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8.2.3Acids and Bases

Relative Strength of Acids and Bases — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 5 parts Lesson 18 of 21 Unit quiz

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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.
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Part 1 of 5Measuring Strength

Strength describes how far an acid ionizes or a base accepts protons, not how concentrated it is.

Ka and Kb
  • Ka and Kb A larger Ka means a stronger acid and a larger Kb means a stronger base, since more ionization occurs.
pKa
  • pKa pKa = −log Ka, so a smaller pKa corresponds to a stronger acid.
Conjugate relationship
  • Conjugate relationship The stronger an acid, the weaker its conjugate base, and the product Ka × Kb equals Kw for the pair.
Strong species
  • Strong species Strong acids and bases ionize essentially completely, so their conjugates are negligible bases or acids.
Strength versus concentration
  • Strength versus concentration A dilute solution of a strong acid can have a higher pH than a concentrated solution of a weak acid.

Part 2 of 5Why Acids Differ in Strength

Three linked ideas explain most trends, and all concern how easily the proton leaves.

Why Acids Differ in Strength
Bond polarity
  • Bond polarity In a series with the same type of bond, a more polar H–A or O–H bond releases the proton more easily.
Bond strength
  • Bond strength Down Group 17 the H–X bond lengthens and weakens, so acid strength rises: HF is weak but HCl, HBr and HI are strong.
Conjugate base stability
  • Conjugate base stability If the conjugate base spreads its negative charge well, the acid is stronger, as in oxyacids with more oxygen atoms.
Inductive effect
  • Inductive effect Electronegative atoms near the acidic group pull electron density away and stabilise the conjugate base.
AcidApproximate pKaReason for trend
Ethanoic acid4.8Methyl group donates electron density
Chloroethanoic acid2.9One electronegative Cl withdraws charge
Trichloroethanoic acid0.7Three Cl atoms stabilise the anion
HClO / HBrO / HIO7.5 / 8.7 / 10.7Electronegativity of the halogen falls

Part 3 of 5Comparing Acids and Bases in Practice

Follow these steps to rank substances or to predict the direction of a proton transfer.

1
Identify the acid and base in each species and write the conjugate pair that each forms after donating or accepting a proton.
2
Compare Ka values or pKa values, remembering that the lower pKa belongs to the stronger acid.
3
For two bases, compare Kb values, or use the fact that the base whose conjugate acid is weaker is the stronger base.
4
Predict that the equilibrium lies on the side of the weaker acid and weaker base.
5
For oxyacids, count the oxygen atoms not bonded to hydrogen: the more there are, the stronger the acid, as in HClO₄ over HClO.

Part 4 of 5Worked Example

Question

  • A weak acid HA has Ka = 4.0 × 10⁻⁵ at 25 °C.
  • Find the pKa of HA and the Kb and pKb of A⁻.

  1. pKa = −log(4.0 × 10⁻⁵) = 4.40.
  2. Kb = Kw ÷ Ka = 1.0 × 10⁻¹⁴ ÷ 4.0 × 10⁻⁵ = 2.5 × 10⁻¹⁰.
  3. pKb = 14.00 − 4.40 = 9.60, which agrees with −log(2.5 × 10⁻¹⁰).

Answer

  • pKa = 4.40;
    • Kb of A⁻ = 2.5 × 10⁻¹⁰;
    • pKb = 9.60

Part 5 of 5Exam Checklist & Tips

8.2.3 – Key Points for the Exam

  • Ka The equilibrium constant for the ionization of an acid in water.
  • Kb The equilibrium constant for a base reacting with water.
  • pKa The negative logarithm of Ka, where lower values mean stronger acids.
  • Conjugate base The species left after an acid donates a proton.
  • Inductive effect The withdrawal or donation of electron density through sigma bonds by nearby atoms.

Common Mistakes

  • Assuming a concentrated solution means a strong acid, when strength refers to the extent of ionization.
  • Reversing the pKa trend and calling the acid with the larger pKa the stronger.
  • Judging HF as the strongest hydrohalic acid because of its high bond polarity, ignoring its strong H–F bond.
  • Forgetting that a strong acid has a very weak conjugate base.

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