6.1.8Additional lessons
pH Curves — IB Chemistry SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 83 of 98
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Reading a pH Curve
The curve plots pH against the volume of titrant added from a burette.
Starting pH
- Starting pH A strong acid starts at a low pH, such as 1, while a weak acid of the same concentration starts higher.
Buffer region
- Buffer region A weak acid titrated with a strong base shows a gradual rise, where pH changes little as base is added.
Equivalence point
- Equivalence point The point where the moles of acid and base are exactly equal, found at the middle of the steep section.
End point
- End point The point at which the indicator changes colour, chosen to coincide with the equivalence point.
Part 2 of 5pH at Equivalence
pH at Equivalence
Strong acid and strong base
- Strong acid and strong base The pH at equivalence is 7, and the vertical section is large, covering about pH 3 to 11.
Weak acid and strong base
- Weak acid and strong base The pH at equivalence is above 7 because the conjugate base of the acid is basic.
Strong acid and weak base
- Strong acid and weak base The pH at equivalence is below 7 because the conjugate acid of the base is acidic.
Weak acid and weak base
- Weak acid and weak base There is no sharp vertical section, so no indicator gives a clear end point.
| Titration | pH at equivalence | Suitable indicator |
|---|---|---|
| Strong acid + strong base | 7 | Methyl orange or phenolphthalein |
| Weak acid + strong base | above 7 | Phenolphthalein |
| Strong acid + weak base | below 7 | Methyl orange |
| Weak acid + weak base | about 7 | None suitable |
Part 3 of 5Choosing an Indicator
1
Decide the type of titration and estimate the pH at equivalence.
2
Find an indicator whose colour-change range lies on the steep part of the curve.
3
Phenolphthalein changes colour at roughly pH 8–10, so suits weak acid with strong base.
4
Methyl orange changes colour at roughly pH 3–4.5, so suits strong acid with weak base.
5
A pH meter or data logger can be used when no indicator is suitable.
Part 4 of 5Worked Example
Question
- A student titrates 25.0 cm³ of 0.100 mol dm⁻³ ethanoic acid with 0.100 mol dm⁻³ NaOH.
- Predict the volume at equivalence, the pH region and a suitable indicator.
- EquationCH₃COOH + NaOH → CH₃COONa + H₂O, a 1:1 ratio, with equal concentrations.
- So the equivalence volume of NaOH is 25.0 cm³.
- The salt contains ethanoate ions, a weak base, so the pH at equivalence is above 7.
Answer
- 25.0 cm³ of NaOH;
- pH above 7;
- phenolphthalein is suitable.
Part 5 of 5Exam Checklist & Tips
6.1.8 – Key Points for the Exam
- Equivalence point The point at which the acid and base have reacted in exactly the amounts shown by the equation.
- Buffer region The gently sloping part of a weak acid curve where pH changes little.
- Indicator A weak acid or base whose conjugate forms have different colours.
- End point The volume at which the indicator changes colour.
Common Mistakes
- Stating that the equivalence point of every titration is pH 7.
- Using phenolphthalein for strong acid with weak base, where the colour change comes too late.
- Starting a weak acid curve at pH 1.
- Confusing end point with equivalence point; the indicator is chosen so they coincide.
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