Oxides

Acidic oxides are usually formed by NON-METALS combining with oxygen.

Examples and the acids they form in water:

• $\text{CO}_{2} + \text{H}_{2}\text{O} \rightleftharpoons \text{H}_{2}\text{CO}_{3}$ (carbonic acid).
• $\text{SO}_{2} + \text{H}_{2}\text{O} \to \text{H}_{2}\text{SO}_{3}$ (sulfurous acid).
• $\text{SO}_{3} + \text{H}_{2}\text{O} \to \text{H}_{2}\text{SO}_{4}$ (sulfuric acid).
• $\text{NO}_{2}$: dissolves in water to give a mixture of HNO₂ and HNO₃.

Reaction with bases. Acidic oxides neutralise bases: $\text{CO}_{2} + 2\text{NaOH} \to \text{Na}_{2}\text{CO}_{3} + \text{H}_{2}\text{O}.$

Acid rain. SO₂ from burning fossil fuels (sulfur impurity) reacts with O₂ and water in the atmosphere to make H₂SO₃ / H₂SO₄ → acid rain. Damages buildings (limestone), kills aquatic life, harms forests.

Worked qualitative. Why is sulfur dioxide a pollutant? It's an acidic oxide → forms acid rain → environmental damage.

Basic oxides are formed by METALS combining with oxygen.

Reaction with acids: $\text{MgO} + 2\text{HCl} \to \text{MgCl}_{2} + \text{H}_{2}\text{O}.$ $\text{CuO} + \text{H}_{2}\text{SO}_{4} \to \text{CuSO}_{4} + \text{H}_{2}\text{O}.$

Soluble basic oxides + water → alkali.

• $\text{Na}_{2}\text{O} + \text{H}_{2}\text{O} \to 2\text{NaOH}$.
• $\text{CaO} + \text{H}_{2}\text{O} \to \text{Ca(OH)}_{2}$.

(Most metal oxides are insoluble; only Group 1 and some Group 2 oxides are soluble.)

Worked qualitative. A student adds black copper(II) oxide powder to dilute sulfuric acid. The black solid disappears; the solution turns blue. Why? CuO reacts with H₂SO₄ to give CuSO₄ (blue solution).

Amphoteric oxides. React with BOTH acids AND bases.

• Examples: $\text{Al}_{2}\text{O}_{3}$, $\text{ZnO}$.

Reactions of $\text{ZnO}$:

• With acid: $\text{ZnO} + 2\text{HCl} \to \text{ZnCl}_{2} + \text{H}_{2}\text{O}$ (acts as a base).
• With alkali: $\text{ZnO} + 2\text{NaOH} + \text{H}_{2}\text{O} \to \text{Na}_{2}[\text{Zn(OH)}_{4}]$ (acts as an acid; forms a zincate complex). Sometimes simplified to $\text{ZnO} + 2\text{NaOH} \to \text{Na}_{2}\text{ZnO}_{2} + \text{H}_{2}\text{O}$ (sodium zincate).

Reactions of $\text{Al}_{2}\text{O}_{3}$: Similar — reacts with both HCl and NaOH.

Cambridge tip. "Amphoteric" comes from Greek for "both". It's a property that some metalloid-region elements have because their oxides have a balance of properties.

Neutral oxides. Don't react with acids OR bases. Often non-metal oxides where the oxidation state is unusual.

• $\text{CO}$ — carbon monoxide.
• $\text{NO}$ — nitric oxide.
• $\text{H}_{2}\text{O}$ — water (debatable; doesn't classically react as an acid or base, but is amphoteric in the modern sense).