Amphiprotic Species and Oxides
Specification Reference R3.1.3
Quick Notes
- Amphiprotic species can act as both a Brønsted–Lowry acid and a base.
Example H₂O can donate H⁺ (acid) or accept H⁺ (base). - Metal oxides tend to be basic.
- Non-metal oxides tend to be acidic.
- Oxides of nitrogen (NO, NO₂) and sulfur (SO₂, SO₃) form acidic solutions in water, meaning they can lead to acid rain.
Full Notes
Amphiprotic Species
Amphiprotic species can act as both an acid (donate a H⁺ ion) and a base (accept a H⁺ ion).
Example Water (H₂O)
- As an acid: H₂O + NH₃ → OH⁻ + NH₄⁺
- As a base: H₂O + HCl → H₃O⁺ + Cl⁻
Example Hydrogencarbonate (HCO₃⁻)
- As an acid: HCO₃⁻ → CO₃²⁻ + H⁺
- As a base: HCO₃⁻ + H⁺ → H₂CO₃
Acid Rain from Nitrogen and Sulfur Oxides
Combustion in vehicles and power plants releases NO, NO₂, SO₂, and SO₃. These oxides dissolve in atmospheric moisture to form acids, which fall as acid rain.
Key reactions:
- SO₂ + H₂O → H₂SO₃
- SO₃ + H₂O → H₂SO₄
- 2NO₂ + H₂O → HNO₂ + HNO₃
These acids fall as acid rain, damaging ecosystems, corroding buildings, and affecting human health.
Summary
- Amphiprotic species act as both acids and bases.
- Examples include water and hydrogencarbonate ions.
- Metal oxides are basic, non-metal oxides are acidic.
- Across a period, oxides become more acidic.
- Oxides of nitrogen and sulfur form acids in water, causing acid rain.
Linked Course Question
Structure 3.1 — Linked Course Question
What is the periodic trend in the acid–base properties of metal and non-metal oxides?
Across a period (left to right), oxides become more acidic.
- Metal oxides (e.g. Na₂O, CaO) are basic.
They react with water to form alkaline solutions.
Na₂O + H₂O → 2NaOH - Non-metal oxides (e.g. CO₂, SO₂, NO₂) are acidic.
They react with water to form acidic solutions.
CO₂ + H₂O → H₂CO₃