📋 Structured Questions
Complete each question on paper, then check your answers against the mark scheme.
Question 1: Structure and Water Reactions
8 marks(a) Write balanced chemical equations for the reactions of sodium oxide and sulfur trioxide with water. State the approximate pH of the resulting solution in each case. [4]
(b) Explain why sodium oxide and phosphorus(V) oxide behave differently when added to water. Refer to their structure, bonding, and the nature of the solutions formed in your answer. [4]
Show Mark Scheme
(a)
- Na2O(s) + H2O(l) → 2NaOH(aq) [1]; pH = 13 or 14 [1]
- SO3(g) + H2O(l) → H2SO4(aq) [1]; pH = 0, 1 or 2 [1]
(b)
- Sodium oxide is a giant ionic lattice containing O2- ions [1]
- The oxide ions are strongly basic and react with water to form hydroxide ions (OH-), resulting in an alkaline solution [1]
- Phosphorus(V) oxide is a simple molecular structure with covalent bonds [1]
- It reacts with water to form phosphoric acid (H3PO4) which dissociates to release hydrogen ions (H+), resulting in an acidic solution [1]
Question 2: Physical Properties and Bonding
7 marks(a) Explain why the melting point of magnesium oxide is much higher than that of sodium oxide. [3]
(b) Silicon dioxide has a melting point of 1713 degrees Celsius. State its structure and bonding, and explain why its melting point is so high. [2]
(c) Explain, in terms of structure and intermolecular forces, why sulfur dioxide is a gas at room temperature. [2]
Show Mark Scheme
(a)
- Both sodium oxide and magnesium oxide exist as giant ionic lattices [1]
- The Mg2+ ion has a higher positive charge than the Na+ ion and is smaller [1]
- This results in much stronger electrostatic forces of attraction between the Mg2+ and O2- ions, requiring more energy to break the lattice [1]
(b)
- Silicon dioxide has a giant covalent (macromolecular) structure [1]
- Melting requires breaking many strong covalent bonds throughout the giant network, which requires a huge amount of thermal energy [1]
(c)
- Sulfur dioxide exists as simple molecules [1]
- It has only weak intermolecular forces (van der Waals and dipole-dipole forces) between molecules, which require very little energy to overcome [1]
Question 3: Amphoteric Nature of Aluminium Oxide
6 marks(a) Define the term amphoteric. [1]
(b) Write a balanced chemical equation, including state symbols, for the reaction of aluminium oxide with hydrochloric acid. [2]
(c) Write a balanced chemical equation, including state symbols, for the reaction of aluminium oxide with aqueous sodium hydroxide to form sodium tetrahydroxoaluminate. [3]
Show Mark Scheme
(a)
- An oxide that can react as both an acid and a base [1]
(b)
- Al2O3(s) + 6HCl(aq) → 2AlCl3(aq) + 3H2O(l)
- Award [1] for correct formulas, [1] for correct state symbols.
(c)
- Al2O3(s) + 2NaOH(aq) + 3H2O(l) → 2NaAl(OH)4(aq)
- Award [1] for correct reactants and products, [1] for correct balancing, and [1] for correct state symbols.