6.1.3Additional lessons
Amphiprotic Species — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 118 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4Amphiprotic and Amphoteric
Some species can play either role depending on what they react with.
Amphiprotic
- Amphiprotic A species that can both donate and accept a proton, so it can act as a Brønsted–Lowry acid or as a base.
Amphoteric
- Amphoteric A wider term for a substance that can react both as an acid and as a base, such as Al₂O₃ or ZnO.
Relationship
- Relationship All amphiprotic species are amphoteric, but amphoteric substances such as metal oxides need not be able to transfer protons.
Requirement
- Requirement An amphiprotic species must contain a hydrogen atom to donate and a lone pair to accept a proton.
Part 2 of 4Common Examples
Common Examples
Water
- Water H₂O accepts a proton from HCl to form H₃O⁺, and donates a proton to NH₃ to form OH⁻.
Hydrogencarbonate
- Hydrogencarbonate HCO₃⁻ donates a proton to form CO₃²⁻ and accepts one to form H₂CO₃.
Hydrogensulfate
- Hydrogensulfate HSO₄⁻ can donate to give SO₄²⁻ and can accept a proton to give H₂SO₄.
Amino acids
- Amino acids These molecules contain both a basic and an acidic group, so they can accept or donate a proton.
| Species | Acts as an acid to give | Acts as a base to give |
|---|---|---|
| H₂O | OH⁻ | H₃O⁺ |
| HCO₃⁻ | CO₃²⁻ | H₂CO₃ |
| HSO₄⁻ | SO₄²⁻ | H₂SO₄ |
| H₂PO₄⁻ | HPO₄²⁻ | H₃PO₄ |
Part 3 of 4Writing Equations
1
Identify the amphiprotic species and decide which reagent it is meeting, a stronger acid or a stronger base.
2
With a stronger acid the species accepts a proton, so add H⁺ to it to find the product.
3
With a stronger base the species donates a proton, so remove H⁺ from it to find the product.
4
Balance the charges, and check that the total number of hydrogens is the same on both sides.
5
State clearly the role of the species in each case, as the question may ask for this.
Part 4 of 4Exam Checklist & Tips
6.1.3 – Key Points for the Exam
- Amphiprotic A species that can act as both a proton donor and a proton acceptor.
- Amphoteric A substance that can react with both acids and bases.
- Hydrogencarbonate The ion HCO₃⁻, which acts as an acid or a base.
- Lone pair A pair of non-bonding electrons used to accept a proton.
Common Mistakes
- Confusing amphiprotic with amphoteric; the first refers to proton transfer only.
- Believing that any compound with hydrogen is amphiprotic.
- Writing the product for acting as an acid when the question asks for the role as a base.
- Forgetting that H₂O is itself amphiprotic.
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