Periodicity of chemical properties of the elements in Period 3 — practice questions

Amphoteric means a substance can react with both acids and bases.

1. Reaction as a base: Aluminium oxide contains oxide ions (O²⁻) which can accept protons from an acid, such as hydrochloric acid. It neutralises the acid to form a salt and water. Equation: Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l) Ionic equation: Al₂O₃(s) + 6H⁺(aq) → 2Al³⁺(aq) + 3H₂O(l)
2. Reaction as an acid: Aluminium oxide can also react with a strong alkali, such as sodium hydroxide. In this reaction, it behaves as an acid, forming a soluble aluminate complex. Equation: Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2NaAl(OH)₄ (sodium tetrahydroxoaluminate(III)) Ionic equation: Al₂O₃(s) + 2OH⁻(aq) + 3H₂O(l) → 2[Al(OH)₄]⁻(aq)

Because it reacts with both acids and bases, Al₂O₃ is classified as amphoteric.

Observations:

1. A white precipitate/solid is formed (this is hydrated silicon dioxide).
2. Steamy/white fumes are seen (this is hydrogen chloride gas being produced in an exothermic reaction).

Equation: SiCl₄(l) + 2H₂O(l) → SiO₂(s) + 4HCl(aq)

Explanation of acidity: The reaction is a hydrolysis reaction which produces two products: silicon dioxide and hydrogen chloride. Hydrogen chloride is a strong acid which dissolves in the water and fully dissociates into H⁺(aq) and Cl⁻(aq) ions. The high concentration of H⁺(aq) ions makes the resulting mixture strongly acidic.