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Questions on the Period 3 oxides: structure, bonding and acid–base character

The melting points of Na₂O, MgO, Al₂O₃, SiO₂, P₄O₁₀, SO₂ and SO₃ explained by structure and bonding; the reactions of the oxides with water and the pH of each solution; the structures of the acids and anions formed from P₄O₁₀, SO₂ and SO₃; basic, amphoteric and acidic oxides; equations for each oxide with acids and bases, including aluminium oxide and aluminium hydroxide both ways; and, for Cambridge, the reactions of NaCl, MgCl₂, AlCl₃, SiCl₄ and PCl₅ with water and the pH of each solution.

19 original questions · 71 marks · the Period 3 oxides: structure, bonding and acid–base character notes · Periodicity and Period 3 · Flashcards for this unit

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• 6 marks · Discuss · POS-Q17 AQA · CAIE

  Discuss how the bonding in the oxides of Period 3 changes from Na₂O to SO₃, and how the change accounts for their reactions with water.

  Mark scheme

  Indicative content

  1. Na₂O and MgO are ionic, Al₂O₃ is ionic with covalent character, SiO₂ is giant covalent, and P₄O₁₀, SO₂ and SO₃ are simple molecular covalent
  2. the change follows the fall in electronegativity difference between the element and oxygen across the period
  3. the ionic oxides contain O²⁻, which accepts protons from water to form OH⁻: Na₂O gives NaOH at pH 13 to 14, MgO gives sparingly soluble Mg(OH)₂ at pH 9 to 10
  4. Al₂O₃ and SiO₂ do not react with water, the lattice and the covalent network being too strong, so the pH stays at 7
  5. the molecular oxides react with water to form oxoacids: P₄O₁₀ gives H₃PO₄ and SO₃ gives H₂SO₄, both at pH 0 to 1
  6. SO₂ gives H₂SO₃, a weak acid at pH 2 to 3; in each oxoacid the O–H bonds break to release H⁺

  Levels

  • Level 3 (5–6 marks) the change in bonding across the full set of oxides is stated and explained through electronegativity difference, and every reaction with water is matched to the bonding, the ionic oxides giving hydroxide, the two that do not react, and the molecular oxides giving oxoacids, with approximate pH values
  • Level 2 (3–4 marks) the change from ionic to covalent is described and most reactions with water are right; the reason for the change in terms of electronegativity, the two oxides that do not react, or the pH values are not there
  • Level 1 (1–2 marks) the oxides are said to go from basic to acidic; silicon dioxide is typically said to dissolve to give an acid, and bonding is not linked to pH

• 5 marks · Explain · POS-Q3 AQA · CAIE

  Explain, using the melting points in the table, why magnesium oxide melts at a higher temperature than sodium oxide, and why aluminium oxide melts lower than magnesium oxide although Al³⁺ is more highly charged than Mg²⁺.

  Oxide	Na₂O	MgO	Al₂O₃
  Melting point / K	1548	3125	2345

  Mark scheme

  1. B1 all three are ionic, giant lattices held by electrostatic attraction between oppositely charged ions in every direction
  2. B1 Mg²⁺ has a greater charge than Na⁺ and is smaller
  3. B1 so the attraction between Mg²⁺ and O²⁻ is stronger and more energy is needed to overcome it; reject any reference to molecules or intermolecular forces
  4. B1 the small, highly charged Al³⁺ ion distorts, or polarises, the electron cloud of the oxide ion
  5. B1 so the bonding in Al₂O₃ has some covalent character and the ionic attraction is weaker than in a purely ionic lattice

• 4 marks · Give · POS-Q1 AQA · CAIE

  Give an equation, with state symbols, for the reaction of each of sodium oxide, phosphorus(V) oxide and sulfur dioxide with water, the approximate pH of each solution, and the classification of each oxide as basic or acidic.

  Mark scheme

  1. B1 Na₂O(s) + H₂O(l) → 2NaOH(aq), giving pH 13 to 14; allow pH 12 to 14
  2. B1 P₄O₁₀(s) + 6H₂O(l) → 4H₃PO₄(aq), giving pH 0 to 1; allow pH 0 to 2
  3. B1 SO₂(g) + H₂O(l) → H₂SO₃(aq), giving pH 2 to 3
  4. B1 sodium oxide is a basic oxide; phosphorus(V) oxide and sulfur dioxide are acidic oxides

• 4 marks · Explain · POS-Q4 AQA · CAIE

  Explain why silicon dioxide has a high melting point but phosphorus(V) oxide has a low one.

  Mark scheme

  1. B1 silicon dioxide is giant covalent, each Si bonded to four O and each O to two Si throughout the structure
  2. B1 melting silicon dioxide breaks many strong covalent bonds, which needs a lot of energy
  3. B1 phosphorus(V) oxide is simple molecular, made of P₄O₁₀ molecules
  4. B1 it changes state by overcoming only the weak van der Waals forces between molecules; reject breaking covalent bonds in P₄O₁₀

• 4 marks · Explain · POS-Q6 AQA · CAIE

  Explain, in terms of bonding, why sodium oxide forms an alkaline solution in water and sulfur trioxide forms an acidic one.

  Mark scheme

  1. B1 sodium oxide is ionic and contains oxide ions, O²⁻
  2. B1 the oxide ion accepts a proton from water: O²⁻ + H₂O → 2OH⁻, so hydroxide ions make the solution alkaline
  3. B1 sulfur trioxide is covalent and reacts with water to form sulfuric(VI) acid: SO₃ + H₂O → H₂SO₄
  4. B1 the O–H bonds in the acid break, releasing H⁺ ions, so the solution is acidic

• 4 marks · Explain · POS-Q7 AQA · CAIE

  Explain why adding aluminium oxide or silicon dioxide to water does not change its pH, and describe one reaction of each oxide that shows its acid–base character.

  Mark scheme

  1. B1 neither oxide reacts with water or dissolves in it, so no H⁺ or OH⁻ ions are released
  2. B1 the aluminium oxide lattice is too strong for water to break up, and silicon dioxide is a giant covalent network with no ions to release
  3. B1 aluminium oxide reacts with acid, Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l), and also with alkali, so it is amphoteric
  4. B1 silicon dioxide reacts with hot, concentrated sodium hydroxide, SiO₂(s) + 2NaOH(aq) → Na₂SiO₃(aq) + H₂O(l), so it is acidic

• 4 marks · Calculate · POS-Q11 AQA · CAIE

  Calculate the volume of 0.200 mol dm⁻³ sodium hydroxide needed to neutralise completely, to Na₃PO₄, the acid formed when 1.42 g of P₄O₁₀ is added to water.

  Mark scheme

  1. M1 amount of P₄O₁₀ = 1.42 ÷ 284.0 = 0.00500 mol
  2. M1 amount of H₃PO₄ = 4 × 0.00500 = 0.0200 mol
  3. M1 amount of NaOH = 3 × 0.0200 = 0.0600 mol; error carried forward from M2
  4. A1 volume = 0.0600 ÷ 0.200 = 0.300 dm³, or 300 cm³, to three significant figures

• 4 marks · Suggest · POS-Q14 AQA · CAIE

  Suggest whether beryllium oxide, BeO, is basic, amphoteric or acidic, given that the electronegativity of beryllium is 1.5 and of oxygen 3.5, and give equations, with state symbols, for its reactions with hydrochloric acid and with sodium hydroxide solution.

  Mark scheme

  1. B1 the electronegativity difference, 2.0, is the same as aluminium's with oxygen, so BeO is ionic with covalent character
  2. B1 so beryllium oxide is amphoteric, like aluminium oxide
  3. B1 BeO(s) + 2HCl(aq) → BeCl₂(aq) + H₂O(l)
  4. B1 BeO(s) + 2NaOH(aq) + H₂O(l) → Na₂Be(OH)₄(aq); allow BeO(s) + 2NaOH(aq) → Na₂BeO₂(aq) + H₂O(l)

• 4 marks · Evaluate · POS-Q15 AQA · CAIE

  Evaluate the statement that across Period 3 the metal oxides are basic and the non-metal oxides are acidic.

  Mark scheme

  1. B1 it holds for sodium oxide and magnesium oxide, which are ionic, basic and form alkaline solutions
  2. B1 it holds for the non-metal oxides P₄O₁₀, SO₂ and SO₃, which are covalent and form acidic solutions
  3. B1 it fails for aluminium oxide, a metal oxide that is amphoteric, reacting with both acids and bases
  4. B1 silicon, a metalloid or non-metal, has an oxide that is acidic although insoluble in water, so the statement is a useful outline that needs aluminium oxide as an exception

• 4 marks · Explain · POS-Q19 CAIE

  Explain why sodium chloride gives a solution of pH 7 in water but silicon tetrachloride gives a solution of pH 1 to 2. Include an equation, with state symbols, for the reaction of silicon tetrachloride.

  Mark scheme

  1. B1 sodium chloride is ionic; it dissolves and the ions are hydrated, but neither Na⁺ nor Cl⁻ reacts with water, so the solution is neutral
  2. B1 silicon tetrachloride is covalent, and water attacks the partially positive silicon atom, breaking the Si–Cl bonds: it is hydrolysed
  3. B1 SiCl₄(l) + 2H₂O(l) → SiO₂(s) + 4HCl(aq)
  4. B1 the hydrogen chloride formed dissolves to give hydrochloric acid, a strong acid, so the pH is low

• 4 marks · Calculate · POS-Q20 CAIE

  Calculate the concentration of hydrogen chloride, and the pH, of the solution made when 1.043 g of phosphorus(V) chloride, PCl₅, is added to water and made up to 500 cm³. Treat the hydrogen chloride as the only acid. The equation is PCl₅(s) + 4H₂O(l) → H₃PO₄(aq) + 5HCl(aq).

  Mark scheme

  1. M1 Mr of PCl₅ = 31.0 + 5(35.5) = 208.5, so amount of PCl₅ = 1.043 ÷ 208.5 = 0.00500 mol
  2. M1 amount of HCl = 5 × 0.00500 = 0.0250 mol
  3. M1 [H⁺] = [HCl] = 0.0250 ÷ 0.500 = 0.0500 mol dm⁻³, since HCl is fully dissociated
  4. A1 pH = −log(0.0500) = 1.30, to two decimal places; error carried forward from an incorrect [H⁺]

• 3 marks · Give · POS-Q8 AQA · CAIE

  Give the meaning of the term amphoteric, and give equations, with state symbols, for the reactions of aluminium oxide with hydrochloric acid and with sodium hydroxide solution.

  Mark scheme

  1. B1 amphoteric describes a substance that can react both as a base, with acids, and as an acid, with bases
  2. B1 Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l)
  3. B1 Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2NaAl(OH)₄(aq); allow Al₂O₃(s) + 2NaOH(aq) → 2NaAlO₂(aq) + H₂O(l)

• 3 marks · Describe · POS-Q10 AQA · CAIE

  Describe the structure of a molecule of phosphoric(V) acid, H₃PO₄, and of the phosphate(V) ion formed from it.

  Mark scheme

  1. B1 phosphoric(V) acid has a central phosphorus atom bonded to four oxygen atoms
  2. B1 one oxygen is joined by a P=O double bond and three are in P–O–H groups, so all three hydrogens are on oxygen atoms
  3. B1 the phosphate(V) ion, PO₄³⁻, is phosphorus bonded to four oxygens, formed when all three H⁺ ions are lost, with a 3− charge

• 3 marks · Calculate · POS-Q12 AQA · CAIE

  Calculate the mass of magnesium oxide that exactly neutralises 25.0 cm³ of 2.00 mol dm⁻³ hydrochloric acid.

  Mark scheme

  1. M1 amount of HCl = 2.00 × 25.0 ÷ 1000 = 0.0500 mol
  2. M1 MgO + 2HCl → MgCl₂ + H₂O, so amount of MgO = 0.0500 ÷ 2 = 0.0250 mol
  3. A1 mass of MgO = 0.0250 × 40.3 = 1.01 g, to three significant figures

• 3 marks · Suggest · POS-Q13 AQA · CAIE

  Suggest why a slurry of calcium oxide removes sulfur dioxide from power-station waste gases, and give an equation, with state symbols, for the reaction.

  Mark scheme

  1. B1 sulfur dioxide is an acidic oxide, because it is a covalent non-metal oxide
  2. B1 calcium oxide is a basic, ionic oxide, so the two react as an acid and a base to form a salt
  3. B1 CaO(s) + SO₂(g) → CaSO₃(s)

• 3 marks · Compare · POS-Q16 AQA · CAIE

  Compare the behaviour of aluminium oxide and silicon dioxide with water, with acids and with bases.

  Mark scheme

  1. B1 neither reacts with or dissolves in water, so neither changes the pH
  2. B1 aluminium oxide reacts with acids, forming an aluminium salt and water, whereas silicon dioxide does not
  3. B1 both react with sodium hydroxide, aluminium oxide to give NaAl(OH)₄ and silicon dioxide, when the alkali is hot and concentrated, to give Na₂SiO₃

• 3 marks · Give · POS-Q18 AQA · CAIE

  Give equations, with state symbols, for the reactions of phosphorus(V) oxide, sulfur dioxide and sulfur trioxide with excess sodium hydroxide solution.

  Mark scheme

  1. B1 P₄O₁₀(s) + 12NaOH(aq) → 4Na₃PO₄(aq) + 6H₂O(l)
  2. B1 SO₂(g) + 2NaOH(aq) → Na₂SO₃(aq) + H₂O(l)
  3. B1 SO₃(l) + 2NaOH(aq) → Na₂SO₄(aq) + H₂O(l); allow SO₃(g)

• 3 marks · Suggest · POS-Q21 CAIE

  Suggest explanations for two observations. A few drops of water added to solid aluminium chloride give steamy fumes. Aluminium chloride dissolved in a large volume of water gives no fumes, but the solution has a pH of about 3.

  Mark scheme

  1. B1 with a little water, aluminium chloride is hydrolysed and hydrogen chloride gas escapes as steamy fumes: AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl
  2. B1 in a large volume of water the aluminium chloride dissolves to give [Al(H₂O)₆]³⁺ ions, and any HCl dissolves rather than escaping
  3. B1 the small, highly charged Al³⁺ ion polarises the O–H bonds of its water ligands, which release some H⁺, so the solution is acidic, about pH 3

• 3 marks · Show · POS-Q22 AQA · CAIE

  Show that 1.20 dm³ of sulfur dioxide, measured at room temperature and pressure, needs 100 cm³ of 1.00 mol dm⁻³ sodium hydroxide solution to convert it completely to sodium sulfite, Na₂SO₃. Molar gas volume = 24.0 dm³ mol⁻¹.

  Mark scheme

  1. M1 n(SO₂) = 1.20 ÷ 24.0 = 0.0500 mol
  2. M1 SO₂(g) + 2NaOH(aq) → Na₂SO₃(aq) + H₂O(l), so n(NaOH) = 2 × 0.0500 = 0.100 mol
  3. A1 V = n ÷ c = 0.100 ÷ 1.00 = 0.100 dm³ = 100 cm³, with the working shown in full because the answer is given

Practise the Period 3 oxides: structure, bonding and acid–base character one question at a time