Chemistry • Year 12 • Module 6 • Lesson 17
Titration & Indicator Mastery
Consolidate the core vocabulary, indicator selection rules, and five-region curve identification needed before tackling data and extended-response questions.
1. Term–definition match
Match each term to its definition. Write the correct term in the right-hand column. Terms (use each once): equivalence point, endpoint, half-equivalence point, pKa, phenolphthalein, methyl orange, buffer region, indicator transition range, base hydrolysis, concordant results. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | The stoichiometric point at which moles of acid exactly equal moles of base; identified by volume as the midpoint of the steepest section of the pH jump. | |
| 1.2 | The experimentally observed colour change of an indicator during a titration; in an ideal titration this coincides with the equivalence point. | |
| 1.3 | The point on a weak acid titration curve where exactly half the acid has been neutralised; at this point [HA] = [A⁻] and pH = pKa. | |
| 1.4 | The negative logarithm of the acid dissociation constant; read from the pH at VEP/2 on a weak acid titration curve. | |
| 1.5 | An acid–base indicator with transition range pH 8.3–10.0; suitable when the equivalence point pH is above 7, such as in a weak acid + strong base titration. | |
| 1.6 | An acid–base indicator with transition range pH 3.1–4.4; suitable when the equivalence point pH is below 7, such as in a strong acid + weak base titration. | |
| 1.7 | The flat plateau region on a weak acid titration curve before the equivalence point where both HA and A⁻ coexist and pH changes slowly. | |
| 1.8 | The pH range over which an indicator changes colour; must encompass the equivalence point pH for accurate endpoint detection. | |
| 1.9 | The reaction of a conjugate base anion with water: A⁻(aq) + H₂O(l) ⇌ HA(aq) + OH⁻(aq); explains why weak acid + strong base EP pH is above 7. | |
| 1.10 | Two or more titre readings that agree to within 0.10 mL of each other; used to calculate a reliable mean titre in a standard titration procedure. |
2. True or false — with correction
Circle T or F. If the statement is false, write the corrected version on the line provided. 10 marks (1 T/F + 1 correction where needed)
2.1 The equivalence point of a weak acid + strong base titration always occurs at pH 7 because the reaction produces a neutral salt. T / F
2.2 The pKa of a weak acid is found by reading the pH at the half-equivalence point volume on the titration curve. T / F
2.3 For a weak acid + strong base titration, methyl orange (pH 3.1–4.4) is a suitable indicator because it gives a vivid colour change that is easy to see. T / F
2.4 The buffer region for a weak acid + strong base titration appears before the equivalence point on the curve. T / F
2.5 The endpoint of a titration and the equivalence point are the same thing, so the terms can be used interchangeably in a practical report. T / F
3. Complete the passage
Fill each blank with the correct word or phrase from the word bank. Each term is used once. 8 marks
Word bank: phenolphthalein • base hydrolysis • equivalence point • half-equivalence point • buffer region • pKa • methyl orange • pH 7
A weak acid titrated with a strong base produces a characteristic S-shaped curve. Before the , the curve shows a where pH changes only slowly because both HA and A⁻ are present. When exactly half the acid has been neutralised, the curve is at the , and the pH at this point equals the of the acid. At the equivalence point the pH is above because the salt of a weak acid undergoes , producing OH⁻ ions. The appropriate indicator for this titration is (range 8.3–10.0), not (range 3.1–4.4) which transitions far below the equivalence point.
Stuck? Work through the Six-Question Framework in Card 2 of Lesson 17.4. Function recall
Answer each in 1–2 sentences using precise terms. 8 marks (2 each)
4.1 What is the function of the buffer region on a weak acid titration curve, and why does it disappear after the equivalence point?
4.2 What does it mean for an indicator’s transition range to “encompass” the equivalence point pH, and why is visual colour clarity irrelevant to indicator selection?
4.3 What is the function of the three-step indicator justification required in HSC answers, and what are the three steps?
4.4 Explain in one sentence why a strong acid + strong base titration allows any of the three standard indicators (methyl orange, BTB, phenolphthalein) to be used, whereas a weak acid + strong base titration does not.
5. Build a concept map
Draw labelled arrows between the six terms below to show how they connect in a weak acid + strong base titration. Each arrow must carry a linking phrase. Aim for at least 6 labelled arrows. 6 marks
Supplied terms: equivalence point • half-equivalence point • pKa • indicator transition range • base hydrolysis • EP pH above 7.
Q1 — Term–definition matches
1.1 equivalence point • 1.2 endpoint • 1.3 half-equivalence point • 1.4 pKa • 1.5 phenolphthalein • 1.6 methyl orange • 1.7 buffer region • 1.8 indicator transition range • 1.9 base hydrolysis • 1.10 concordant results.
Q2 — True/false with correction
2.1 False. The equivalence point of a weak acid + strong base titration occurs above pH 7 because the conjugate base (A⁻) undergoes base hydrolysis, producing OH⁻ ions. Only a strong acid + strong base titration gives an EP at pH 7 (neutral salt, no hydrolysis).
2.2 True.
2.3 False. Visual clarity is irrelevant to indicator selection. Methyl orange is unsuitable because its transition range (pH 3.1–4.4) does not encompass the equivalence point pH, which is above 7 for a weak acid + strong base titration. Using methyl orange would give a false endpoint far below the equivalence point, causing severe underestimation of acid concentration.
2.4 True.
2.5 False. The equivalence point is the calculated stoichiometric point; the endpoint is the experimentally observed indicator colour change. A good indicator makes these coincide in practice, but they are conceptually distinct. Confusing them in a practical report is an error.
Q3 — Cloze answers (in order)
equivalence point • buffer region • half-equivalence point • pKa • pH 7 • base hydrolysis • phenolphthalein • methyl orange.
Q4 — Function recall
4.1 The buffer region contains both HA and A⁻, which resist pH changes by consuming added OH⁻ or H⁺; after the equivalence point all HA has been converted to A⁻ and excess NaOH dominates, so there is no HA/A⁻ pair to buffer pH changes.
4.2 “Encompass” means the indicator’s transition range includes the pH value at the equivalence point, so the colour change occurs when stoichiometric neutralisation is complete. Visual clarity is irrelevant because a vivid colour change at the wrong pH gives an incorrect titre; the only criterion is whether the transition range overlaps the EP pH.
4.3 The three-step justification provides the reasoning chain examiners require: (1) State the EP pH and explain why it is above/at/below 7 using salt hydrolysis; (2) Name the indicator and its transition range; (3) Confirm the transition range overlaps the EP pH. This prevents marks being lost for selecting the right indicator without proof of reasoning.
4.4 A strong acid + strong base titration produces a very large pH jump (spanning approximately pH 4–10), so all three indicator ranges fall within the jump; for a weak acid + strong base titration the jump is smaller and centred above pH 7, so only indicators with ranges above 7 (e.g. phenolphthalein) are valid.
Q5 — Sample concept map arrows
- equivalence point —pH at EP is determined by→ base hydrolysis
- base hydrolysis —produces OH⁻, making→ EP pH above 7
- EP pH above 7 —dictates that→ indicator transition range must be above 7
- equivalence point —volume is doubled to find→ half-equivalence point
- half-equivalence point —pH at this volume equals→ pKa
- pKa —is an intrinsic property distinct from→ EP pH above 7
Award 1 mark per correctly labelled arrow with valid linking phrase and correct causal direction. Six marks for six valid arrows.