Carboxylic Acids — Edexcel GCSE Chemistry Revision Notes

What you'll learn

Carboxylic acids form a homologous series of organic compounds containing the –COOH functional group. This topic examines their structural formulae, physical properties including solubility and boiling points, their characteristic weak acid behaviour, and their reaction with alcohols to produce esters. Questions on carboxylic acids regularly appear in Edexcel GCSE Chemistry papers, particularly in the organic chemistry sections worth 6-8 marks.

Key terms and definitions

Carboxylic acid — an organic compound containing the –COOH functional group (carboxyl group), which gives the compound acidic properties.

Functional group — the atom or group of atoms responsible for the characteristic chemical reactions of a compound; in carboxylic acids this is –COOH.

Homologous series — a family of organic compounds with the same general formula, similar chemical properties and a gradual trend in physical properties.

Weak acid — an acid that only partially dissociates (ionises) in aqueous solution, releasing relatively few H⁺ ions.

Esterification — the reaction between a carboxylic acid and an alcohol to produce an ester and water, typically requiring an acid catalyst.

Aqueous solution — a solution where water is the solvent; carboxylic acids dissolve in water to form acidic solutions.

Dissociation — the process by which an acid releases hydrogen ions (H⁺) when dissolved in water; carboxylic acids dissociate only partially.

Ester — an organic compound formed from the reaction between a carboxylic acid and an alcohol, with the general structure RCOOR'.

Core concepts

Structure of carboxylic acids

The carboxyl functional group (–COOH) consists of a carbonyl group (C=O) joined to a hydroxyl group (–OH) on the same carbon atom. This structural arrangement is written as –COOH or –CO₂H in structural formulae.

The general formula for carboxylic acids is CₙH₂ₙ₊₁COOH or CₙH₂ₙO₂ when simplified. The simplest members include:

Methanoic acid (HCOOH) — 1 carbon atom, formula CH₂O₂
Ethanoic acid (CH₃COOH) — 2 carbon atoms, formula C₂H₄O₂
Propanoic acid (CH₃CH₂COOH) — 3 carbon atoms, formula C₃H₆O₂
Butanoic acid (CH₃CH₂CH₂COOH) — 4 carbon atoms, formula C₄H₈O₂

When drawing displayed formulae for Edexcel GCSE Chemistry exams, the carboxyl group must show:

A double bond between carbon and one oxygen atom (C=O)
A single bond between the same carbon and an –OH group
All bonds clearly represented

The naming system follows the pattern: count the carbon atoms in the longest chain (including the carboxyl carbon), then use the appropriate prefix (meth-, eth-, prop-, but-) followed by '-oic acid'.

Physical properties of carboxylic acids

Carboxylic acids display characteristic physical properties that distinguish them from other organic compounds:

Boiling points:

Higher than alkanes and alcohols with similar molecular mass
Increase as the carbon chain lengthens
Methanoic acid: 101°C
Ethanoic acid: 118°C
Propanoic acid: 141°C

These relatively high boiling points result from hydrogen bonding between carboxylic acid molecules. The –COOH group can form two hydrogen bonds per molecule, creating stronger intermolecular forces that require more energy to overcome.

Solubility:

Short-chain carboxylic acids (C₁–C₄) dissolve readily in water
Solubility decreases as carbon chain length increases
Water solubility results from hydrogen bonding between –COOH groups and water molecules
Longer hydrocarbon chains become increasingly non-polar, reducing water solubility

State at room temperature:

Methanoic, ethanoic, propanoic and butanoic acids exist as liquids
Ethanoic acid freezes at 17°C, earning it the common name "glacial acetic acid" when pure

Chemical properties: behaviour as weak acids

Carboxylic acids function as weak acids in aqueous solution. When dissolved in water, they partially dissociate to release hydrogen ions:

CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)

The reversible arrow (⇌) indicates that only a small proportion of molecules ionise at any moment. This contrasts sharply with strong acids like hydrochloric acid, which dissociate completely:

HCl(aq) → H⁺(aq) + Cl⁻(aq)

For ethanoic acid at typical concentrations, fewer than 1% of molecules dissociate. This produces a higher pH (typically 3-4 for 0.1 mol/dm³ ethanoic acid) compared to strong acids of the same concentration (pH 1).

Reactions characteristic of acids:

Carboxylic acids undergo the typical reactions of acids, but more slowly than strong mineral acids:

With metals (above hydrogen in reactivity series):
- 2CH₃COOH(aq) + Mg(s) → (CH₃COO)₂Mg(aq) + H₂(g)
- Produces a salt (magnesium ethanoate) and hydrogen gas
- Effervescence observed; hydrogen identified with a lit splint (squeaky pop)
With metal oxides:
- 2CH₃COOH(aq) + CuO(s) → (CH₃COO)₂Cu(aq) + H₂O(l)
- Black copper oxide dissolves to form blue copper ethanoate solution
With metal hydroxides (bases):
- CH₃COOH(aq) + NaOH(aq) → CH₃COONa(aq) + H₂O(l)
- Neutralisation reaction producing sodium ethanoate and water
With metal carbonates:
- 2CH₃COOH(aq) + Na₂CO₃(s) → 2CH₃COONa(aq) + H₂O(l) + CO₂(g)
- Effervescence occurs; carbon dioxide turns limewater milky
- 2CH₃COOH(aq) + CaCO₃(s) → (CH₃COO)₂Ca(aq) + H₂O(l) + CO₂(g)

The salts formed are named by replacing '-oic acid' with '-oate' and adding the metal name: ethanoic acid forms ethanoates, propanoic acid forms propanoates.

Formation of esters

Esterification represents the key synthetic reaction of carboxylic acids tested at Edexcel GCSE level. When a carboxylic acid reacts with an alcohol in the presence of an acid catalyst (typically concentrated sulfuric acid), an ester and water form:

General equation: Carboxylic acid + Alcohol ⇌ Ester + Water

Specific example: CH₃COOH + CH₃OH ⇌ CH₃COOCH₃ + H₂O Ethanoic acid + Methanol ⇌ Methyl ethanoate + Water

Reaction conditions:

Concentrated sulfuric acid catalyst (H₂SO₄)
Gentle heating (typically 60-70°C)
Reversible reaction indicated by ⇌ symbol

Naming esters: Esters are named in two parts:

First word: derived from the alcohol (replace '-ol' with '-yl')
Second word: derived from the acid (replace '-oic acid' with '-oate')

Examples:

Ethanoic acid + methanol → methyl ethanoate
Propanoic acid + ethanol → ethyl propanoate
Methanoic acid + propanol → propyl methanoate

Properties and uses of esters:

Sweet, fruity odours (banana, pear, apple scents)
Used as flavourings in food industry
Used as fragrances in perfumes and cosmetics
Solvents in industrial processes
Lower boiling points than corresponding carboxylic acids (no hydrogen bonding)

The reversible nature of esterification means that esters can undergo hydrolysis back to carboxylic acids and alcohols when heated with dilute acid or alkali.

Laboratory preparation and testing

Testing for carboxylic acids:

Universal indicator or pH meter shows pH 3-5 for dilute solutions
React with carbonates producing carbon dioxide (limewater test)
Distinctive sharp, vinegary odours (especially ethanoic acid)

Distinguishing weak from strong acids: Students must recognise that carboxylic acids react more slowly than strong acids of equal concentration because fewer H⁺ ions are present at any moment. The pH will also be higher (less acidic) for the same molar concentration.

Worked examples

Example 1: Naming and structural formulae

Question: Butanoic acid has the molecular formula C₄H₈O₂. (a) Draw the displayed formula of butanoic acid, showing all bonds. [2 marks] (b) Explain why butanoic acid is less soluble in water than propanoic acid. [2 marks]

Solution: (a)

       H   H   H   O
       |   |   |   ||
   H — C — C — C — C — O — H
       |   |   |
       H   H   H

[1 mark for correct carbon chain; 1 mark for correct –COOH group with C=O and –OH shown]

(b) Butanoic acid has a longer hydrocarbon chain than propanoic acid [1 mark]. The longer non-polar chain reduces the proportion of the molecule that can form hydrogen bonds with water molecules [1 mark].

Example 2: Reactions of carboxylic acids

Question: A student adds 2.0 g of calcium carbonate to 50 cm³ of ethanoic acid solution. (a) Write a balanced symbol equation for this reaction. [3 marks] (b) Describe what the student would observe. [2 marks] (c) Name the salt formed. [1 mark]

Solution: (a) 2CH₃COOH + CaCO₃ → (CH₃COO)₂Ca + H₂O + CO₂ [1 mark for correct formulae; 1 mark for products; 1 mark for balancing]

(b) Effervescence/fizzing/bubbles observed [1 mark]. The solid calcium carbonate dissolves/disappears [1 mark].

Example 3: Ester formation

Question: Propanoic acid reacts with methanol to form an ester. (a) Name the ester formed. [1 mark] (b) Write a balanced symbol equation for this reaction. [3 marks] (c) State the conditions required for this reaction. [2 marks]

Solution: (a) Methyl propanoate [1 mark]

(b) CH₃CH₂COOH + CH₃OH ⇌ CH₃CH₂COOCH₃ + H₂O [1 mark for correct formulae; 1 mark for products including water; 1 mark for reversible arrow]

Common mistakes and how to avoid them

Mistake: Drawing the –COOH group incorrectly as C–O–O–H Correction: The carboxyl group must show a carbon atom double-bonded to one oxygen (C=O) AND single-bonded to –OH. Both oxygens attach to the same carbon atom, not in a chain.

Mistake: Naming carboxylic acids incorrectly by missing the carboxyl carbon in the count Correction: When counting carbons for naming, always include the carbon in the –COOH group. CH₃CH₂COOH has three carbons total, making it propanoic acid (not ethanoic acid).

Mistake: Stating that carboxylic acids are strong acids Correction: Carboxylic acids are weak acids because they only partially dissociate in water. Use the reversible arrow (⇌) in equations showing their ionisation. Strong acids like HCl dissociate completely.

Mistake: Reversing the ester name (writing "ethyl methanoate" instead of "methyl ethanoate") Correction: The first part of the ester name comes from the alcohol (change -ol to -yl), and the second part comes from the acid (change -oic acid to -oate). Methanol + ethanoic acid = methyl ethanoate.

Mistake: Forgetting water as a product in esterification reactions Correction: Esterification always produces two products: the ester AND water. The general equation is: acid + alcohol ⇌ ester + water. This is a condensation reaction.

Mistake: Writing a single arrow instead of reversible arrows for esterification Correction: Esterification is reversible, so use ⇌ not →. Esters can hydrolyse back to carboxylic acids and alcohols, establishing an equilibrium.

Exam technique for "Carboxylic acids: structure, properties and reactions"

Command word "explain": When asked to explain why carboxylic acids are weak acids or why their boiling points are high, provide both the observation and the underlying reason. State that partial dissociation occurs, then explain this means fewer H⁺ ions are present. For boiling points, identify hydrogen bonding and explain why this creates stronger intermolecular forces.

Drawing displayed formulae: Show all atoms and all bonds clearly. Examiners deduct marks for missing bonds or incorrectly drawn functional groups. The –COOH group commonly loses marks—ensure the C=O double bond and –OH group are both shown attached to the same carbon.

Balancing equations with carboxylic acids: Remember that two carboxylic acid molecules react with one metal carbonate or one magnesium atom. The coefficient 2 appears frequently: 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂. Check that oxygen and hydrogen balance on both sides.

Naming organic compounds: Write the full systematic name: "ethanoic acid" not "ethanoate" or "ethanol". For esters, both words are needed (methyl propanoate, not just methyl or propanoate). Spelling counts—"ethanoic" not "ethanoatic".

Quick revision summary

Carboxylic acids contain the –COOH functional group and follow the general formula CₙH₂ₙO₂. They are weak acids that partially dissociate in water, producing pH 3-5 solutions. They react with metals, bases, and carbonates like other acids but more slowly. Hydrogen bonding between molecules causes high boiling points and water solubility for short-chain acids. Esterification occurs when carboxylic acids react with alcohols in the presence of concentrated sulfuric acid catalyst and heat, producing sweet-smelling esters and water. Name esters using the alcohol-derived prefix first (changing -ol to -yl), followed by the acid-derived suffix (changing -oic acid to -oate).