4.2Unit 2 · Organic Chemistry
Carboxylic acids — Edexcel IGCSE Chemistry Modular - UNIT 2 Revision Notes & Topical Questions
5 min read 7 parts Lesson 2 of 4 1 quiz
Lesson summary
Recognise carboxylic acids from names, formulae and the –COOH group.Functional group–COOH; names end in -oic acid.Ethanoic acid is a weak acidCH3COOH ⇌ CH3COO− + H+.2CH3COOH + Mg → (CH3COO)2Mg + H2.2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2.CH3COOH + NaOH → CH3COONa + H2O.Salts of…
- What are carboxylic acids?
- Weak acids
- Reaction with metals
- Reaction with carbonates
- Reaction with bases
- Making and using ethanoic acid
Section 1 of 7Carboxylic acids
Essential knowledge
What are carboxylic acids?
Carboxylic acids are a homologous series with the carboxyl functional group, –COOH. Names end in –oic acid: methanoic, ethanoic, propanoic and butanoic acid. The general formula is CnH2n+1COOH.
Structure
Structure in –COOH the carbon is double bonded to one oxygen and single bonded to an –OH group. The carbon of the COOH group counts in the name, so CH3COOH (2 carbons) is ethanoic acid.
Weak acids
Weak acids in water, carboxylic acids only partially ionise to release H+ ions. Their solutions have a pH of about 3–5 and they react more slowly than strong acids of the same concentration.
Reaction with metals
Reaction with metals carboxylic acids react with reactive metals to form a salt and hydrogen, e.g. ethanoic acid + magnesium → magnesium ethanoate + hydrogen. Bubbling is slower than with hydrochloric acid.
Reaction with carbonates
Reaction with carbonates carboxylic acids react with metal carbonates to give a salt, water and carbon dioxide, e.g. with sodium carbonate forming sodium ethanoate. Effervescence is seen and the gas turns limewater milky.
Reaction with bases
Reaction with bases they are neutralised by alkalis such as sodium hydroxide to form a salt and water, e.g. sodium ethanoate.
Making and using ethanoic acid
Making and using ethanoic acid ethanoic acid is produced by oxidising ethanol, either with acidified potassium dichromate(VI) or by bacteria in air. Vinegar is dilute ethanoic acid, used as a flavouring and preservative.
Reaction with alcohols
Reaction with alcohols with an alcohol and a concentrated sulfuric acid catalyst, carboxylic acids form esters (e.g. ethyl ethanoate).
Salt names
Salt names the salt is named from the acid with -oate, e.g. butanoic acid with potassium carbonate gives potassium butanoate: 2C3H7COOH + K2CO3 → 2C3H7COOK + H2O + CO2.
Vinegar strength
Vinegar strength vinegar is a dilute aqueous solution containing about 5% ethanoic acid. The general formula of carboxylic acids is often shortened to RCOOH.
Section 2 of 7Process & sequence
1
Pour a sample of dilute ethanoic acid into test tubes.
2
Add universal indicator: orange/yellow shows a weak acid (pH about 3–4).
3
Add a piece of magnesium ribbon and test the gas with a lighted splint (squeaky pop).
4
Add sodium carbonate and bubble the gas through limewater (turns milky).
5
Compare the rate of fizzing with hydrochloric acid of the same concentration: ethanoic acid is slower.
6
Conclude that ethanoic acid shows typical but weaker acid reactions.
Reactions of ethanoic acidwrite these steps in this order; each step is a marking point.
1
Ethanoic acid, weak acid
2
With metal: salt and hydrogen
3
With carbonate: salt, water, CO2
4
With alkali: salt and water
5
With alcohol: ester and water
Section 3 of 7Compare
| Name | Formula | Carbons |
|---|---|---|
| Methanoic acid | HCOOH | 1 |
| Ethanoic acid | CH3COOH | 2 |
| Propanoic acid | C2H5COOH | 3 |
| Butanoic acid | C3H7COOH | 4 |
General formula CnH2n+1COOH (n starts at 0). The carbon in COOH counts in the name.
| Reacts with | Products | Observation |
|---|---|---|
| Magnesium | Magnesium ethanoate + hydrogen | Slow fizzing, metal dissolves |
| Sodium carbonate | Sodium ethanoate + water + CO2 | Fizzing; gas turns limewater milky |
| Sodium hydroxide | Sodium ethanoate + water | No visible change; temperature rises |
| Ethanol (+ conc. H2SO4) | Ethyl ethanoate + water | Fruity smell |
Section 4 of 7Worked examples
Worked examples
Write a balanced equation for propanoic acid reacting with sodium carbonate and name the salt.
- 2C2H5COOH + Na2CO3 → 2C2H5COONa + H2O + CO2. The salt is sodium propanoate.
Explain why magnesium fizzes more slowly in ethanoic acid than in hydrochloric acid of equal concentration.
- Ethanoic acid is a weak acid and only partially ionises, so its solution contains a lower concentration of H+ ions.
- Fewer successful collisions per second between H+ ions and magnesium mean a slower rate.
Section 5 of 7Key terms
Section 6 of 7Command terms
Command terms
Give / State / Name
Recall a fact; one idea per mark.
Define
Give the exact meaning using the specification keywords.
Describe
Give an account of what happens / the pattern in data, quoting values; no reasons needed.
Explain
Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.
Suggest
Use knowledge in a new context; more than one answer may be accepted.
Compare
Similarities and differences; each point must refer to both items.
Calculate
Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.
Discuss
Explore the issue from different angles and link the points.
Evaluate / Assess
Weigh strengths and weaknesses and reach a supported judgement.
Deduce
Draw a conclusion from the information given.
Justify
Support a statement/choice with evidence.
Section 7 of 7Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Carboxylic acid · Carboxyl group · Ethanoic acid · Weak acid · Salt · Ethanoate ion · Neutralisation · Homologous series
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
How to get full marks (examiner guidance)
Use specification keywords exactly (e.g. electronegativity, nucleophile, dynamic equilibrium, activation energy).
Link ideas
Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.
Use the information in the question
“discuss/evaluate” answers must use the data provided and match the number of marks.
Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.
Avoid vague words
“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.
Do not extrapolate a graph line beyond the data or to zero unless asked.
Six-mark extended responses are marked on content AND a logical, linked line of reasoning.
Marked practiceQuizzes for this lesson
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