Chemistry • Year 12 • Module 7 • Lesson 14

Carboxylic Acids: Structure, Properties & Reactions

Lock in the carboxyl group structure, IUPAC naming, boiling point reasoning, and the four core reactions before moving on to Apply.

Build · Band 3–4 · Recall & Vocab

1. Label the carboxyl group diagram

The diagram below shows the structural features of the carboxyl group (−COOH) with boxes A–G left blank. Write the correct label or value for each box. 7 marks

Diagram pending — see image-prompts/chem-y12-m7.md #chem-y12-m7-l14-w01-fig1 The diagram will show a condensed structural formula of the carboxyl group (−COOH) with bond-type labels and blank boxes: A = name of the C=O bond, B = name of the O−H bond, C = hybridisation of the carboxyl carbon, D = approximate bond angle at the carboxyl carbon, E = the term for a molecule with this group at both ends, F = the functional group suffix for IUPAC naming, G = the role of the C=O oxygen in H-bonding (donor / acceptor).

A — Name of the C=O bond in −COOH _______________________
B — Name of the O−H bond in −COOH _______________________
C — Hybridisation of the carboxyl carbon _______________________
D — Approximate bond angle at the carboxyl carbon _______________________
E — Term for two −COOH molecules locked together by H-bonds _______________________
F — IUPAC suffix for carboxylic acids _______________________
G — Role of the C=O oxygen in hydrogen bonding (donor / acceptor) _______________________

Stuck? Revisit lesson Card 1 (carboxyl structure) and the Formula Panel.

2. Term–definition matching

Match each term in the right-hand column to its definition. Choose from: carboxyl group, carboxylate anion, dimer, Ka, weak acid, esterification, ionisation equilibrium, saponification, inductive effect, pKa. 10 marks

#	Definition	Matching term
2.1	The functional group −COOH consisting of a carbonyl and a hydroxyl on the same carbon.
2.2	An acid that only partially ionises in water; Ka « 1.
2.3	The equilibrium constant for the partial ionisation of a weak acid in water; equals [RCOO−][H₃O⁺] / [RCOOH].
2.4	The anion formed when a carboxylic acid loses a proton; resonance-stabilised with charge on both oxygens.
2.5	An associated pair of carboxylic acid molecules held together by two simultaneous H-bonds in a cyclic ring.
2.6	The negative logarithm of Ka; lower value means stronger acid.
2.7	The reversible partial ionisation of a weak acid in water, represented with a double arrow (↔).
2.8	Reaction of a carboxylic acid with an alcohol under acid catalyst to form an ester and water.
2.9	Base hydrolysis of an ester yielding a carboxylate salt and an alcohol (soap-making).
2.10	The withdrawal of electron density through sigma bonds by an electronegative substituent, increasing acid strength.

Stuck? Revisit the lesson Key Terms panel and Cards 3–4.

3. True or false — with correction

Circle T or F. If false, write the corrected statement on the line below. 10 marks (1 T/F, 1 correction each)

3.1 The carboxyl group (−COOH) contains two O−H bonds, one on the carbonyl oxygen and one on the hydroxyl oxygen. T / F

3.2 Carboxylic acids have higher boiling points than alcohols of the same chain length because the −COOH group forms dimers with two simultaneous hydrogen bonds per pair. T / F

3.3 Ethanoic acid is a strong acid because it fully ionises in water. T / F

3.4 The NaHCO₃ test distinguishes carboxylic acids from alcohols and phenols because only carboxylic acids produce CO₂ gas. T / F

3.5 Acid strength follows the order: alcohol > phenol > carboxylic acid. T / F

Stuck? Revisit Cards 1, 2, 3 and 4 and the misconceptions box.

4. Function recall

Answer each prompt in 1–2 sentences using precise chemical terms. 8 marks (2 each)

4.1 What is the role of the C=O oxygen in the −COOH group with respect to hydrogen bonding?

4.2 Why does adding NaHCO₃ to a carboxylic acid produce effervescence (bubbles)?

4.3 What products form when a carboxylic acid reacts with a reactive metal such as magnesium?

4.4 Why do short-chain carboxylic acids (C1–C4) dissolve readily in water?

Stuck? Revisit Cards 1 and 3.

5. Fill-the-blank paragraph

Complete the passage using the word bank below. Each word is used once. 10 marks

Word bank: carboxyl • dimer • weak • resonance • alkoxide • effervescence • NaHCO₃ • pKa • esterification • partially

The __________ group (−COOH) consists of a carbonyl and a hydroxyl on the same carbon. Carboxylic acids are classified as __________ acids because they only __________ ionise in water; their Ka value is much less than 1. The __________ scale provides a convenient measure of acid strength — a lower value means a stronger acid. The carboxylate anion (RCOO⁻) is more stable than the __________ (RO⁻) because __________ delocalises the negative charge across both oxygen atoms. When added to a solution containing __________, carboxylic acids produce CO₂ gas (__________ ), which is the key diagnostic test. Two −COOH molecules can associate as a __________, held together by two simultaneous H-bonds. The reaction of a carboxylic acid with an alcohol in the presence of concentrated H₂SO₄ is called __________.

Stuck? Revisit Cards 1, 3 and 4 and the Formula Panel.

6. Build a concept map

Draw labelled arrows between the six terms to show how they connect. Each arrow must carry a linking phrase. Aim for at least 6 labelled arrows. 5 marks

Supplied terms: −COOH group • dimer • high boiling point • carboxylate anion (RCOO⁻) • resonance stabilisation • weak acid

−COOH group

dimer

high boiling point

RCOO⁻

resonance stabilisation

weak acid

Think: −COOH group → forms dimer → requires more energy to vaporise → high BP; and −COOH → ionises partially → forms RCOO⁻ → stabilised by resonance → makes it a weak acid.

Answers — Do not peek before attempting

Q1 — Labelled diagram

A: carbonyl bond (C=O). B: O−H bond (hydroxyl). C: sp² (trigonal planar). D: approximately 120°. E: dimer. F: −oic acid. G: acceptor (the C=O oxygen accepts H-bonds; it does not donate).

Q2 — Term–definition matches

2.1 carboxyl group • 2.2 weak acid • 2.3 Ka • 2.4 carboxylate anion • 2.5 dimer • 2.6 pKa • 2.7 ionisation equilibrium • 2.8 esterification • 2.9 saponification • 2.10 inductive effect.

Q3 — True / False

3.1 False. The carboxyl group has only ONE O−H bond. The carbonyl (C=O) oxygen has no attached hydrogen — it accepts H-bonds only.

3.2 True.

3.3 False. Ethanoic acid is a weak acid — it only partially ionises in water (Ka = 1.8 × 10⁻⁵; approximately 1% ionised at 0.1 mol/L).

3.4 True.

3.5 False. The correct order is: carboxylic acid > phenol > alcohol (pKa ~5 < ~10 < ~16). A lower pKa means a stronger acid.

Q4 — Function recall

4.1 The C=O oxygen acts as a hydrogen-bond acceptor — its lone pairs attract the δ+ hydrogen of another molecule's O−H bond. It cannot donate H-bonds because it has no attached hydrogen.

4.2 The carboxylic acid (pKa ~5) donates H⁺ to HCO₃⁻, which forms carbonic acid (H₂CO₃) that immediately decomposes to H₂O and CO₂ gas — this is the effervescence.

4.3 A metal carboxylate salt (e.g. magnesium ethanoate) and hydrogen gas (H₂). For Mg: 2RCOOH + Mg → (RCOO)₂Mg + H₂.

4.4 The −COOH group forms strong hydrogen bonds with water molecules and the carboxylate ion produced by partial ionisation is fully solvated by water. The short alkyl chain (low carbon number) does not provide enough non-polar character to overcome the H-bonding with water.

Q5 — Cloze answers (in order of blanks)

carboxyl • weak • partially • pKa • alkoxide • resonance • NaHCO₃ • effervescence • dimer • esterification.

Q6 — Sample concept map

Correct maps should include arrows such as:

−COOH group — forms → dimer
dimer — requires more energy to vaporise, causing → high boiling point
−COOH group — partially ionises to give → RCOO⁻
RCOO⁻ — is stabilised by → resonance stabilisation
resonance stabilisation — makes −COOH a → weak acid
weak acid — only partially ionises, coming from → −COOH group

Any biologically valid linking phrases are accepted. Award full marks for at least 6 correctly labelled arrows that respect causal direction.