OxfordAQA A-Level Chemistry: Properties of Period 3 elements and their oxides and chlorides (9620) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

This practice set covers Properties of Period 3 elements and their oxides and chlorides, section 3.2.4 (subsection 3.2.4.1) of the OxfordAQA International AS and A-level Chemistry (9620) specification, Version 5.3, for International AS and A-level exams May/June 2020 onwards. Because the specification puts it in International A2, the whole topic is International A-level only. Its home is Unit 3 (Inorganic 2 and Physical 2), though the synoptic Unit 5 may include it too.

Work through the study guide and revision notes before trying these. Useful links: the OxfordAQA A-Level Chemistry hub, the printable checklist and the free 10-minute diagnostics.

Use these relative atomic masses where needed: H = 1.0, O = 16.0, Mg = 24.3, Al = 27.0, Si = 28.1, P = 31.0, Cl = 35.5.

Questions

1. Which chloride dissolves in water to give a solution with a pH of about 6?

• A NaCl
• B MgCl₂
• C SiCl₄
• D PCl₅

[1]

2. A strip of magnesium ribbon is left in cold water for a day. A few bubbles of gas collect and the water reaches a pH of about 10.

(a) Give the equation for this reaction. [1] (b) Explain why the pH is lower than that of the solution formed when sodium reacts with water. [1] (c) The experiment is repeated by heating magnesium in steam. Give the formula of the magnesium-containing product and one observation. [2]

3. Several Period 3 elements burn or react on heating in chlorine.

(a) Write equations for the reaction of silicon with chlorine and of phosphorus, P₄, with excess chlorine. [2] (b) Aluminium reacts with chlorine to form Al₂Cl₆. Explain how two AlCl₃ units are joined in this molecule. [2]

4. The melting points of three Period 3 chlorides are: NaCl 801 °C, MgCl₂ 714 °C, SiCl₄ −69 °C.

(a) Explain, in terms of structure and bonding, why SiCl₄ has a much lower melting point than NaCl. [3] (b) Mg²⁺ has a higher charge than Na⁺, yet MgCl₂ melts at a lower temperature than NaCl. Suggest why. [2]

5. This question is about the reactions of Period 3 chlorides with water.

(a) Write an equation for the reaction of SiCl₄ with water and give one observation. [2] (b) Write equations for the reaction of PCl₅ with a small amount of cold water and with excess water. [2] (c) Aluminium chloride dissolves in water to form [Al(H₂O)₆]³⁺ ions. Explain, with an equation, why the solution is acidic. [2]

6. This question is about the structures of acids and anions formed from Period 3 compounds.

(a) Draw the structure of sulfuric(VI) acid, H₂SO₄. [1] (b) Draw the dihydrogenphosphate ion, H₂PO₄⁻. [1] (c) Draw the structure of sulfuric(IV) acid, H₂SO₃, and state the shape around the sulfur atom. [2]

7. Silicon tetrachloride lowers the pH of water sharply, but silicon dioxide has no effect on the pH of water. Explain this difference in terms of the bonding and structure of each compound. [4]

8. Write an equation for each reaction.

(a) Sodium oxide with dilute nitric acid. [1] (b) Aluminium oxide with potassium hydroxide solution, forming KAl(OH)₄. [1] (c) Sulfur trioxide with potassium hydroxide solution. [1] (d) Aluminium chloride with excess sodium hydroxide solution. [1]

9. A 2.085 g sample of PCl₅ is hydrolysed completely in water, and the solution is diluted to 250 cm³ in a volumetric flask. Then 25.0 cm³ of this solution is titrated with 0.250 mol dm⁻³ sodium hydroxide.

(a) Write an equation for the overall reaction of PCl₅ with sodium hydroxide. [1] (b) Calculate the volume of sodium hydroxide needed to neutralise the 25.0 cm³ portion. [4]

10. Silicon (1.405 g) is heated in a stream of dry chlorine and 7.65 g of SiCl₄ is collected.

(a) Give the equation for the formation of SiCl₄. [1] (b) Calculate the percentage yield of SiCl₄. [3]

11. Three Period 3 chlorides, A, B and C, are tested.

• A: white solid, melts at 714 °C, dissolves in water without fuming; pH of the solution about 6.
• B: colourless liquid at room temperature; reacts violently with water, giving steamy fumes and a white solid.
• C: very pale yellow solid that turns to vapour at about 180 °C; hisses with water; pH of the solution about 3.

(a) Identify A, B and C. [3] (b) Write an equation for the reaction of B with water. [1] (c) Write an equation for C dissolving in excess water to form a complex ion. [1] (d) Explain why C turns to vapour at such a low temperature. [2]

12. A 0.852 g sample of phosphorus(V) oxide, P₄O₁₀, is added to water.

(a) State the equation for the reaction of P₄O₁₀ with water. [1] (b) Calculate the amount, in moles, of H₃PO₄ formed. [2] (c) The solution is reacted with 0.300 mol dm⁻³ sodium hydroxide until all of the acid is converted to Na₂HPO₄. Write an equation for this reaction and calculate the volume of sodium hydroxide used. [3] (d) Explain why P₄O₁₀ has a much lower melting point than sodium oxide. [2]

Answers

1. B [1] Examiner insight: Give one letter only; two letters leave your answer ambiguous.

2. (a) Mg + 2H₂O → Mg(OH)₂ + H₂ [1] (b) Magnesium hydroxide is only slightly soluble, so the concentration of OH⁻ ions is much lower than in NaOH solution [1] (c) MgO [1]; burns with a bright white light, or a white solid forms [1] Examiner insight: Swapping the products (MgO with cold water, Mg(OH)₂ with steam) is a chemistry error that balancing cannot rescue.

3. (a) Si + 2Cl₂ → SiCl₄ [1]; P₄ + 10Cl₂ → 4PCl₅ [1] (b) Al in AlCl₃ has only six electrons in its outer shell (it is electron-deficient) [1]; a lone pair on a chlorine atom of the other unit is donated to aluminium, forming a co-ordinate bond, and two such bonds bridge the units [1] Examiner insight: Say which atom donates the lone pair and which accepts it; “a dative bond forms” alone does not explain the dimer.

4. (a) NaCl is a giant ionic lattice with strong attraction between oppositely charged ions [1]; SiCl₄ is simple molecular, with weak van der Waals forces between molecules [1]; much less energy is needed to overcome these intermolecular forces [1] (b) The small, highly charged Mg²⁺ ion polarises (distorts) the large Cl⁻ ions [1]; this gives the bonding some covalent character, so it is not purely ionic and the lattice is weakened [1] Examiner insight: In (a), stating that Si–Cl bonds break on melting contradicts the simple molecular structure and loses the SiCl₄ point.

5. (a) SiCl₄ + 2H₂O → SiO₂ + 4HCl [1]; steamy (misty) fumes, or a white solid forms, or the mixture gets hot [1] (b) PCl₅ + H₂O → POCl₃ + 2HCl [1]; PCl₅ + 4H₂O → H₃PO₄ + 5HCl [1] (c) The small, 3+ aluminium ion polarises the O–H bonds in its water ligands, so H⁺ is released [1]; [Al(H₂O)₆]³⁺ ⇌ [Al(H₂O)₅(OH)]²⁺ + H⁺ [1] Examiner insight: An observation is something you see, such as fumes; “HCl is produced” is a deduction.

6. (a) Sulfur with two S=O and two S–O–H bonds [1]

        O
        ‖
  H–O – S – O–H
        ‖
        O

(b) Phosphorus with one P=O, two P–O–H and one P–O⁻ [1]

        O
        ‖
  H–O – P – O⁻
        |
        O–H

(c) Sulfur with one S=O, two S–O–H and a lone pair on S [1]; pyramidal [1]

        O
        ‖
  H–O – S – O–H
        ..

Examiner insight: In (b), put the 1− charge on a single-bonded oxygen, not on phosphorus.

7. SiCl₄ is simple molecular (covalent) [1]; water hydrolyses the Si–Cl bonds, forming HCl, an acid that releases H⁺ ions [1]; SiO₂ has a giant covalent (macromolecular) structure with strong Si–O bonds throughout [1]; it does not dissolve or react with water, so the pH stays at 7 [1] Examiner insight: Name the structure of each compound; describing only what you see does not answer “in terms of bonding and structure”.

8. (a) Na₂O + 2HNO₃ → 2NaNO₃ + H₂O [1] (b) Al₂O₃ + 2KOH + 3H₂O → 2KAl(OH)₄ [1] (c) SO₃ + 2KOH → K₂SO₄ + H₂O [1] (d) AlCl₃ + 4NaOH → NaAl(OH)₄ + 3NaCl [1] Examiner insight: In (b), leaving out the three water molecules leaves the equation unbalanced in H and O, so check atom counts before moving on.

9. (a) PCl₅ + 8NaOH → Na₃PO₄ + 5NaCl + 4H₂O [1] (b) Mr(PCl₅) = 31.0 + 5(35.5) = 208.5, so n = 2.085 / 208.5 = 0.0100 mol [1]; in 25.0 cm³: 0.0100 / 10 = 1.00 × 10⁻³ mol [1]; n(NaOH) = 8 × 1.00 × 10⁻³ = 8.00 × 10⁻³ mol [1]; volume = 8.00 × 10⁻³ / 0.250 × 1000 = 32.0 cm³ [1] Examiner insight: Show the 1 : 8 ratio and the scaling to 25.0 cm³ as separate lines so each step of method is visible.

10. (a) Si + 2Cl₂ → SiCl₄ [1] (b) n(Si) = 1.405 / 28.1 = 0.0500 mol [1]; theoretical mass of SiCl₄ = 0.0500 × 170.1 = 8.505 g [1]; yield = 7.65 / 8.505 × 100 = 89.9% [1] Examiner insight: Compare actual and theoretical masses of SiCl₄; dividing by the mass of silicon is a common slip.

11. (a) A is MgCl₂ [1]; B is SiCl₄ [1]; C is aluminium chloride (AlCl₃ / Al₂Cl₆) [1] (b) SiCl₄ + 2H₂O → SiO₂ + 4HCl [1] (c) AlCl₃ + 6H₂O → [Al(H₂O)₆]³⁺ + 3Cl⁻ [1] (d) Its bonding is largely covalent and the vapour consists of Al₂Cl₆ molecules [1]; only weak intermolecular forces hold these together, so little energy is needed [1] Examiner insight: In (a), use the stem: a high melting point and no fuming mean ionic; fuming in water means covalent.

12. (a) P₄O₁₀ + 6H₂O → 4H₃PO₄ [1] (b) Mr(P₄O₁₀) = 4(31.0) + 10(16.0) = 284.0, so n = 0.852 / 284.0 = 3.00 × 10⁻³ mol [1]; n(H₃PO₄) = 4 × 3.00 × 10⁻³ = 0.0120 mol [1] (c) H₃PO₄ + 2NaOH → Na₂HPO₄ + 2H₂O [1]; n(NaOH) = 2 × 0.0120 = 0.0240 mol [1]; volume = 0.0240 / 0.300 × 1000 = 80.0 cm³ [1] (d) P₄O₁₀ consists of separate molecules attracted to each other only by weak van der Waals forces, while Na₂O is a giant ionic lattice with strong attraction between ions [1]; much less energy is needed to overcome the intermolecular forces, so P₄O₁₀ melts lower [1] Examiner insight: In (c), Na₂HPO₄ fixes a 1 : 2 ratio; using 1 : 3 (as for Na₃PO₄) gives the wrong volume.

Where marks are usually lost

• Writing AlCl₃, not Al₂Cl₆, for the product of aluminium and chlorine.
• Saying SiCl₄ “dissolves”; it is hydrolysed, and the fumes are hydrogen chloride.
• Explaining the acidity of aluminium chloride solution without mentioning polarisation by the small, 3+ ion.
• Breaking covalent bonds when describing the melting of simple molecular chlorides or oxides.
• Wrong mole ratios for chloride neutralisation (PCl₅ needs 8 NaOH).
• Losing the lone pair on sulfur, or placing a charge on P or S, when drawing acids and anions.

Next steps

• Period 3 oxides and chlorides revision notes
• Period 3 oxides and chlorides study guide
• OxfordAQA A-Level Chemistry hub
• Printable checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

OxfordAQA, International AS and A-level Chemistry (9620) specification, Version 5.3, for International AS and A-level exams May/June 2020 onwards: section 3.2.4 Properties of Period 3 elements and their oxides and chlorides (International A2).