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8.5Acids and Bases

Acid-Base Titrations — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 10 parts Lesson 10 of 16 2 quizzes

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Lesson summary

8.5 Acid-Base Titrations — AP CHEMISTRY notes: Interpret titration curves, equivalence and half-equivalence points and choose indicators.Shape — S-shaped plot of pH against volume of titrantEquivalence point — moles of titrant equal the moles of analyte, at the middle of the stee…

  • Calculation
  • Buffer region
  • Ionisation
  • Weak acid indicator
  • Weak acid + strong base
  • Strong acid + weak base

Section 1 of 10Features of a titration curve

Features of a titration curve
Shape
  • S-shaped plot of pH against volume of titrant
Equivalence point
  • Equivalence point moles of titrant equal the moles of analyte, at the middle of the steep section
Readings
  • Readings starting pH, equivalence volume, pH at equivalence and the range of the vertical section
Calculation
  • Calculation for monoprotic acid and base, C1V1 = C2V2

Section 2 of 10Types of titration

TitrationStarting pHpH at equivalence
Strong acid + strong baseAbout 17
Weak acid + strong baseAbout 3Above 7
Weak base + strong acidAbout 11Below 7
Weak acid + weak base—About 7; no sharp change, not performed

Section 3 of 10Half-equivalence point

Weak acid
  • [HA] = [A−], so pH = pKa
Weak base
  • [B] = [BH+], so pOH = pKb
Buffer region
  • Buffer region the gradual part of the curve before equivalence

Section 4 of 10Polyprotic acids

Ionisation
  • H3PO4 ionises in three steps
Curve
  • Curve one equivalence point per ionisable H, with buffer regions between them
Midpoints
  • Midpoints pH = pKa at each half-equivalence point

Section 5 of 10Indicators

Weak acid indicator
  • HIn ⇌ H+ + In−, with different colours for each form
Range
  • Range colour changes over about pKa ± 1; at the end point pH = pKa
Choice
  • Choice the indicator range must lie in the steep section of the titration curve

Section 6 of 10Examples of indicators

IndicatorpKaColour change range
Methyl orange3.73.1–4.4
Methyl red5.14.4–6.2
Phenolphthalein9.38.3–10.0

Section 7 of 10Matching indicators

1Weak acid + strong base
  • Weak acid + strong base phenolphthalein
2Strong acid + weak base
  • Strong acid + weak base methyl red
3Strong acid + strong base
  • Strong acid + strong base methyl red or phenolphthalein
4Weak acid + weak base
  • Weak acid + weak base no suitable indicator

Section 8 of 10More from the specification

1Titration of polyprotic acids in solution
  • Titration of polyprotic acids in solution titrating polyprotic acids with standard basic solution produces multiple equivalence points corresponding to successive proton losses.
2Polyprotic acid
  • Acid with several ionisable hydrogens.
3Alkali
  • A soluble base that produces OH− ions in water.
4Hydroxide
  • A compound containing the OH− ion, formed by Group I metals with water.

Section 9 of 10Key terms

TermMeaning
Equivalence pointMoles of titrant equal to moles of analyte.
Half-equivalence pointHalf the analyte has reacted; pH = pKa for a weak acid.
End pointPoint at which the indicator changes colour.
Polyprotic acidAcid with several ionisable hydrogens.
IndicatorWeak acid or base with differently coloured forms.

Section 10 of 10Exam technique

Avoid these mistakes
Assuming equivalence point at pH 7 for all titrations.
Reading pKa from the equivalence point rather than the half-equivalence point.
Choosing an indicator whose range misses the vertical section.

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