O Level Combined Science Chemistry Materials Quiz

O-Level Combined Science Quiz - Chemistry Materials - Answer Key

Total Marks: 40

Section A: Structured Questions (10 marks)

1. (a) Effervescence / bubbles of gas produced / magnesium dissolves / magnesium disappears / colourless gas evolved [1]

(b) The rate of reaction would increase [1] because magnesium powder has a larger surface area than magnesium ribbon, so there is more frequent collision between reactant particles / greater frequency of effective collisions [1].

2. (a) In solid sodium chloride, the ions (Na⁺ and Cl⁻) are held in fixed positions in the giant ionic lattice and cannot move [1]. In molten sodium chloride, the ions are free to move and carry electric charge [1].

(b) Graphite conducts electricity [1] because each carbon atom is bonded to three other carbon atoms, leaving one delocalised electron per carbon atom. These delocalised electrons are free to move throughout the structure and carry electric charge [1].

3. (a) Rate = change in volume / change in time = (50 - 24) cm³ / (90 - 30) s [1] = 26 / 60 = 0.43 cm³ s⁻¹ [1]

(b) As the reaction proceeds, the concentration of hydrochloric acid decreases / the amount of calcium carbonate decreases [1]. This means there are fewer reactant particles per unit volume, so the frequency of effective collisions decreases, reducing the rate of reaction [1].

4. (a) Substance W [1] because it has a high melting point (characteristic of giant ionic structures), does not conduct electricity when solid (ions fixed in lattice), but conducts when molten (ions free to move) [1].

(b) Metallic bonding [1]

5. (a) pH 3-5 (accept any value in this range) [1]

(b) A weak acid is an acid that undergoes partial ionisation / dissociation in water [1]. It exists in equilibrium between the molecular form and the ionised form / only a small proportion of acid molecules dissociate to form H⁺ ions [1].

(c) CH₃COOH + NaOH → CH₃COONa + H₂O [1]

Section B: Data Interpretation and Application (10 marks)

6. (a) In pure copper, all atoms are the same size and arranged in regular layers that can slide over each other easily when a force is applied [1]. In brass, zinc atoms are a different size to copper atoms, which disrupts the regular arrangement of layers, making it more difficult for layers to slide over each other [1].

(b) Any suitable use with explanation, e.g.:

• Musical instruments [1] because brass is hard and resistant to corrosion / produces good sound [1]
• Door handles / taps [1] because brass is hard, durable, and resistant to corrosion [1]
• Electrical connectors [1] because brass is hard and conducts electricity [1]

7. (a) Mass of oxygen = 0.80 - 0.48 = 0.32 g [1]

(b) Moles of Mg = mass / Aᵣ = 0.48 / 24 = 0.020 mol [1]

(c) Moles of O atoms = mass / Aᵣ = 0.32 / 16 = 0.020 mol [1]

(d) Ratio Mg : O = 0.020 : 0.020 = 1 : 1 [1] Empirical formula = MgO [1]

8. (a) Substance Q [1] because its melting point (-7 °C) is below room temperature (25 °C) and its boiling point (58 °C) is above room temperature, so it exists as a liquid at 25 °C [1].

(b) Simple molecular structure / simple covalent molecules with weak intermolecular forces (van der Waals' forces) [1]

(c) Substance R has a giant ionic structure [1]. When solid, the ions are held in fixed positions in the lattice and cannot move. When molten, the ions are free to move and can carry electric charge [1].

9. (a) Experiment B could have used zinc powder with 1.0 mol/dm³ sulfuric acid at a lower temperature (e.g., 20 °C) [1] OR zinc granules/ribbon with 1.0 mol/dm³ sulfuric acid at 30 °C [1]. The lower rate (less steep initial gradient) indicates either lower temperature, larger particle size, or lower concentration compared to experiment A [1].

(b) Experiments A and B used the same amount of zinc and same volume and concentration of acid, so the same amount of hydrogen gas is produced when the limiting reactant is completely used up [1]. Experiment C produced less hydrogen because either less zinc was used or a smaller volume / lower concentration of acid was used, meaning the limiting reactant was present in a smaller amount [1].

10. (a)

    H   H
    |   |
  — C — C —
    |   |
    H   H

[1] for correct structure with single bonds between carbon atoms; [1] for brackets or indication of repeating unit with continuation bonds.

(b) Poly(ethene) is an addition polymer because it is formed by the addition reaction of ethene monomers where the carbon-carbon double bond breaks and monomers join together without the loss of any small molecules / no other products are formed [1].

(c) Any one of:

• Non-biodegradable / does not decompose easily in the environment [1]
• Produces toxic gases when burned [1]
• Contributes to plastic pollution / landfill problems [1]
• Harmful to wildlife if ingested [1]

Section C: Extended Questions (10 marks)

11. (a) Anode: Bromine / Br₂ [1]; Cathode: Lead / Pb [1]

(b) Lead(II) bromide must be molten so that the ions (Pb²⁺ and Br⁻) are free to move [1]. In solid state, the ions are held in fixed positions in the giant ionic lattice and cannot move, so no electrolysis can occur [1].

12. (a) Solution B [1] because pure water has a neutral pH of 7 [1].

(b) pH = -log[H⁺]; 2 = -log[H⁺]; [H⁺] = 10⁻² = 0.01 mol/dm³ [1] for correct calculation; [1] for correct units.

(c) HCl + NaOH → NaCl + H₂O [1]

13. (a) Mᵣ of CaCO₃ = 40 + 12 + (3 × 16) = 100 [1]; Moles = mass / Mᵣ = 10.0 / 100 = 0.10 mol [1]

(b) Moles of CaO = moles of CaCO₃ = 0.10 mol [1]; Mᵣ of CaO = 40 + 16 = 56; Mass = moles × Mᵣ = 0.10 × 56 = 5.6 g [1]

(c) Any one of: manufacture of cement / manufacture of glass / neutralising acidic soils in agriculture / drying agent in industry [1]

14. (a) Covalent bonding [1]

(b) Ammonia has a simple molecular structure with weak intermolecular forces (van der Waals' forces) between molecules [1]. Only a small amount of energy is needed to overcome these weak forces, resulting in a low boiling point [1].

(c) NH₃ + HCl → NH₄Cl [1]

15. (a) The iron nail in test tube 2 is exposed to both water and oxygen (from air) [1], which are both necessary for rusting to occur. The iron oxidises to form hydrated iron(III) oxide (rust) [1].

(b) In test tube 3, the boiled water has had dissolved oxygen removed, and the oil layer prevents oxygen from the air dissolving into the water [1]. Without oxygen, rusting cannot occur [1].

(c) Any one of: painting / oiling / greasing / galvanising / sacrificial protection / alloying with chromium to make stainless steel [1]

Section D: Application and Analysis (10 marks)

16. (a) A yellow precipitate (sulfur) forms / the solution becomes cloudy [1]

(b) The cross disappears because the reaction produces a precipitate of sulfur which makes the solution cloudy / opaque [1]. As the precipitate forms, the solution becomes increasingly opaque until the cross can no longer be seen [1].

(c) The time taken for the cross to disappear would decrease [1] because at a higher temperature, the particles have more kinetic energy, leading to more frequent collisions and a greater proportion of collisions having energy greater than the activation energy, increasing the rate of reaction [1].

17. (a) Substance X (silicon dioxide is not in the table; the answer should refer to the substance with a high melting point and poor electrical conductivity) [1]. Silicon dioxide has a giant covalent structure with strong covalent bonds throughout, resulting in a high melting point, and no free electrons or ions, so it does not conduct electricity [1].

(b) Sodium chloride conducts electricity when dissolved in water because the ions (Na⁺ and Cl⁻) dissociate and are free to move in solution [1]. In solid state, the ions are held in fixed positions in the giant ionic lattice and cannot move [1].

(c) Graphite has a high melting point because it has a giant covalent structure with strong covalent bonds between carbon atoms [1]. A large amount of energy is needed to overcome these strong bonds.

18. (a) Bright white flame / white solid (magnesium oxide) formed / colourless gas (hydrogen) evolved [1]

(b) Moles of Mg = mass / Aᵣ = 0.48 / 24 = 0.020 mol [1]; From equation, moles of H₂ = moles of Mg = 0.020 mol [1]; Volume of H₂ = moles × molar volume = 0.020 × 24 = 0.48 dm³ [1]

(c) Steam provides more energy (higher temperature) than cold water [1], so the reaction is more vigorous.

19. (a) Diamond is very hard because each carbon atom is bonded to four other carbon atoms by strong covalent bonds in a tetrahedral arrangement [1]. This forms a rigid, three-dimensional giant covalent structure that is very difficult to break [1].

(b) Diamond does not conduct electricity because all four valence electrons of each carbon atom are used in covalent bonding [1]. There are no delocalised electrons or free ions to carry electric charge.

(c) Any one of: cutting tools / drill bits / jewellery (due to hardness and brilliance) [1]

20. (a) A reddish-brown solid (copper) deposits on the zinc / the blue colour of copper(II) sulfate solution fades / the zinc dissolves [1]

(b) This is a redox reaction because zinc loses electrons (oxidation) and copper ions gain electrons (reduction) [1]. Zn → Zn²⁺ + 2e⁻ (oxidation); Cu²⁺ + 2e⁻ → Cu (reduction) [1].

(c) Moles of Zn = mass / Aᵣ = 3.27 / 65.4 = 0.0500 mol [1]; From equation, moles of Cu = moles of Zn = 0.0500 mol; Mass of Cu = moles × Aᵣ = 0.0500 × 63.5 = 3.18 g [1]

END OF ANSWER KEY