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LearnYCH11WCH14 · Unit 4: Rates, Equilibria and Further Organic ChemistryAcid-base equilibriaBrønsted–Lowry acids and bases
Acid-base equilibria · Subtopic 2 of 2

Brønsted–Lowry Acids and Bases

A Brønsted–Lowry acid is a proton (H⁺) donor; a base is a proton acceptor. Every acid–base reaction has two conjugate pairs: in HCl+H2O→Cl−+H3O+\mathrm{HCl+H_2O\rightarrow Cl^-+H_3O^+}, HCl/Cl⁻ and H₃O⁺/H₂O. Strong acids dissociate fully (HCl→H++Cl−\mathrm{HCl\rightarrow H^++Cl^-}); weak acids partially (CH3COOH⇌CH3COO−+H+\mathrm{CH_3COOH\rightleftharpoons CH_3COO^-+H^+}). pH=−log⁡10[H+]\mathrm{pH}=-\log_{10}[\mathrm{H^+}]; for a strong monoprotic acid [H+]=c[\mathrm{H^+}]=c. Water is amphoteric: it can act as an acid or a base.

Cambridge 9701 Paper 1Cambridge 9701 Paper 2Edexcel YCH11 WCH14AQA 7405 Paper 1≈ 40 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Define acids and bases using the Brønsted–Lowry theory.
  • Identify conjugate acid–base pairs.
  • Distinguish strong and weak acids and bases.
  • Calculate pH of strong acids.
Pause and think

Ammonia gas and hydrogen chloride gas react to form a white solid without any water. Is that an acid–base reaction?

Section 2 of 14

Core idea

Start hereA proton passed along

An acid-base reaction is a game of pass-the-proton. Brønsted–Lowry theory tracks who gives and who takes.

The big idea

Acids donate protons; bases accept them. Each reaction has two conjugate pairs. Strong acids donate completely; weak acids only partly.

1. Conjugate pairs

Proton transfer

Each acid becomes its conjugate base.

Worked examplepH of a strong acid

Calculate the pH of 0.150 mol dm⁻³ HNO₃.

Goal
pH.
Strategy
Strong monoprotic: [H+]=c[\mathrm{H^+}]=c.
  1. [H+]=0.150[\mathrm{H^+}]=0.150
    Complete dissociation.
  2. pH=−log⁡0.150=0.82\mathrm{pH}=-\log0.150=0.82
    Definition.
AnswerpH 0.82.
✓ CheckBelow 1 for concentration above 0.1 ✓.
Common misconceptionWeak vs dilute A 10 mol dm⁻³ ethanoic acid solution is concentrated but still weak. Strength is about the extent of dissociation.
Pause and think

What is the pH of 1.00 mol dm⁻³ HCl?

Summary

  • Acid donates H⁺; base accepts.
  • Conjugate pairs.
  • pH = −log[H⁺].
Key words
Conjugate base
Species formed when an acid loses a proton.
Strong acid
An acid that fully dissociates in water.
pH
−log₁₀ of the hydrogen ion concentration.
HistoryTwo chemists, one idea. Johannes Brønsted (Denmark) and Thomas Lowry (England) independently proposed the proton-transfer definition in the same year, 1923.
Fun factSuperacids. "Magic acid" (HSO₃F + SbF₅) is so strong it can protonate candle wax. Its name came from a Christmas party experiment.
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