Chemistry • Year 12 • Module 7 • Lesson 18
Organic Acids & Bases: pKa, Strength & Reactions
Lock in the pKa scale, Brønsted–Lowry definitions, diagnostic reactions, and the base-strength ranking of amines before tackling more complex problems.
1. Term–definition match
The definitions below are shuffled. Write the matching term from this list in the right-hand column: pKa, Ka, Brønsted–Lowry acid, Brønsted–Lowry base, conjugate base, inductive effect, carboxylate ion, zwitterion, amide, pKb. 10 marks (1 each)
| # | Definition | Your term |
|---|---|---|
| 1.1 | A species that donates a proton (H⁺) to another species in a chemical reaction. | |
| 1.2 | A species that accepts a proton (H⁺) from another species in a chemical reaction. | |
| 1.3 | The equilibrium constant for the partial ionisation of a weak acid: HA + H₂O ⇌ A⁻ + H₃O⁺. | |
| 1.4 | A logarithmic measure of acid strength: equals −log₁₀(Ka). A lower value means a stronger acid. | |
| 1.5 | The species remaining after an acid donates its proton; for ethanoic acid, this is the ethanoate ion CH₃COO⁻. | |
| 1.6 | The RCOO⁻ ion produced when a carboxylic acid loses its acidic proton; stabilised by resonance across two equivalent oxygen atoms. | |
| 1.7 | The withdrawal of electron density through sigma bonds by electronegative atoms (e.g. Cl, F) that stabilises the conjugate base of a nearby carboxylic acid, increasing its acid strength. | |
| 1.8 | A molecule carrying both a positive and a negative charge simultaneously; amino acids exist in this form at their isoelectric point. | |
| 1.9 | An organic functional group in which N is bonded directly to a carbonyl carbon (C=O); the lone pair on N is fully delocalised into the C=O, making it essentially non-basic. | |
| 1.10 | A logarithmic measure of base strength: equals −log₁₀(Kb). A lower value means a stronger base. |
2. True or false — with correction
Circle T or F. If false, write the corrected statement. 10 marks (1 T/F + 1 correction where needed)
2.1 Carboxylic acids are classified as strong acids because they react with NaHCO₃ solution to produce CO₂. T / F
2.2 A lower pKa value means a stronger acid — each unit decrease in pKa represents a 10-fold increase in acid strength. T / F
2.3 Phenol (pKa ≈ 10) reacts with sodium hydrogen carbonate solution to produce CO₂ gas, just like ethanoic acid. T / F
2.4 Sodium metal reacts with all three classes of organic compound — carboxylic acids, phenols, and alcohols — to produce hydrogen gas. T / F
2.5 Arylamines (e.g. aniline) are stronger bases than alkylamines (e.g. ethylamine) because the benzene ring donates electrons to nitrogen. T / F
3. Fill-the-blank paragraph
Complete the passage by writing one word or value from the word bank into each blank. Each word is used once. 8 marks
Word bank: lower, resonance, stronger, stabilised, 6.4, delocalised, pKa, weaker
The acid strength of an organic compound is measured by its __________ value. A __________ pKa corresponds to a stronger acid. Carboxylic acids (pKa ~4–5) are the __________ organic acid class in Module 7. The reason for their relatively low pKa is that the conjugate base (carboxylate, RCOO⁻) is __________ by __________: the negative charge is __________ across two equivalent oxygen atoms. This makes the carboxylate a __________ base than the alkoxide (RO⁻), where no resonance is possible. The NaHCO₃ test distinguishes carboxylic acids from other organic acids because only acids stronger than H₂CO₃ (pKa __________) produce CO₂ gas.
4. Rank the compounds by acid strength
Complete the table by writing the approximate pKa value and placing each compound in the correct position in the acid-strength ranking (1 = strongest acid, 6 = weakest). Identify the compound class in the final column. 12 marks (2 per row)
| Compound | Approximate pKa | Rank (1–6) | Class |
|---|---|---|---|
| Chloroacetic acid (ClCH₂COOH) | |||
| Ethanoic acid (CH₃COOH) | |||
| Propanoic acid (CH₃CH₂COOH) | |||
| Aspirin (acetylsalicylic acid) | |||
| Phenol (C₆H₅OH) | |||
| Ethanol (CH₃CH₂OH) |
Reference pKa values: chloroacetic acid 2.86; ethanoic acid 4.76; propanoic acid 4.87; aspirin 3.5; phenol ~10; ethanol ~16.
Stuck? Revisit lesson Key Formulas panel — lower pKa = stronger acid. Note that electron-withdrawing Cl lowers pKa.5. Build a concept map
Draw labelled arrows between the six terms below to show cause-and-effect relationships. Each arrow must carry a linking phrase. Aim for at least 5 labelled arrows. 5 marks
Terms: electron-withdrawing group · conjugate base stability · pKa value · acid strength · resonance delocalisation · NaHCO₃ reaction
Q1 — Term–definition matches
1.1 Brønsted–Lowry acid • 1.2 Brønsted–Lowry base • 1.3 Ka • 1.4 pKa • 1.5 conjugate base • 1.6 carboxylate ion • 1.7 inductive effect • 1.8 zwitterion • 1.9 amide • 1.10 pKb
Q2 — True / false
2.1 False. Carboxylic acids are weak acids (Ka << 1, only ~1–2% ionised). They are the strongest organic acid class in Module 7, but reacting with NaHCO₃ does not make them strong acids by definition.
2.2 True.
2.3 False. Phenol (pKa ≈ 10) is a weaker acid than H₂CO₃ (pKa 6.4), so the reaction is thermodynamically unfavourable — no CO₂ is produced. Only acids stronger than H₂CO₃ (pKa < 6.4) react with NaHCO₃.
2.4 True. Na metal reacts with any O–H bond to produce H₂ gas. All three classes contain O–H bonds, so Na cannot distinguish between them.
2.5 False. Arylamines are weaker bases than alkylamines. The benzene ring withdraws the nitrogen lone pair into its pi system via resonance (delocalisation), making it less available to accept a proton. This reduces base strength — aniline Kb ≈ 4 × 10⁻¹⁰ vs ethylamine Kb ≈ 4 × 10⁻⁴.
Q3 — Cloze answers (in order)
pKa; lower; stronger; stabilised; resonance; delocalised; weaker; 6.4
Q4 — pKa ranking
| Compound | pKa | Rank | Class |
|---|---|---|---|
| Chloroacetic acid | 2.86 | 1 (strongest) | Halogenated carboxylic acid |
| Aspirin | 3.5 | 2 | Aromatic carboxylic acid |
| Ethanoic acid | 4.76 | 3 | Carboxylic acid |
| Propanoic acid | 4.87 | 4 | Carboxylic acid |
| Phenol | ~10 | 5 | Phenol |
| Ethanol | ~16 | 6 (weakest) | Alcohol |
Note: Chloroacetic acid has a lower pKa than ethanoic acid because the Cl atom withdraws electron density inductively through the carbon chain, stabilising the conjugate base (ClCH₂COO⁻) more than plain CH₃COO⁻.
Q5 — Sample concept map
Accept any combination of five or more biologically valid labelled arrows, e.g.:
- electron-withdrawing group — increases → conjugate base stability
- resonance delocalisation — increases → conjugate base stability
- conjugate base stability — lowers → pKa value
- pKa value — determines → acid strength
- acid strength — predicts → NaHCO₃ reaction (only if pKa < 6.4)