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6.1.2Additional lessons

Conjugate Acids and Bases — IB Chemistry SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 78 of 98

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Curriculum Equation Key pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: pH + pOH = 14
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Part 1 of 6What Is a Conjugate Pair

6.1.2 Conjugate Acids and Bases

Because proton transfer is reversible, each acid has a conjugate base and each base has a conjugate acid.

Conjugate acid
  • The species formed when a base accepts a proton; it can donate that proton back.
Conjugate base
  • The species left after an acid donates a proton; it can accept a proton again.
One proton difference
  • The two members of a pair differ by exactly one H⁺, so the acid has one more H and one more positive charge.
Two pairs
  • Every Brønsted–Lowry reaction contains two conjugate pairs, one on each side of the ⇌ sign.

Part 2 of 6Common Conjugate Pairs

Common Conjugate Pairs
HCl and Cl⁻
  • Hydrochloric acid loses a proton to leave the chloride ion as its conjugate base.
H₃O⁺ and H₂O
  • The hydronium ion is the conjugate acid of water.
NH₄⁺ and NH₃
  • The ammonium ion is the conjugate acid of ammonia.
CH₃COOH and CH₃COO⁻
  • Ethanoic acid pairs with the ethanoate ion, its conjugate base.
H₂O and OH⁻
  • Water is the conjugate acid of the hydroxide ion.
AcidConjugate base
HNO₃NO₃⁻
H₂CO₃HCO₃⁻
HCO₃⁻CO₃²⁻
NH₄⁺NH₃

Part 3 of 6Finding the Pairs in an Equation

1
Write the reaction, for example CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺.
2
Pick the reactant on the left that loses H⁺: CH₃COOH becomes CH₃COO⁻, forming the first pair.
3
The other reactant gains H⁺: H₂O becomes H₃O⁺, forming the second pair.
4
State each pair as acid/base with the acid first, and check the charges differ by one unit.

Part 4 of 6Strength and Conjugate Pairs

Strength and Conjugate Pairs
Strong acid
  • A strong acid has a very weak conjugate base, because it gives up its proton very readily.
Weak acid
  • A weak acid has a conjugate base that is stronger than the base from a strong acid.
Weak base
  • A weak base has a conjugate acid that is a stronger acid than that of a strong base.

Part 5 of 6Worked Example

Question Identify the two conjugate acid–base pairs in NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq).

  1. NH₃ gains a proton to become NH₄⁺, so NH₄⁺ is the conjugate acid of the base NH₃.
  2. H₂O loses a proton to become OH⁻, so OH⁻ is the conjugate base of the acid H₂O.
  3. Write each pair with the acid first.

Answer Pair 1: NH₄⁺/NH₃. Pair 2: H₂O/OH⁻.

Part 6 of 6Exam Checklist & Tips

6.1.2 – Key Points for the Exam

  • Conjugate acid The species formed when a base gains a proton.
  • Conjugate base The species formed when an acid loses a proton.
  • Conjugate pair Two species that differ by one proton.
  • Proton donor Another name for a Brønsted–Lowry acid.

Common Mistakes

  • Labelling the two reactants as a conjugate pair;
    • the pairs are acid with its own product and base with its own product.
  • Changing the charge by the wrong amount; the acid always has one more positive charge than its base.
  • Writing the conjugate base of H₂SO₄ as SO₄²⁻ rather than HSO₄⁻ when only one proton is lost.
  • Saying that the conjugate base of a strong acid is strong.

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