Mini Chemistry
Part of Mini EducationSwitch subject

Acids & Bases

Acid–base models, conjugate pairs and Lewis adducts.

  • GCE A-Level H2 Chemistry 9476-2027
  • 3 lessons
Start topic

Before you begin

Acid–base models, conjugate pairs and Lewis adducts.

Learning goals
  • Arrhenius, Brønsted–Lowry, Lewis Definitions
  • Conjugate Pairs and Amphoteric Species
  • Lewis Adducts and Exam Phrasing
Syllabus statements covered
  • show understanding of, and apply the Arrhenius theory of acids and bases
  • show understanding of, and apply the Brønsted-Lowry theory of acids and bases, including the concept of conjugate acids and conjugate bases
  • show understanding of, and apply the Lewis theory of acids and bases (including non-aqueous system e.g. reaction between BF3 and NH3)

Lessons

Work through them in order.

  1. Arrhenius, Brønsted–Lowry, Lewis DefinitionsApply the Arrhenius, Brønsted–Lowry and Lewis definitions, and see where they overlap.
  2. Conjugate Pairs and Amphoteric SpeciesIdentify conjugates and prove amphiprotic behaviour with equations.
  3. Lewis Adducts and Exam PhrasingShow how a Lewis adduct forms with a correctly drawn arrow, naming the donor and acceptor.

Practise and check

Recommended nextAcid–Base Theories topic check

Or choose

Topic reference

This topic covers the Arrhenius, Brønsted–Lowry and Lewis theories of acids and bases. The models overlap, so apply the theory named in the question and use its evidence and vocabulary.

Acid strength, pH and equilibrium calculations are in Chemistry of Aqueous Solutions.

Be comfortable with:

Quick Reference

Three acid-base theory lensesThree stacked panels compare Arrhenius aqueous ion production, Brønsted-Lowry proton transfer, and Lewis electron-pair transfer. A final note explains that one reaction may fit more than one theory.1ArrheniusLens: ion production in aqueous solutionAcid produces H+ / H3O+ in water.Base produces OH− in water.Evidence: name the ion produced.2Brønsted–LowryLens: proton transferAcid donates H+.Base accepts H+.Evidence: identify who loses and gains H+.3LewisLens: electron-pair transferAcid accepts an electron pair.Base donates an electron pair.Trace the electron pair: donor → acceptor.Theories can overlapNH3 accepting H+ is a Brønsted baseand a Lewis base: it accepts the protonwhile donating the new bonding pair.
Use the evidence named by each theory. The models overlap: proton-transfer reactions also involve electron-pair donation, while Arrhenius is restricted to aqueous ion production.
  • Arrhenius (aqueous only): an acid produces H⁺ or H₃O⁺ in water; a base produces OH⁻ in water. State the ion produced.
  • Brønsted–Lowry: an acid donates a proton; a base accepts a proton. Show which species loses or gains one H⁺.
  • Lewis: an acid accepts an electron pair; a base donates an electron pair. Draw any coordinate-bond arrow from the donor to the acceptor.

The theories can overlap. For example, NH₃ is a Brønsted–Lowry base when it accepts a proton and a Lewis base because its lone pair forms the new bond.

Quick default examples (quote these under exam pressure):

  • Brønsted–Lowry: NH₃ + H₂O ⇌ NH₄ + + OH⁻
  • Lewis adduct: :NH₃ → BF₃

What You Must Memorise

  • Arrhenius acid/base (aq): produces H⁺ / OH⁻ in water.
  • Brønsted–Lowry acid/base: proton donor / proton acceptor.
  • Lewis acid/base: electron-pair acceptor / electron-pair donor.
  • Conjugate pair: differs by exactly one H⁺.
  • Amphiprotic: can donate and accept H⁺. Such a species is amphoteric under the Brønsted–Lowry model; show both roles with equations.

Common Exam Traps

  • Treating the theories as mutually exclusive instead of applying the theory named in the question.
  • Using Arrhenius language when the reaction is not in aqueous solution.
  • Saying “acid = has H” or “base = has OH” (not A Level reasoning).
  • Not writing an equation to justify Brønsted–Lowry classification.
  • Conjugate pairs: choosing species that don’t differ by exactly one H⁺ (charge changes by 1; the rest stays the same).
  • Lewis questions: using “proton donor/acceptor” when there is no proton transfer.