4.8 Acid-Base Reactions

Acid-Base Reactions

Before and after compound cards show H⁺ transferring from acid to base for 9 reactions. Conjugate pairs are labeled below, with pH outcome explained by reactant strength.

H⁺ TransferBrønsted-Lowry4 ReactionsConjugate Pairs
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 →