Acids and bases
Compare H2 Chemistry Arrhenius, Brønsted–Lowry and Lewis models, identify conjugate pairs and explain electron-pair donation in Lewis adducts.
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Before you begin
This topic compares three definitions of acids and bases: Arrhenius, Brønsted–Lowry and Lewis. It then uses the Brønsted–Lowry definition to find conjugate pairs and amphiprotic species, and the Lewis definition to explain adducts. Acid strength, pH and Kₐ are in aqueous equilibria.
Be comfortable with: dative bonding, lone pairs in molecules and ions, and writing equations that balance charge.
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.
- Arrhenius, Brønsted–Lowry and Lewis acids and basesApply the Arrhenius, Brønsted–Lowry and Lewis definitions, and see where they overlap.
- Conjugate pairs and amphoteric speciesIdentify conjugates and prove amphiprotic behaviour with equations.
- Lewis acids, bases and adductsShow how a Lewis adduct forms with a correctly drawn arrow, naming the donor and acceptor.
Practise and check
Or choose
Topic reference
Quick reference
| Definition | Acid | Base | Evidence to give |
|---|---|---|---|
| Arrhenius | produces H⁺ in water | produces OH⁻ in water | the ion formed in aqueous solution |
| Brønsted–Lowry | proton donor | proton acceptor | an equation showing which species loses H⁺ |
| Lewis | electron-pair acceptor | electron-pair donor | the lone pair and where it forms the new bond |
Core knowledge to remember
- The definitions overlap: every Brønsted–Lowry base is also a Lewis base, because it accepts a proton with a lone pair.
- The Arrhenius definition applies only to aqueous solutions.
- A conjugate acid–base pair differs by exactly one proton, and so by one unit of charge.
- An amphiprotic species can both donate and accept a proton, like water and HCO₃⁻. Show both roles with two equations.
- Amphoteric is the wider term: amphiprotic species are amphoteric, but Al₂O₃ is amphoteric without being amphiprotic.
- A Lewis base donates a lone pair to a Lewis acid, forming a coordinate bond drawn from donor to acceptor.
- Electron-deficient molecules such as BF₃ and AlCl₃, the proton and metal ions are Lewis acids.
Common mistakes
- Calling a substance an acid because it contains H, or a base because it contains OH.
- Using the Arrhenius definition for a reaction that is not in water.
- Pairing two reactants as a conjugate pair.
- Forgetting to change the charge by one when a proton is added or removed.
- Showing only one role when asked to show that a species is amphiprotic.
- Drawing a coordinate arrow from the acceptor to the donor.
- Writing “electron donor” instead of “electron-pair donor”.