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

This study guide teaches alcohols, carboxylic acids and esters for OxfordAQA International GCSE Chemistry (9202). The source document is the OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards. Every lettered statement of section 3.10.3 is taught here: 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 student sits the same two papers. Either paper may test this unit, since the specification says each paper can assess any part of the course.

Useful pages: 9202 chemistry hub · topic checklist · condensed notes on alcohols, acids and esters · question set on alcohols, acids and esters · free diagnostics

What this unit covers

Spec statement	You need to be able to
3.10.3.1 a	Recognise alcohols by the –OH group; know methanol, ethanol and propanol; read structural and displayed formulae
3.10.3.1 b	Describe how the first three alcohols behave in water, with sodium and in air; give their uses
3.10.3.1 c	Describe the oxidation of ethanol to ethanoic acid, chemically or by microbes; link ethanoic acid to vinegar
3.10.3.2 a	Recognise carboxylic acids by the –COOH group; know methanoic, ethanoic and propanoic acid
3.10.3.2 b	Describe their reactions in water, with carbonates and with alcohols; explain why they are weak acids; write balanced equations
3.10.3.3 a	Recognise esters by the –COO– group; name ethyl ethanoate and write the equation for making it
3.10.3.3 b	State that esters are volatile, have distinctive smells and are used as flavourings and perfumes

None of the required practicals in section 6.2 sits in 3.10.3. (The organic required practical, testing for a double bond, belongs to 3.10.1.3.) You will need moles and the molar gas volume from quantitative chemistry; revise both in OxfordAQA quantitative chemistry.

Families of organic compounds

A functional group is the part of a molecule responsible for how it reacts. Compounds sharing one functional group form a homologous series: neighbouring members differ by CH₂, and they all react in similar ways.

3.10.3.1 a: recognising alcohols

Alcohols contain the –OH functional group bonded to a carbon atom. The first three members are:

Name	Carbon atoms	Structural formula	Molecular formula
Methanol	1	CH₃OH	CH₄O
Ethanol	2	CH₃CH₂OH	C₂H₆O
Propanol	3	CH₃CH₂CH₂OH	C₃H₈O

The name ends in -ol; the stem (meth-, eth-, prop-) gives the number of carbon atoms. Ethanol is often written C₂H₅OH.

Displayed formula of ethanol, with every bond drawn:

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

Worked example: spotting the alcohols. Which of these are alcohols? P: C₄H₉OH. R: CH₃CH₂CH₃. S: CH₃OH. T: CH₃CH₂COOH.

• P ends in –OH attached to a carbon chain: an alcohol. You need only recognise it, not name it.
• R has only carbon and hydrogen: a hydrocarbon.
• S is methanol: an alcohol.
• T contains –COOH. The O–H here is part of a carboxyl group, so T is a carboxylic acid.

Answer: P and S.

3.10.3.1 b: properties and uses of alcohols

Methanol, ethanol and propanol share four behaviours.

1. They dissolve in water to form a neutral solution. The –OH group does not release H⁺ or OH⁻ ions into the water. Universal indicator stays green, pH 7.

2. They react with sodium to produce hydrogen. A small piece of sodium fizzes, gets smaller and disappears, more gently than with water. The gas gives a squeaky pop with a lighted splint. A sodium compound of the alcohol forms.

2CH₃OH + 2Na → 2CH₃ONa + H₂

3. They burn in air. With plenty of oxygen, complete combustion gives carbon dioxide and water, and releases energy.

4. Uses. They are used as fuels and solvents. Ethanol is the main alcohol in alcoholic drinks.

Worked example: balancing the combustion of methanol.

1. Write the formulae: CH₃OH + O₂ → CO₂ + H₂O.
2. Balance carbon: one C each side, so 1 CO₂.
3. Balance hydrogen: CH₃OH has 4 H, so 2 H₂O.
4. Count oxygen on the right: 2 + 2 = 4. The left already has 1 O in methanol, so O₂ must supply 3 atoms, which is 1½ O₂.
5. Double everything to clear the half:

2CH₃OH + 3O₂ → 2CO₂ + 4H₂O

Check: C 2 = 2; H 8 = 8; O 2 + 6 = 8 and 4 + 4 = 8.

For ethanol the same method gives C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.

Worked example: gas volume from sodium. 0.69 g of sodium reacts completely with excess methanol. Calculate the volume of hydrogen formed at room temperature and pressure. (Ar Na = 23; molar gas volume 24 dm³.)

1. Moles of sodium = 0.69 ÷ 23 = 0.030 mol.
2. The equation shows 2 Na → 1 H₂, so moles of H₂ = 0.030 ÷ 2 = 0.015 mol.
3. Volume = 0.015 × 24 = 0.36 dm³ (360 cm³).

3.10.3.1 c: oxidation of ethanol

Ethanol can be oxidised to ethanoic acid, a carboxylic acid. There are two routes in the specification.

• Chemical oxidising agents. Warming ethanol with an oxidising agent converts it to ethanoic acid. A common laboratory choice is acidified potassium dichromate, which turns from orange to green as it reacts.
• Microbial action. Microbes use oxygen from the air to oxidise ethanol, which is why opened wine turns sour.

Ethanoic acid is the main acid in vinegar.

Overall, for oxidation by oxygen:

C₂H₅OH + O₂ → CH₃COOH + H₂O

Oxidation keeps the two-carbon chain. Combustion breaks the molecule down completely to carbon dioxide and water.

Worked example: mass of acid from ethanol. 6.9 g of ethanol is fully oxidised to ethanoic acid. Calculate the mass of ethanoic acid formed. (Ar H = 1, C = 12, O = 16.)

1. Mr ethanol, C₂H₅OH = 24 + 6 + 16 = 46. Moles = 6.9 ÷ 46 = 0.15 mol.
2. The ratio is 1 : 1, so 0.15 mol of ethanoic acid forms.
3. Mr ethanoic acid, CH₃COOH = 24 + 4 + 32 = 60. Mass = 0.15 × 60 = 9.0 g.

3.10.3.2 a: recognising carboxylic acids

Carboxylic acids contain the –COOH functional group. The carbon in –COOH counts towards the name.

Name	Carbon atoms	Structural formula
Methanoic acid	1	HCOOH
Ethanoic acid	2	CH₃COOH
Propanoic acid	3	CH₃CH₂COOH

Displayed formula of ethanoic acid. The carboxyl carbon has a double bond to one oxygen and a single bond to an O–H:

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

Recognise a carboxylic acid from its name (ending -oic acid) or formula; you only need to name these three.

3.10.3.2 b: reactions and strength of carboxylic acids

In water. Carboxylic acids dissolve to give acidic solutions. Universal indicator turns orange or red.

With carbonates. Like other acids, they react with metal carbonates to give a salt, water and carbon dioxide. You see fizzing, and the gas turns limewater milky. The salts take the ending -oate: methanoates, ethanoates, propanoates. You must be able to write the balanced equations.

Worked example: propanoic acid and potassium carbonate.

1. Each acid molecule gives up one H, and the carbonate ion is 2–, so two acid molecules are needed.
2. The salt is potassium propanoate, CH₃CH₂COOK.

2CH₃CH₂COOH + K₂CO₃ → 2CH₃CH₂COOK + H₂O + CO₂

3. Check: C 6 + 1 = 7 and 6 + 1 = 7; H 12 = 10 + 2; O 4 + 3 = 7 and 4 + 1 + 2 = 7; K 2 = 2.

Now the amounts. 2.76 g of potassium carbonate reacts with excess propanoic acid. What volume of carbon dioxide forms at room temperature and pressure? (Ar K = 39, C = 12, O = 16.)

• Mr K₂CO₃ = 78 + 12 + 48 = 138. Moles = 2.76 ÷ 138 = 0.020 mol.
• 1 K₂CO₃ gives 1 CO₂, so 0.020 mol CO₂.
• Volume = 0.020 × 24 = 0.48 dm³ (480 cm³).

With alcohols. With an acid catalyst, a carboxylic acid reacts with an alcohol to make an ester and water. See 3.10.3.3 below.

Why carboxylic acids are weak acids. When a carboxylic acid dissolves, only a small fraction of its molecules ionise. Most stay as whole molecules. The forward and backward reactions both happen, so use the ⇌ sign:

CH₃COOH ⇌ CH₃COO⁻ + H⁺

A strong acid such as hydrochloric acid ionises completely. At the same concentration, the strong acid releases far more H⁺ ions, and more H⁺ means a lower pH. So, at equal concentrations, the solution of the weak acid has the higher pH.

Worked example: explaining pH data. A student tests 0.10 mol/dm³ hydrochloric acid and 0.10 mol/dm³ ethanoic acid with universal indicator. She records pH 1 and pH 3. Which is which, and why?

1. Hydrochloric acid ionises completely, giving the higher H⁺ concentration and the lower pH (1).
2. Ethanoic acid only partly ionises, giving fewer H⁺ ions and the higher pH (3).

A weak acid is not the same as a dilute acid. “Weak” describes how much the acid ionises. “Dilute” means there is only a small amount of acid in each dm³ of solution.

3.10.3.3 a: esters and ethyl ethanoate

Esters contain the –COO– functional group, with a carbon atom on both sides. The only ester you must name is ethyl ethanoate, CH₃COOCH₂CH₃, made from ethanol and ethanoic acid:

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

An acid catalyst is needed. Displayed formula:

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

Recognising an ester. From a name: two words, the second ending in -oate (for example methyl propanoate). From a formula: –COO– sits in the middle of the chain, not at the end. Compare CH₃CH₂COOH (ends in –COOH, an acid) with CH₃COOCH₃ (carbon on both sides of –COO–, an ester). Both are C₃H₆O₂, so a molecular formula alone cannot decide.

Worked example: mass of ester. 9.0 g of ethanoic acid reacts with excess ethanol. Calculate the maximum mass of ethyl ethanoate. (Ar H = 1, C = 12, O = 16.)

1. Moles of ethanoic acid = 9.0 ÷ 60 = 0.15 mol.
2. The ratio is 1 : 1, so at most 0.15 mol ester.
3. Mr CH₃COOC₂H₅ = 48 + 8 + 32 = 88. Mass = 0.15 × 88 = 13.2 g.

You will collect less than this. Because the reaction is reversible it never goes to completion, and separating the ester from the mixture loses some.

3.10.3.3 b: properties and uses of esters

Esters are volatile: they evaporate easily at room temperature. They have distinctive, often fruity, smells. Together these explain their use as flavourings and in perfumes: the molecules escape from food or skin and reach your nose.

Common errors

• Calling C₂H₅OH a base or an alkali because it contains OH. Alcohol solutions are neutral.
• Writing CO₂ and H₂O as the products of oxidising ethanol with an oxidising agent or microbes. That is combustion; oxidation gives ethanoic acid.
• Counting the carbons in propanoic acid as two by forgetting the –COOH carbon.
• Using → instead of ⇌ for the ionisation of a weak acid.
• Saying a weak acid has a higher pH “because it is more dilute”. The concentrations are the same; the difference is the degree of ionisation.
• Leaving the acid catalyst out when describing ester formation.

Next steps

Move on to the condensed notes for fast recall, then try the practice questions. For command words, see the OxfordAQA chemistry exam preparation guide; a free diagnostic will flag your weaker areas. Related: acids, bases and salts.

Official syllabus

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