Brønsted–Lowry theory of acids and bases

AS Level Chemistry · Equilibria. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

In the Brønsted–Lowry theory an acid is a proton (H+) donor and a base is a proton acceptor. An acid can only give a proton if a base is there to take it, so every acid–base reaction is a proton transfer.

A strong acid or base is fully dissociated into ions in aqueous solution. A weak acid or base is only partially dissociated, so an equilibrium exists and far fewer ions are present. Strong and weak describe the degree of dissociation; concentrated and dilute describe how much is dissolved.

In neutralisation, H+(aq) and OH−(aq) form water, and a salt is left. The shape of the pH curve in a titration depends on whether the acid and the alkali are strong or weak.

Rules to remember

Common mistake

Students think a weak acid needs less alkali to neutralise it. Equal moles of HCl and CH3COOH need the same amount of NaOH; only the pH and the rate of reaction differ.

Worked example

A student wants to titrate 0.1 mol dm−3 ethanoic acid with 0.1 mol dm−3 aqueous ammonia using an indicator. Is methyl orange or phenolphthalein suitable?

  1. Ethanoic acid is a weak acid and ammonia is a weak base.
  2. For a weak acid with a weak alkali the pH changes gradually near the end-point: the curve has no clear vertical section.
  3. An indicator needs a vertical section that covers its whole colour-change range, so both indicators would change colour slowly over a large volume.

Answer: Neither is suitable; no indicator gives a sharp end-point for a weak acid with a weak alkali.

Practice questions

Try each one, then open the answer.

1. Which of these is a weak base?

  1. A
    NaOH
  2. B
    KOH
  3. C
    NH3
  4. D
    CH3COOH
Show answer

Answer: C. Ammonia is only partially converted to NH4+ and OH− in water, so it is a weak base. Sodium and potassium hydroxide are fully dissociated strong bases; ethanoic acid is a weak acid.

2. In the equilibrium NH3 + H2O ⇌ NH4+ + OH−, water is acting as

  1. A
    a Brønsted–Lowry base, because it accepts a proton
  2. B
    an oxidising agent
  3. C
    a Brønsted–Lowry acid, because it donates a proton to ammonia
  4. D
    neither an acid nor a base, because water is neutral
Show answer

Answer: C. A Brønsted–Lowry acid is a proton donor. Water gives H+ to NH3, leaving OH−, so here it is the acid and NH3 is the base. Water can act as either, depending on its partner.

3. Which indicator is suitable for titrating ethanoic acid with sodium hydroxide?

  1. A
    methyl orange, pH range 3.1–4.4
  2. B
    bromophenol blue, pH range 3.0–4.6
  3. C
    methyl red, pH range 4.2–6.3
  4. D
    phenolphthalein, pH range 8.3–10.0
Show answer

Answer: D. For a weak acid–strong base titration the vertical section lies at about pH 7–11, so the indicator must change colour within that range. Phenolphthalein fits; the others change colour too early.

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