OxfordAQA International GCSE Chemistry: Organic compounds: their structure and reactions (9202) – Revision Notes

These revision notes condense section 3.10.3 Organic compounds – their structure and reactions of the OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards. Included: 3.10.3.1 Alcohols (a to c), 3.10.3.2 Carboxylic acids (a and b) and 3.10.3.3 Esters (a and b). Every candidate needs every point, and none of the section 6.2 required practicals falls in 3.10.3. Go to the full guide to alcohols, acids and esters when you need the full teaching.

Links: 9202 chemistry hub · topic checklist · question set on alcohols, acids and esters · free diagnostics

The three families at a glance

Family	Functional group	Name pattern	Members you must name	Solution in water
Alcohols	–OH	ends -ol	methanol, ethanol, propanol	neutral, pH 7
Carboxylic acids	–COOH	ends -oic acid	methanoic, ethanoic, propanoic acid	acidic, weak acid
Esters	–COO–	two words, second ends -oate	ethyl ethanoate only	–

Homologous series: same functional group, similar reactions, each member differs from the next by CH₂.

Recognition rule: you must recognise any alcohol, carboxylic acid or ester from its name or formula, even ones you are not asked to name. Butanol is still an alcohol; methyl propanoate is still an ester.

Formulae to know

Compound	Structural formula	Mr (H = 1, C = 12, O = 16)
Methanol	CH₃OH	32
Ethanol	CH₃CH₂OH or C₂H₅OH	46
Propanol	CH₃CH₂CH₂OH or C₃H₇OH	60
Methanoic acid	HCOOH	46
Ethanoic acid	CH₃COOH	60
Propanoic acid	CH₃CH₂COOH	74
Ethyl ethanoate	CH₃COOCH₂CH₃ or CH₃COOC₂H₅	88

Displayed formula of propanoic acid (three carbons, including the –COOH carbon):

    H   H   O
    |   |   ||
H - C - C - C - O - H
    |   |
    H   H

Method in steps: which family is it?

1. Find the oxygen atoms.
2. One O as –OH on a carbon with no C=O: alcohol.
3. C=O and O–H on the same carbon, at the end of the chain (–COOH): carboxylic acid.
4. C=O with the second O joined to another carbon (–COO– in the middle): ester.

Alcohols (3.10.3.1 b)

Test or condition	What happens
Added to water	dissolves; neutral solution; universal indicator green
Small piece of sodium	fizzing; sodium shrinks and disappears; hydrogen given off (squeaky pop)
Burned in air	complete combustion → carbon dioxide + water; energy released

Uses: fuels, solvents; ethanol is the main alcohol in alcoholic drinks.

Complete combustion equations:

Alcohol	Complete combustion
Methanol	2CH₃OH + 3O₂ → 2CO₂ + 4H₂O
Ethanol	C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
Propanol	2C₃H₇OH + 9O₂ → 6CO₂ + 8H₂O

With sodium: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂. Two moles of alcohol (or sodium) give one mole of hydrogen.

Oxidation of ethanol (3.10.3.1 c)

Ethanol → ethanoic acid, by either:

• a chemical oxidising agent (for example acidified potassium dichromate, orange → green), or
• microbial action using oxygen from the air (why opened wine turns sour).

Ethanoic acid is the main acid in vinegar. Overall: C₂H₅OH + O₂ → CH₃COOH + H₂O.

Carboxylic acids (3.10.3.2 b)

Reacts with	Products	What you see
Water	acidic solution (partly ionised)	universal indicator orange/red
Metal carbonate	salt + water + carbon dioxide	fizzing; gas turns limewater milky
Alcohol + acid catalyst	ester + water	sweet or fruity smell

Salt names end in -oate: sodium methanoate, calcium ethanoate, potassium propanoate.

Method in steps: balancing acid + carbonate

1. Write the acid, carbonate, salt, H₂O and CO₂.
2. A carbonate ion (2–) takes two H⁺, so a metal carbonate M₂CO₃ or MCO₃ needs two carboxylic acid molecules.
3. For a 1+ metal (Na, K) write two salt molecules, RCOONa; for a 2+ metal (Ca, Mg) write one salt, (RCOO)₂Ca.
4. Count C, H and O on each side.

Examples:

2CH₃COOH + K₂CO₃ → 2CH₃COOK + H₂O + CO₂
2HCOOH + CaCO₃ → (HCOO)₂Ca + H₂O + CO₂

Weak acids and pH

• Carboxylic acids do not ionise completely in water: CH₃COOH ⇌ CH₃COO⁻ + H⁺. That makes them weak acids.
• Strong acids (hydrochloric, nitric, sulfuric) ionise completely.
• Same concentration → weak acid gives fewer H⁺ ions → higher pH.

Esters (3.10.3.3)

ethanol + ethanoic acid → ethyl ethanoate + water
C₂H₅OH + CH₃COOH → CH₃COOC₂H₅ + H₂O      (acid catalyst)

Displayed formula of ethyl ethanoate:

    H   O       H   H
    |   ||      |   |
H - C - C - O - C - C - H
    |           |   |
    H           H   H

• Name: “ethyl” from ethanol, “ethanoate” from ethanoic acid.
• Properties: volatile (evaporate easily), distinctive smells.
• Uses: flavourings and perfumes.
• The reaction is reversible, so you never get the full calculated mass of ester.

How the three families link

ethanol --(oxidising agent or microbes)--> ethanoic acid
ethanol + ethanoic acid --(acid catalyst)--> ethyl ethanoate + water

One alcohol can therefore supply both halves of an ester: oxidise some of it to the acid, then react the acid with more of the alcohol. If you are given an unknown compound and its reactions, work back to the family. Use these clues:

Clue in the question	Points to
Neutral, but fizzes with sodium	alcohol
Acidic, fizzes with a carbonate, gas turns limewater milky	carboxylic acid
Made from an acid and an alcohol; fruity smell; evaporates easily	ester
Formed when wine is left open to the air	ethanoic acid

Method in steps: mole calculations in this unit

1. Write or read the balanced equation.
2. Work out Mr for the substance whose mass you know, using the Ar values given.
3. Moles = mass ÷ Mr (or volume in dm³ ÷ 24 for a gas at room temperature and pressure).
4. Use the equation ratio to get moles of the substance you want.
5. Mass = moles × Mr, or gas volume = moles × 24 dm³.
6. Give units, and check you used the ratio from the equation, not 1 : 1 by habit.

Must-know distinctions

• Oxidation vs combustion of ethanol. Oxidation (oxidising agent or microbes) → ethanoic acid. Combustion (burning) → CO₂ + H₂O.
• Weak vs dilute. Weak = partly ionised. Dilute = little acid per dm³. A dilute strong acid still ionises fully.
• Alcohol –OH vs hydroxide OH⁻. An alcohol has a covalent O–H; it gives a neutral solution, not an alkaline one.
• Acid vs ester with the same formula. CH₃CH₂COOH (acid) and CH₃COOCH₃ (ester) are both C₃H₆O₂. Look at where –COO– sits.

Small worked reminders

• Moles of hydrogen from sodium: 1.15 g Na = 0.050 mol; H₂ = 0.025 mol; volume = 0.025 × 24 = 0.60 dm³.
• Mass of acid from alcohol: 1 mol ethanol → 1 mol ethanoic acid, so mass scales by 60/46.
• Gas from a carbonate: 1 mol of any metal carbonate gives 1 mol CO₂, so 0.050 mol carbonate gives 1.2 dm³.

Quick self-test

1. Give the functional group of a carboxylic acid.
2. Is CH₃CH₂CH₂CH₂OH an alcohol, an acid or an ester?
3. State the colour of universal indicator in aqueous propanol.
4. Name the gas made when sodium reacts with methanol, and give its test.
5. Name two ways of oxidising ethanol to ethanoic acid.
6. Calculate the Mr of propanoic acid.
7. Write a balanced equation for ethanoic acid reacting with potassium carbonate.
8. Why does 0.1 mol/dm³ ethanoic acid have a higher pH than 0.1 mol/dm³ nitric acid?
9. Which word in “propyl methanoate” shows it is an ester?
10. 3.2 g of methanol burns completely. Calculate the mass of carbon dioxide formed.
11. What mass of calcium carbonate gives 1.2 dm³ of carbon dioxide at room temperature and pressure with excess acid? (Ca = 40)
12. Give two properties of esters that make them useful in perfumes.

Answers

1. –COOH.
2. An alcohol (–OH on a carbon chain, no C=O).
3. Green (pH 7, neutral).
4. Hydrogen; a lighted splint gives a squeaky pop.
5. A chemical oxidising agent; microbial action (oxygen from the air).
6. 3 × 12 + 6 × 1 + 2 × 16 = 74.
7. 2CH₃COOH + K₂CO₃ → 2CH₃COOK + H₂O + CO₂.
8. Ethanoic acid is weak and only partly ionises; nitric acid ionises completely, so ethanoic acid gives a lower H⁺ concentration.
9. “Methanoate” (the -oate ending of a two-word name).
10. 3.2 ÷ 32 = 0.10 mol methanol; 2CH₃OH → 2CO₂ is 1 : 1, so 0.10 mol CO₂ × 44 = 4.4 g.
11. 1.2 ÷ 24 = 0.050 mol CO₂ = 0.050 mol CaCO₃; × 100 = 5.0 g.
12. Volatile; distinctive (pleasant) smell.

Where marks are usually lost

• Calling an alcohol solution alkaline because the formula contains OH.
• Writing “gas given off” with sodium but not naming hydrogen, or giving the limewater test instead of the squeaky pop.
• Giving CO₂ and water as the product of microbial oxidation of ethanol.
• Forgetting the carbon in –COOH, so propanoic acid is drawn with two carbons.
• Writing a single acid molecule with a carbonate, leaving the hydrogen unbalanced.
• Using → for weak-acid ionisation where ⇌ is expected, or not linking weak to partial ionisation.
• Explaining the higher pH of a weak acid by saying it is “more dilute”.
• Missing the acid catalyst when describing how an ester is made.
• Naming ethyl ethanoate as “ethanoyl ethanol”, or drawing it with an O–H left on the end, which makes it an acid.

Official syllabus

OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards, published by OxfordAQA. Notes condensed from: 3.10.3 Organic compounds – their structure and reactions (alcohols, carboxylic acids, esters).