Secondary 4 Combined Science Chemistry Organic Chemistry Quiz

Secondary 4 Combined Science Chemistry Quiz - Organic Chemistry - ANSWER KEY

Total Marks: 40

Section A: Multiple Choice (10 marks)

1. B. CₙH₂ₙ Alkenes have the general formula CₙH₂ₙ due to the presence of one C=C double bond. [1 mark]

2. C. C₄H₈ Empirical formula: C:H = 85.7/12 : 14.3/1 = 7.14 : 14.3 ≈ 1:2 → CH₂. Empirical formula mass = 14. n = 56/14 = 4. Molecular formula = C₄H₈. [1 mark]

3. B. They have the same chemical properties. Members of a homologous series have the same functional group and therefore similar chemical properties. Physical properties (e.g., boiling point) vary with chain length. [1 mark]

4. A. Hydrogen gas with nickel catalyst at 150°C This is a hydrogenation/addition reaction: C₂H₄ + H₂ → C₂H₆. Nickel catalyst and heat are required. [1 mark]

5. B. But-2-ene But-1-ene and but-2-ene are structural isomers; both have molecular formula C₄H₈ but differ in the position of the double bond. [1 mark]

6. C. Ethanoic acid Ethanol is oxidised by acidified potassium dichromate(VI) to ethanoic acid. The oxidising agent changes from orange to green. [1 mark]

7. C. Protein Proteins are natural condensation polymers made from amino acids. Polyethene, nylon, and Terylene are synthetic polymers. [1 mark]

8. B. Addition Bromine adds across the C=C double bond; this is an addition reaction. The orange/brown colour of bromine is decolorised. [1 mark]

9. C. It produces carbon dioxide as a by-product. Fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. It uses yeast at about 37°C and does not require high pressure or an acid catalyst. [1 mark]

10. B. –CH(CH₃)–CH₂– Propene is CH₃–CH=CH₂. During addition polymerisation, the double bond opens to form the repeating unit –CH(CH₃)–CH₂–. [1 mark]

Section B: Short Answer (14 marks)

11. (a) The hydrocarbons in crude oil have different boiling points. During fractional distillation, the crude oil is heated and the vapours rise up a fractionating column. Substances with lower boiling points rise higher before condensing, allowing separation based on boiling point ranges. [1 mark]

(b) Bitumen is used for road surfacing / roofing. [1 mark]

12. (a) Add bromine water (or bromine dissolved in an organic solvent) to ethene. The orange/brown colour of bromine decolorises (turns colourless). This shows that ethene is unsaturated because it contains a C=C double bond that undergoes an addition reaction with bromine. [2 marks: 1 for reagent, 1 for observation]

(b)

   H   H   H   H
   |   |   |   |
–[–C–––C–––C–––C–]–
   |   |   |   |
   H   H   H   H

Two repeating units of –CH₂–CH₂– shown. Accept any clear representation of the polyethene chain. [2 marks: 1 for correct repeating unit, 1 for showing two units]

13. (a) C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ [1 mark]

(b) Fermentation uses glucose from plants, which is a renewable resource. Hydration of ethene uses ethene from crude oil, which is non-renewable. / Fermentation occurs at lower temperature and pressure, requiring less energy input. [1 mark for any valid advantage]

14. (a) CₙH₂ₙ₊₂ [1 mark]

(b)(i) Substitution (or free radical substitution) [1 mark]

(b)(ii) C₃H₈ + Cl₂ → C₃H₇Cl + HCl Accept any monosubstituted product (1-chloropropane or 2-chloropropane). Equation must be balanced. [2 marks: 1 for correct reactants and products, 1 for balanced equation]

15. (a) The boiling point increases as the number of carbon atoms increases (or as the molecular size increases down the series). [1 mark]

(b) As the molecular size increases, the relative molecular mass increases. The strength of the intermolecular forces of attraction (van der Waals' forces) between molecules increases. More energy is required to overcome these stronger forces, so the boiling point increases. [1 mark]

Section C: Data Analysis and Extended Response (16 marks)

16. (a)

Empirical formula = C₂H₆O [2 marks: 1 for correct mole calculation, 1 for correct ratio and formula]

(b) Empirical formula mass = (2×12) + (6×1) + 16 = 46. n = 46/46 = 1. Molecular formula = C₂H₆O. [1 mark]

(c)(i) Esterification (or condensation) [1 mark]

(c)(ii) Ethyl ethanoate [1 mark]

(c)(iii) C₂H₅OH + CH₃COOH ⇌ CH₃COOC₂H₅ + H₂O Accept structural formulae. Equation must be balanced. [2 marks: 1 for correct reactants and products, 1 for balanced equation]

17. (a) Aluminium oxide (or silicon dioxide / broken porcelain / zeolites) [1 mark]

(b)(i) Addition polymerisation [1 mark]

(b)(ii) Polyethene is non-biodegradable, so it persists in the environment for many years, causing land pollution / litter problems. / When burned, it can release toxic gases. [1 mark for any valid environmental problem]

(c)(i) 2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O Equation must be balanced. [2 marks: 1 for correct products, 1 for correct balancing]

(c)(ii) Incomplete combustion produces carbon monoxide (CO) and/or soot (carbon particles). Carbon monoxide binds irreversibly to haemoglobin in red blood cells, reducing the blood's ability to carry oxygen. This can cause fatigue, headaches, loss of consciousness, and even death at high concentrations. Soot particles can cause respiratory problems. [2 marks: 1 for identifying CO/soot, 1 for explaining health effects]

18. (a)

    H   O
    |   ||
H – C – C – O – H
    |
    H

Accept any clear structural formula showing all atoms and bonds. [1 mark]

(b)(i) A weak acid is an acid that partially ionises/dissociates in water to form hydrogen ions (H⁺). Only a small proportion of the acid molecules release H⁺ ions. [1 mark]

(b)(ii) 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂ Equation must be balanced. Accept structural formulae. [2 marks: 1 for correct reactants and products, 1 for balanced equation]

19. (a) They are isomers (or structural isomers / chain isomers). [1 mark]

(b) Compound A (butane) is a straight-chain molecule, while Compound B (2-methylpropane) is branched. Straight-chain molecules have a larger surface area of contact between molecules, resulting in stronger intermolecular forces of attraction (van der Waals' forces). More energy is required to overcome these stronger forces, so Compound A has a higher boiling point. Branched molecules have less surface contact, so intermolecular forces are weaker. [2 marks: 1 for identifying difference in structure, 1 for linking to intermolecular forces and boiling point]

20. (a) Condensation polymerisation is a type of polymerisation in which monomers join together with the elimination of a small molecule (such as water or hydrogen chloride) for each new bond formed. [1 mark]

(b)

• Advantage: Polyesters are strong, lightweight, and durable / can be moulded into various shapes / are cheap to produce / can be recycled. [1 mark for any valid advantage]
• Disadvantage: Polyesters are non-biodegradable, causing long-term environmental pollution / they are made from crude oil, a non-renewable resource / their production and disposal can release harmful chemicals. [1 mark for any valid disadvantage]

END OF ANSWER KEY