reweigh the burner to find the mass of alcohol burned. 8. Repeat with different alcohols (ethanol, propanol, butanol). 9. Calculate energy transferred per mole for each alcohol.
Variables: IV: type of alcohol (chain length), DV: energy released per mole (kJ/mol), Control: volume of water, temperature rise, distance between burner and calorimeter, type of calorimeter
🔢 Maths Skills
Mathematical Skills
You need to calculate energy transferred in combustion experiments using Q = m × c × ΔT, and determine energy per mole from mass of fuel burned.
Maths Example
In an experiment, 0.46 g of ethanol (Mᵣ = 46) heated 100 g of water by 20°C. The specific heat capacity of water is 4.2 J/g°C. Calculate the energy transferred per mole.
Energy = 100 × 4.2 × 20 = 8,400 J. Moles of ethanol = 0.46 ÷ 46 = 0.01 mol. Energy per mole = 8,400 ÷ 0.01 = 840,000 J/mol = 840 kJ/mol
⚠️ Common Misconceptions
Watch Out!
1. Wrong: Carboxylic acids are strong acids because they are acids Correct: Carboxylic acids are WEAK acids — they only partially ionise in water, releasing fewer H⁺ ions than strong acids like HCl
2. Wrong: Addition polymers produce water as a by-product Correct: Addition polymers produce NO by-product — the C=C double bond simply opens and monomers join together. Only CONDENSATION polymers produce water
3. Wrong: Oxidation of ethanol produces water and carbon dioxide Correct: Oxidation of ethanol produces ETHANOIC ACID and water — combustion produces CO₂ and water, but oxidation with an oxidising agent is different from combustion
4. Wrong: A polymer and its monomer have the same properties Correct: Polymers have very different properties from their monomers — e.g. ethene is a gas but polyethene is a solid plastic
✍️ 6-Mark Question
Extended Answer
6 marks: Compare addition polymerisation and condensation polymerisation, including the monomers involved, the products, and the type of bond formed. Give an example of each.
Addition polymerisation uses a single type of monomer — alkenes with a C=C double bond. The double bond opens and the monomers join together in a long chain. No other substance is produced as a by-product. The bond formed between monomers is a C—C single bond from the opened C=C double bond. An example is the polymerisation of ethene to form polyethene: n C₂H₄ → (—CH₂—CH₂—)ₙ. Condensation polymerisation uses two different types of monomer. For polyesters, one monomer is a diol (with two –OH groups) and the other is a dicarboxylic acid (with two –COOH groups). Each time the monomers join, a molecule of water is lost as a by-product. The bond formed between monomers is an ester linkage (—COO—). An example is the formation of a polyester from a diol and a dicarboxylic acid: diol + dicarboxylic acid → polyester + water. The key differences are: addition uses one monomer type and produces no by-product, while condensation uses two monomer types and produces water.
Mark scheme: 1 mark for addition monomer type (alkenes, single type); 1 mark for addition mechanism (C=C opens, no by-product); 1 mark for addition example; 1 mark for condensation monomer types (diol + dicarboxylic acid); 1 mark for condensation mechanism (water lost as by-product, ester linkage); 1 mark for condensation example
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A student carried out the combustion of alcohols practical and obtained the following results:
- Methanol: 0.32 g burned, temperature rise = 24°C, energy per mole = 1008 kJ/mol
- Ethanol: 0.46 g burned, temperature rise = 24°C, energy per mole = 840 kJ/mol
- Propanol: 0.60 g burned, temperature rise = 24°C, energy per mole = 840 kJ/mol
- Butanol: 0.74 g burned, temperature rise = 24°C, energy per mole = 840 kJ/mol
(100 g of water used in each case, SHC = 4.2 J/g°C)
(a) The accepted values for energy per mole are: methanol 726 kJ/mol, ethanol 1367 kJ/mol, propanol 2021 kJ/mol, butanol 2676 kJ/mol. Compare the student's results with the accepted values and suggest why they differ.
(b) Suggest two improvements to the method that would give more accurate results.
(c) Explain the trend in accepted values as chain length increases.
Answers: (a) The student's values are all much lower than the accepted values and show little difference between alcohols, whereas the accepted values increase significantly with chain length. This is because much of the heat energy is lost to the surroundings (air) rather than being transferred to the water. (b) Two improvements: (1) Use a draft shield around the flame to reduce heat loss to the air. (2) Reduce the distance between the flame and the calorimeter. (3) Put a lid on the calorimeter to reduce heat loss from the water. (4) Insulate the calorimeter. (c) As the carbon chain length increases, there are more C—H and C—C bonds to be broken and more CO₂ and H₂O formed, releasing more energy per mole. Each additional CH₂ group adds a similar amount of energy, so the energy per mole increases roughly proportionally with chain length.
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