6.1.2Additional lessons
Conjugate Acids and Bases — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 117 of 172
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
Part 1 of 4Conjugate Pairs
Reversible proton transfer produces pairs of species that are linked by one H⁺.
Definition
- A conjugate pair consists of two species that differ by exactly one proton, H⁺.
Conjugate base
- When an acid loses a proton, the species left behind is its conjugate base.
Conjugate acid
- When a base gains a proton, the species formed is its conjugate acid.
Two pairs
- Every acid–base reaction involves two conjugate pairs, one on each side of the equation.
Part 2 of 4Examples
Examples
Ethanoic acid
- In CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺ the pairs are CH₃COOH/CH₃COO⁻ and H₃O⁺/H₂O.
Ammonia
- In NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ the pairs are NH₃/NH₄⁺ and H₂O/OH⁻.
Water
- Water forms H₃O⁺ as the conjugate acid and OH⁻ as the conjugate base, depending on the partner.
Acid on left
- The acid on the left of the equation always gives a conjugate base on the right.
| Acid | Conjugate base |
|---|---|
| HCl | Cl⁻ |
| H₂SO₄ | HSO₄⁻ |
| H₂CO₃ | HCO₃⁻ |
| NH₄⁺ | NH₃ |
| H₃O⁺ | H₂O |
Part 3 of 4Strength Relationship
1
A strong acid donates its proton very readily, so its conjugate base is a very weak base with little tendency to take a proton back.
2
Chloride ions from HCl are an example: they are so weakly basic that they do not react with water noticeably.
3
A weak acid donates protons only partly, so its conjugate base is a stronger base than the base from a strong acid.
4
A strong base accepts protons strongly, so its conjugate acid is very weak.
5
To write a conjugate partner, add H⁺ to the species to find its conjugate acid, or remove H⁺ to find its conjugate base.
Part 4 of 4Exam Checklist & Tips
6.1.2 – Key Points for the Exam
- Conjugate acid The species formed when a base accepts a proton.
- Conjugate base The species formed when an acid donates a proton.
- Conjugate pair Two species that differ by one H⁺.
- Proton donor Another name for a Brønsted–Lowry acid.
Common Mistakes
- Moving an electron instead of a proton when writing the conjugate partner.
- Forgetting to change the charge when adding or removing H⁺.
- Saying that the conjugate base of a strong acid is also strong.
- Pairing species that differ by more than one proton, such as H₃O⁺ and OH⁻.
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