9701 · 9.2
Periodicity of chemical properties of the elements in Period 3 flashcards
Revision flashcards for Cambridge 9701 Periodicity of chemical properties of the elements in Period 3 (syllabus 9.2). Flip, recall, then mark a real past-paper question.
Card
What is the general trend in the acid-base nature of Period 3 oxides?
They trend from basic (Na₂O, MgO) to amphoteric (Al₂O₃) to acidic (SiO₂, P₄O₁₀, SO₂, SO₃). This reflects the change from metallic to non-metallic character across the period.
Card
Why is sodium oxide (Na₂O) a basic oxide?
It is a giant ionic lattice containing the oxide ion, O²⁻. In water, the O²⁻ ion is a strong proton acceptor, reacting to form hydroxide ions: O²⁻(aq) + H₂O(l) → 2OH⁻(aq).
Card
Write the equation for magnesium oxide reacting with water.
MgO(s) + H₂O(l) ⇌ Mg(OH)₂(aq). Magnesium hydroxide is only sparingly soluble, so the resulting solution is weakly alkaline (pH ~9-10).
Card
What does 'amphoteric' mean in the context of Al₂O₃?
It means aluminium oxide can act as both a base (reacting with acids) and an acid (reacting with bases). For example: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O and Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄].
Card
Why is silicon dioxide (SiO₂) classified as acidic despite being insoluble in water?
It does not react with water due to the high energy required to break its giant covalent structure. However, it is classified as acidic because it reacts with strong bases like hot, concentrated NaOH to form a salt (sodium silicate): SiO₂(s) + 2NaOH(aq) → Na₂SiO₃(aq) + H₂O(l).
Card
Describe the reaction of phosphorus(V) oxide with water.
It reacts vigorously and exothermically with water to form phosphoric(V) acid, a weak acid. The equation is: P₄O₁₀(s) + 6H₂O(l) → 4H₃PO₄(aq).
Card
Compare the reactions of NaCl and SiCl₄ with water.
NaCl dissolves to form a neutral solution (pH 7). SiCl₄ reacts violently with water (hydrolysis) to form a white precipitate of SiO₂ and fumes of HCl gas, resulting in a strongly acidic solution (pH 1-2).
Card
Why does AlCl₃ form an acidic solution in water?
The Al³⁺ ion has a high charge density. It strongly attracts and polarises the water molecules in its hydration shell, weakening the O-H bonds and causing the release of H⁺ ions into the solution. This process is called hydrolysis.
Card
Write the ionic equation for the hydrolysis of the aqueous aluminium ion.
[Al(H₂O)₆]³⁺(aq) ⇌ [Al(H₂O)₅(OH)]²⁺(aq) + H⁺(aq)
Card
What is the maximum oxidation state of sulfur in its common oxides?
+6, which is found in sulfur trioxide (SO₃). The maximum oxidation state generally corresponds to the group number for elements in Groups 1, 2, and 13-16.