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6.1.3Additional lessons

Amphiprotic Species — IB Chemistry SL Revision Notes & Topical Questions

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Part 1 of 5Amphiprotic Species

6.1.3 Amphiprotic Species

Many ions that still contain a hydrogen after partial deprotonation are amphiprotic.

Definition
  • An amphiprotic species can both donate and accept a proton, so it can behave as a Brønsted–Lowry acid or base.
Requirement
  • It must contain a hydrogen atom to give away and a lone pair to accept a proton.
Water
  • H₂O is amphiproticwith HCl it accepts a proton to form H₃O⁺, and with NH₃ it donates one to form OH⁻.
Hydrogencarbonate
  • HCO₃⁻ accepts a proton to form H₂CO₃ and donates one to form CO₃²⁻.

Part 2 of 5Examples with Equations

Examples with Equations
Water as a base
  • Water accepts a proton from hydrogen chloride: H₂O(l) + HCl(aq) → H₃O⁺(aq) + Cl⁻(aq).
Water as an acid
  • Water donates a proton to ammoniaH₂O(l) + NH₃(aq) ⇌ OH⁻(aq) + NH₄⁺(aq).
HCO₃⁻ as an acid
  • With a base it donates a proton: HCO₃⁻(aq) + OH⁻(aq) → CO₃²⁻(aq) + H₂O(l).
HCO₃⁻ as a base
  • With an acid it accepts a proton: HCO₃⁻(aq) + H⁺(aq) → H₂CO₃(aq).
Others
  • H₂PO₄⁻ and HPO₄²⁻ can each gain or lose a proton, as can HSO₃⁻.

Part 3 of 5Amphiprotic versus Amphoteric

Amphoteric
  • A substance that can react as both an acid and a base, not necessarily by proton transfer.
Amphiprotic
  • A substance that can both donate and accept a proton, so every amphiprotic species is amphoteric.
Amphoteric oxides
  • Al₂O₃ reacts with acid and with alkali, but it has no hydrogen to donate, so it is not amphiprotic.
Equations
  • Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l), and Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2Na[Al(OH)₄](aq).
TermMeaningExample
AmphiproticDonates and accepts H⁺H₂O, HCO₃⁻
AmphotericActs as acid and baseAl₂O₃, ZnO, H₂O

Part 4 of 5Using the Idea in Exams

Questions on this topic usually ask for a definition, two equations or a comparison with the term amphoteric.

Using the Idea in Exams
Spot the clue
  • Look for an ion that still carries a hydrogen but also has a negative charge, such as HSO₃⁻ or H₂PO₄⁻.
Two equations
  • A full answer gives one equation as an acid and one as a base, each with the correct conjugate partner.
Water in buffers
  • Amphiprotic ions behave differently depending on whether the partner is a stronger acid or a stronger base.
Explain the term
  • State that the species can both donate and accept a proton, not just that it reacts with acids and bases.

Part 5 of 5Exam Checklist & Tips

6.1.3 – Key Points for the Exam

  • Amphiprotic Able to both donate and accept a proton.
  • Amphoteric Able to react both as an acid and as a base.
  • Hydrogencarbonate ion HCO₃⁻, an amphiprotic ion formed when H₂CO₃ loses one proton.
  • Amphoteric oxide An oxide such as Al₂O₃ that reacts with both acids and bases.

Common Mistakes

  • Assuming amphoteric and amphiprotic mean the same thing; amphoteric is the wider term.
  • Calling Al₂O₃ amphiprotic; it cannot donate a proton.
  • Giving only one equation for an amphiprotic species instead of one showing each behaviour.
  • Using HCO₃⁻ and CO₃²⁻ as examples of the same pair; CO₃²⁻ has no hydrogen to donate.

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