8.1 Acids and Bases

Conjugate Acid-Base Pairs

Step through proton transfer from acid to water with animated arrows and highlighted conjugate pair brackets. Choose from 10 acids and see Kₐ, Kb, and pKₐ resolve.

Proton TransferConjugate PairsKa × Kb = Kw10 Acids
Topic 4.8

Introduction to Acid–Base Reactions

Identify species as Brønsted–Lowry acids, bases, and/or conjugate acid–base pairs, based on proton transfer involving those species.

By definition, a Brønsted–Lowry acid is a proton donor and a Brønsted–Lowry base is a proton acceptor. Everything in Unit 8 rests on that pair of sentences.

Water plays a special role in aqueous solutions: its molecular structure allows it to both accept protons from and donate protons to dissolved species. A substance that can act as either an acid or a base is amphiprotic. Water, HCO₃⁻, HSO₄⁻, and H₂PO₄⁻ are the standard examples.

When an acid or base ionizes in water, the conjugate acid–base pairs can be identified and their relative strengths compared. A conjugate pair differs by exactly one H⁺ — and therefore by exactly one unit of charge.

HA + H₂O ⇌ A⁻ + H₃O⁺
acid base conjugate base conjugate acid

Every proton-transfer reaction has two conjugate pairs: here HA/A⁻ and H₃O⁺/H₂O.

The inverse-strength relationship. The stronger an acid, the weaker its conjugate base. HCl is a strong acid, so Cl⁻ is such a weak base that it is essentially inert in water. Acetic acid is weak, so acetate is a meaningfully basic species. Quantitatively this is Ka × Kb = Kw (see 8.3).

Key points

  • Acid donates H⁺; base accepts H⁺. Conjugates differ by exactly one H⁺ and one unit of charge.
  • Every proton transfer produces two conjugate pairs.
  • Water is amphiprotic and appears on both sides of the definition.
  • Strong acid → very weak conjugate base, and vice versa.

Common mistakes

  • Changing more than the proton. The conjugate base of H₂SO₄ is HSO₄⁻, not SO₄²⁻.
  • Forgetting the charge shift. Removing H⁺ makes the species one unit more negative.
  • Assuming the conjugate base of a strong acid is a strong base. The relationship is inverse.
  • Missing the second conjugate pair. Water/hydronium or water/hydroxide is almost always the other one.

Worked example

For the reaction HCO₃⁻(aq) + HF(aq) ⇌ H₂CO₃(aq) + F⁻(aq), identify the acid, base, and the two conjugate acid–base pairs. Then explain why HCO₃⁻ is described as amphiprotic.

HF is the acid — it donates a proton, becoming F⁻.
HCO₃⁻ is the base — it accepts that proton, becoming H₂CO₃.

Conjugate pairs:
HF / F⁻ (acid / conjugate base)
H₂CO₃ / HCO₃⁻ (conjugate acid / base)

Amphiprotic: HCO₃⁻ acts as a base here by accepting a proton to form H₂CO₃. But it can also act as an acid by donating its own proton:
HCO₃⁻ + OH⁻ → CO₃²⁻ + H₂O

Because it can both donate and accept a proton, HCO₃⁻ is amphiprotic — which is exactly why the carbonate/bicarbonate system buffers blood.

Full notes for topic 4.8 →