Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 14.15 Titration curves-Study Notes - New Syllabus

14.15 Titration Curves (Strong & Weak, Mono- and Diprotic Systems)

Titration curves show how pH changes as a titrant is added. Interpreting them requires understanding acid/base strength, stoichiometry, and (for weak systems) equilibria and buffer regions.

General Features of a Titration Curve

• Initial pH: depends on acid/base strength.
• Buffer region (weak systems): gradual pH change.
• Half-equivalence point: \( \mathrm{pH = pK_a} \) (weak acid).
• Equivalence point: moles acid = moles base.
• Vertical section: rapid pH change near equivalence.

1. Strong Acid vs Strong Base

• Very low initial pH.
• No buffer region.
• Sharp vertical jump.
• Equivalence point at pH = 7.

Interpretation: neutralisation produces neutral salt.

2. Weak Acid vs Strong Base

• Higher initial pH than strong acid.
• Buffer region present.
• Half-equivalence: \( \mathrm{pH = pK_a} \).
• Equivalence point at pH > 7.

Explanation: conjugate base formed hydrolyses to produce \( \mathrm{OH^-} \).

3. Weak Base vs Strong Acid

• High initial pH (but lower than strong base).
• Buffer region present.
• Equivalence point at pH < 7.

Explanation: conjugate acid formed produces \( \mathrm{H^+} \).

4. Strong Base vs Strong Acid

• Mirror of strong acid vs strong base curve.
• Initial pH very high.
• Equivalence at pH = 7.

Diprotic Acids (e.g. \( \mathrm{H_2A} \))

Diprotic acids donate two protons, so titration curves show two stages.

Key Features

• Two equivalence points.
• Two buffer regions.
• Two \( \mathrm{pK_a} \) values ( \( \mathrm{pK_{a1}} \), \( \mathrm{pK_{a2}} \) ).

Equations

\( \mathrm{H_2A \rightleftharpoons H^+ + HA^-} \)

\( \mathrm{HA^- \rightleftharpoons H^+ + A^{2-}} \)

Diprotic Bases

• Accept two protons.
• Show two equivalence points when titrated with acid.

Interpreting Curves

• Identify number of equivalence points → number of ionisable protons.
• Determine acid strength from initial pH and buffer region.
• Use half-equivalence to estimate \( \mathrm{pK_a} \).
• Compare equivalence pH to identify strong/weak systems.

Indicator Choice

• Choose indicator whose pH range lies within steep vertical region.
• Strong acid–base: wide choice.
• Weak systems: must match equivalence pH.

Key Features

• Curve shape depends on acid/base strength.
• Weak systems show buffer regions.
• Diprotic systems show multiple equivalence points.
• Half-equivalence gives \( \mathrm{pK_a} \).