Period 3 oxides: structure and acid-base characterAQA A-Level Chemistry: Subtopic test

1. 1
   A student is given samples of four Period 3 oxides, sodium oxide, magnesium oxide, silicon dioxide and phosphorus(V) oxide, and is asked to compare their melting points.

   (a)

   Which statement correctly explains the very high melting point of magnesium oxide?

   [1 mark]

   • AStrong covalent bonds between magnesium and oxygen atoms must be broken
   • BWeak London forces between MgO molecules must be overcome
   • CStrong electrostatic attraction between Mg²⁺ and O²⁻ ions in a giant lattice must be overcome
   • DDelocalised electrons are strongly attracted to the Mg²⁺ ions

   (b)

   Which statement explains why the melting point of P₄O₁₀ is much lower than that of SiO₂?

   [1 mark]

   • AP₄O₁₀ is simple molecular so only weak intermolecular forces are overcome, whereas SiO₂ has many strong covalent bonds to break
   • BP₄O₁₀ is ionic with weaker ionic bonds than SiO₂
   • CThe P–O bonds break more easily than the Si–O bonds when P₄O₁₀ melts
   • DP₄O₁₀ has delocalised electrons, which are weaker than covalent bonds

   (c)

   Explain why the melting point of magnesium oxide is higher than that of sodium oxide.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student adds small amounts of sodium oxide, aluminium oxide and phosphorus(V) oxide to separate beakers of water and tests the pH of each mixture with universal indicator.

   (a)

   Which row gives the approximate pH of the mixtures formed with Na₂O, Al₂O₃ and P₄O₁₀ respectively?

   [1 mark]

   • A1, 7, 14
   • B14, 7, 1
   • C14, 14, 1
   • D7, 7, 7

   (b)

   What is the formula of the acid formed when P₄O₁₀ reacts with water?

   [1 mark]

   • AH₃PO₃
   • BH₂SO₄
   • CPH₃
   • DH₃PO₄

   (c)

   Write an equation for the reaction of sodium oxide with water and explain why the resulting solution is alkaline.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A technician is preparing a safety sheet for the oxides of Period 3. She needs equations for the acids formed when phosphorus(V) oxide, sulfur dioxide and sulfur trioxide dissolve in water, and a description of the structures of some of the acids and anions involved.

   (a)

   Write equations for the reactions of phosphorus(V) oxide, sulfur dioxide and sulfur trioxide with water.

   [3 marks]

   (b)

   Describe the structures of phosphoric(V) acid, sulfuric(VI) acid and the sulfate(VI) ion in terms of the bonds around the central atom.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A class is asked to explain the trends in structure, melting point and acid-base character of the highest oxides of the Period 3 elements, sodium to sulfur, using results from experiments with water, acids and bases.

   (a)

   Explain the trend in the melting points of Na₂O, MgO, Al₂O₃, SiO₂, P₄O₁₀ and SO₃ in terms of structure and bonding.

   [6 marks]

   (b)

   Classify sodium oxide, aluminium oxide and sulfur trioxide as basic, amphoteric or acidic. Support your answer with equations for their reactions with water, acids or bases, and relate the trend in acid-base character to the bonding in the oxides.

   [6 marks]

   Total for question 4: 12 marks

End of questions