An acid is not defined by tasting sour or turning litmus red. In the Brønsted–Lowry theory it is anything that donates a proton, and every acid-base reaction is a proton changing hands between two conjugate pairs.
A Brønsted–Lowry acid is a proton (H+) donor; a base is a proton acceptor. When an acid donates a proton it becomes its conjugate base; when a base accepts one it becomes its conjugate acid. Each pair differs by one H+. A strong acid dissociates fully in water (HCl, HNO3, H2SO4); a weak acid dissociates only partially, setting up an equilibrium (CH3COOH). Strength is about the extent of dissociation, not concentration.
In the simulator, drop acids and bases into water and follow the protons. Identify the conjugate pair on each side before the labels appear.
| Reaction | Acid | Base | Conjugate acid | Conjugate base |
|---|---|---|---|---|
| HCl + H2O → H3O+ + Cl− | HCl | H2O | H3O+ | Cl− |
| NH3 + H2O ⇌ NH4+ + OH− | H2O | NH3 | NH4+ | OH− |
| CH3COOH + H2O ⇌ CH3COO− + H3O+ | CH3COOH | H2O | H3O+ | CH3COO− |
Water acts as a base with HCl and as an acid with NH3: it is amphoteric.
Compare 0.1 mol dm−3 hydrochloric acid with 0.1 mol dm−3 ethanoic acid. Same concentration of acid, very different concentration of H+.
| Test (same concentration) | Strong acid (HCl) | Weak acid (CH3COOH) |
|---|---|---|
| pH | about 1 | about 3 |
| electrical conductivity | high | low |
| rate with magnesium | fast fizzing | slow fizzing |
| total H2 from excess Mg | same | same (the equilibrium keeps shifting) |
Strong and concentrated mean different things. Strong means fully dissociated; concentrated means a lot of acid per dm3. A dilute solution of HCl is still a strong acid. Also, in a conjugate pair the formulae differ by exactly one H+: the conjugate base of H2SO4 is HSO4−, not SO42−.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
A Brønsted–Lowry acid is:
What is the conjugate base of H2O?
A weak acid is one that:
In NH3 + H2O ⇌ NH4+ + OH−, water is acting as:
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
In HNO3 + H2SO4 ⇌ H2NO3+ + HSO4−, which species acts as a base?
Which is a conjugate acid-base pair?
Equal volumes of 0.10 mol dm−3 HCl and 0.10 mol dm−3 CH3COOH react with excess magnesium. Which statement is correct?
Which statement best describes 0.001 mol dm−3 nitric acid?
This skill is now recorded on your profile. Keep the chain going.