Chemistry • Year 12 • Module 6 • Lesson 16

Titration Curves: Interpreting & Analysing All Four Types

Build the foundational vocabulary and identification skills for all four titration curve types: strong/strong, weak acid/strong base, strong acid/weak base, and weak/weak.

Build · Band 3–4

1. Label the four-curve diagnostic summary

The diagram below shows a schematic of four unlabelled titration curves (A–D) for 0.10 mol/L acid titrated against 0.10 mol/L NaOH, plus a summary table with eight blank cells. Complete the labels A–H using the terms and values provided. 8 marks

Diagram pending — see image-prompts/chem-y12-m6.md #chem-y12-m6-l16-w01-fig1 The diagram will show four overlaid or side-by-side pH-vs-volume sigmoid curves on shared axes (volume of NaOH on x-axis, pH on y-axis). Curve A is a tall symmetric S-shape centred at pH 7. Curve B starts at intermediate pH, shows a flat buffer plateau, and the jump ends above pH 7. Curve C starts at low pH and has a jump ending below pH 7. Curve D starts at intermediate pH and rises gradually with no sharp jump. Each curve has one blank-labelled callout box lettered A–D identifying curve type. A summary table below has four more blank boxes E–H for equivalence point pH values and jump sizes.

A — Curve type (acid + base combination): _______________________
B — Curve type (acid + base combination): _______________________
C — Curve type (acid + base combination): _______________________
D — Curve type (acid + base combination): _______________________
E — Equivalence point pH for Curve A: _______________________
F — Equivalence point pH for Curve B (above or below 7?): _______________________
G — Equivalence point pH for Curve C (above or below 7?): _______________________
H — Does Curve D have a usable indicator? (Yes / No): _______________________

Label	Your answer
A
B
C
D
E
F
G
H

Stuck? Apply the four diagnostic checks in order: (1) starting pH; (2) buffer region before EP; (3) EP pH; (4) jump size.

2. Term–definition match

The ten definitions below are shuffled. In the right-hand column write the matching term from this list: titration curve, equivalence point, half-equivalence point, buffer region, pK_a, phenolphthalein, methyl orange, strong acid + strong base, weak acid + strong base, weak acid + weak base. 10 marks

#	Definition (shuffled)	Matching term
2.1	A graph of pH versus volume of titrant added; its shape encodes acid/base strength, pK_a, and equivalence point location.
2.2	The point on a titration curve where moles of acid exactly equal moles of base; located at the midpoint of the steepest section of the pH jump.
2.3	The point at which exactly half of the weak acid has been neutralised; pH at this point equals pK_a.
2.4	The flat plateau region of a weak acid + strong base curve, before the equivalence point, where HA and A– coexist and resist pH change.
2.5	The negative logarithm of the acid dissociation constant; readable directly from the titration curve at V_EP/2.
2.6	The combination that gives the largest pH jump and an equivalence point at exactly pH 7.00, with no buffer region before or after.
2.7	The combination that gives an equivalence point above pH 7 because the conjugate base hydrolyses water to produce OH–.
2.8	The combination where dual buffer regions overlap throughout; no sharp equivalence point; standard indicators cannot be used reliably.
2.9	An indicator that changes from colourless to faint pink in the range pH 8.3–10.0; suitable for weak acid + strong base titrations.
2.10	An indicator that changes from red to yellow in the range pH 3.1–4.4; suitable for strong acid + strong base and strong acid + weak base titrations (EP at or below pH 7).

Stuck? Revisit the Key Terms panel and Content Cards 1–2 in lesson 16.

3. True or false — with correction

For each statement, circle T or F. If the statement is false, write the corrected version on the line below. 8 marks (1 for T/F, 1 for correction where needed)

3.1 The equivalence point of any acid-base titration is always at pH 7.00. T / F

3.2 At the half-equivalence point of a weak acid + strong base titration, pH = pK_a. T / F

3.3 The strong acid + strong base titration produces the largest pH jump at the equivalence point because strong acids are more reactive. T / F

3.4 A weak acid + weak base titration has no sharp equivalence point, making standard indicator selection unreliable. T / F

Stuck? Revisit Content Card 1 (curve identification), Card 3 (jump size), and Card 4 (misconceptions box).

4. Function recall

Answer each in 1–2 sentences using precise terms from the lesson. 10 marks (2 each)

4.1 What is the function of the half-equivalence point in analysing a weak acid + strong base titration curve?

4.2 What is the function of the buffer region in a weak acid + strong base curve, and why does it reduce the jump size at the equivalence point?

4.3 Why does the equivalence point pH of a weak acid + strong base titration sit above pH 7?

4.4 What is the function of the starting pH of a titration curve in identifying whether the acid is strong or weak?

4.5 What makes phenolphthalein (pH range 8.3–10.0) a suitable indicator for a weak acid + strong base titration but not for a strong acid + weak base titration?

Stuck? Revisit Content Cards 2 (weak acid curve regions), 3 (jump size), and 4 (reading curves; indicator selection).

5. Fill in the blanks

Complete the paragraph using words from the word bank. Each word is used once. 9 marks

Word bank:

pK_a · equivalence · buffer · hydrolysis · phenolphthalein · half-equivalence · conjugate base · sigmoid · steepest

A titration curve is a -shaped graph of pH versus volume of titrant. The point is identified as the midpoint of the section of the pH jump. For a weak acid titrated with strong base, a flat region precedes the equivalence point. At the point (V_EP/2), pH equals the . Above the equivalence point the of the weak acid undergoes , producing OH– and driving the EP pH above 7. The indicator is appropriate for this titration because its transition range (8.3–10.0) falls within the sharp pH jump above pH 7.

Stuck? Revisit Content Card 2 and the formula panel in the lesson.

Answers — Do not peek before attempting

Q1 — Labelled diagram

A: Strong acid + strong base. B: Weak acid + strong base. C: Strong acid + weak base. D: Weak acid + weak base. E: pH 7.00 (exactly). F: Above 7 (conjugate base hydrolyses; typically ~pH 8–9). G: Below 7 (conjugate acid hydrolyses; typically ~pH 5–6). H: No — no usable standard indicator because there is no sharp equivalence point jump.

Q2 — Term–definition matches

2.1 titration curve • 2.2 equivalence point • 2.3 half-equivalence point • 2.4 buffer region • 2.5 pK_a • 2.6 strong acid + strong base • 2.7 weak acid + strong base • 2.8 weak acid + weak base • 2.9 phenolphthalein • 2.10 methyl orange.

Q3 — True/false with correction

3.1 False. Correction: the equivalence point is at pH 7.00 only for strong acid + strong base. For weak acid + strong base the EP is above 7; for strong acid + weak base it is below 7; for weak acid + weak base there is no sharp equivalence point.

3.2 True.

3.3 False. Correction: the large jump in the strong acid + strong base titration is caused by the complete absence of buffer capacity near the equivalence point — not by reactivity. Weak acid titrations go to the same extent of reaction; the buffer region (HA + A– coexisting) moderates the pH change near the EP, reducing the jump.

3.4 True.

Q4.1 — Function of the half-equivalence point

The half-equivalence point (at V_EP/2) is where n(A–) = n(HA); at this point pH = pK_a exactly. Its function in analysis is to allow direct, graphical extraction of the pK_a (and hence K_a) of the weak acid from the titration curve without any calculation beyond identifying the volume.

Q4.2 — Function of the buffer region

The buffer region is the flat plateau before the equivalence point where both HA and A– coexist. Its function is to resist pH change. This reduces the jump size because even very close to the equivalence point some buffer capacity remains — the pH change per drop of titrant is smaller than it would be with no buffer present.

Q4.3 — Why EP pH is above 7 for weak acid + strong base

At the equivalence point only the conjugate base A– remains in solution. A– undergoes base hydrolysis: A– + H₂O ⇌ HA + OH–. The production of OH– makes the solution basic, so pH is above 7.

Q4.4 — Function of starting pH in identification

If the starting pH is low (~1 for 0.10 mol/L), the acid is fully ionised — it is a strong acid. If the starting pH is intermediate (~2.5–4), the acid is only partially ionised — it is a weak acid. The starting pH thus provides the first diagnostic check for curve type identification.

Q4.5 — Why phenolphthalein suits weak acid + strong base but not strong acid + weak base

Phenolphthalein transitions in the range pH 8.3–10.0. For a weak acid + strong base titration, the equivalence point pH is above 7 (~8–9), so phenolphthalein transitions within the sharp jump — it is suitable. For a strong acid + weak base titration, the equivalence point pH is below 7 (~5–6), which is outside phenolphthalein’s range — it would never change colour near the EP and is therefore unsuitable.

Q5 — Fill-in-the-blank answers (in order)

sigmoid • equivalence • steepest • buffer • half-equivalence • pK_a • conjugate base • hydrolysis • phenolphthalein.