Secondary 4 Combined Science Chemistry Organic Chemistry Quiz

Secondary 4 Combined Science Chemistry Quiz - Organic Chemistry

Answer Key

Section A: Multiple Choice Questions

1. B [1]
Explanation: Alkanes are saturated hydrocarbons with only C–C single bonds. Their general formula is CₙH₂ₙ₊₂. Option A (CₙH₂ₙ) is the general formula for alkenes. Option C (CₙH₂ₙ₋₂) is for alkynes. Option D resembles the general formula for alcohols (CₙH₂ₙ₊₁OH).

2. B [1]
Explanation: CH₃CH₂CH₂CH₃ contains 4 carbon atoms in a continuous chain. The prefix for 4 carbons is "but-" and, as an alkane, the suffix is "-ane," giving butane. Propane has 3 carbons, pentane has 5, and hexane has 6.

3. B [1]
Explanation: Bromine water (orange/brown) is decolorised when it reacts with a carbon–carbon double bond (C=C) in unsaturated compounds. Limewater tests for CO₂. Universal indicator tests for acidity/alkalinity. Silver nitrate tests for halide ions.

4. C [1]
Explanation: Ethene (C₂H₄) undergoes an addition reaction with steam (H₂O) in the presence of a phosphoric acid catalyst to form ethanol (C₂H₅OH). The C=C double bond opens up and a water molecule adds across it. This is not combustion (which produces CO₂ and H₂O), substitution (which replaces an atom/group), or neutralisation (acid + base).

5. C [1]
Explanation: C₂H₄ (ethene) fits the general formula for alkenes, CₙH₂ₙ (where n = 2). C₂H₆ (ethane), C₃H₈ (propane), and CH₄ (methane) are all alkanes with the general formula CₙH₂ₙ₊₂.

Section B: Short-Answer and Structured Questions

6. (a) A homologous series is a family of organic compounds that have the same general formula, the same functional group, and similar chemical properties, with successive members differing by a –CH₂– group (or 14 relative molecular mass units). [2]
Marking: 1 mark for "same general formula / functional group / similar chemical properties" (any one); 1 mark for "successive members differ by CH₂."

(b) Any two of the following: [1 mark each, total 2]
(i) They have the same general formula.
(ii) They show a gradual change in physical properties (e.g., boiling point increases with molecular size).
(iii) They have similar chemical properties.
(iv) Successive members differ by CH₂.
(v) They have the same functional group.

7. Full structural formula of propene (C₃H₆): [2]

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

Or equivalently drawn showing all bonds.
Marking: 1 mark for correct carbon skeleton (3 carbons with one C=C double bond); 1 mark for correct number of hydrogen atoms bonded (6 H atoms total). Deduct 1 mark if any bond is missing or if the double bond is shown as a single bond.

8. (a) Compound Y is unsaturated. [1] The bromine water turned colourless because the C=C double bond in compound Y reacted with bromine in an addition reaction, using up the bromine and causing the orange colour to disappear. [1]
Marking: 1 mark for identifying Y; 1 mark for correct explanation involving C=C / addition reaction.

(b) Carbon–carbon double bond (C=C) [1]

9. (a) 2-methylpropan-2-ol (or tert-butanol) [1]
Marking: Accept "2-methyl-2-propanol" or "tertiary butanol." Deduct if numbering is incorrect.

(b) Tertiary alcohol [1] because the carbon atom bonded to the –OH group is attached to three other carbon atoms. [1]
Marking: 1 mark for classification; 1 mark for correct reasoning.

10. (a) Esterification [1]
(b) Ethyl ethanoate (or ethyl acetate) [1]
(c) Any one of the following: used as a solvent, used in flavourings / perfumes, used in making plastics / adhesives. [1]

11. [1 mark each, total 4]

Marking: 1 mark per correct cell. Accept either molecular formula format for ethanol and ethanoic acid.

12. [3]
Reagent: Bromine water. [1]
Observation with ethane: The orange/brown colour of bromine water remains (no change). [½]
Observation with ethene: The orange/brown colour of bromine water turns colourless (decolorises). [½]
Conclusion: Ethene is unsaturated (contains a C=C double bond) and reacts with bromine water in an addition reaction. Ethane is saturated (contains only C–C single bonds) and does not react with bromine water under these conditions. [1]
Marking: 1 mark for reagent; ½ + ½ for correct observations; 1 mark for conclusion linking unsaturation to the result. Award 2 marks if reagent and observations are correct but conclusion is incomplete.

13. (a) Cracking is the process of breaking down long-chain hydrocarbons (large alkane molecules) into smaller, more useful molecules (shorter-chain alkanes and alkenes) using heat and a catalyst. [1]
(b) Any one of the following: [1]

• To produce smaller, more useful hydrocarbons (e.g., petrol/gasoline).
• To produce alkenes (e.g., ethene) which are used to make polymers.
• Because the demand for short-chain hydrocarbons is greater than what is obtained directly from fractional distillation.

(c) C₆H₁₄ → C₂H₄ + C₄H₁₀ [2]
Marking: 1 mark for correct reactant and products; 1 mark for balancing. Accept other valid cracking products (e.g., C₆H₁₄ → 3C₂H₄ + H₂ or C₆H₁₄ → C₂H₄ + C₄H₁₀) as long as the equation is balanced. Common mistake: students may write C₃H₆ (propene) instead of C₂H₄; accept if balanced correctly.

14. (a) The hydrocarbon contains carbon and hydrogen only. [1]
(b) Complete combustion [1]
(c) Any one of the following: [1]

• Release of CO₂ contributes to global warming / the greenhouse effect.
• Release of CO₂ contributes to climate change.
• If impurities are present, sulfur dioxide may be released, causing acid rain.
  Marking: Accept any valid environmental consequence linked to combustion of fossil fuels.

15. (a) Chloroethene (or vinyl chloride, CH₂=CHCl) [1]
(b) Addition polymerisation [1]
(c) Any one of the following: [1]

• PVC (polyvinyl chloride) is used to make pipes, window frames, insulation for electrical cables, clothing (synthetic leather), or packaging.
  Marking: Accept any valid use of PVC.

Section C: Free-Response and Application Questions

16. (a) The compounds belong to the alkane homologous series. [1]
Evidence: The molecular formulas follow the general formula CₙH₂ₙ₊₂ (e.g., CH₄ where n=1 gives 2(1)+2=4 hydrogen atoms; C₄H₁₀ where n=4 gives 2(4)+2=10 hydrogen atoms). [1]
Alternative evidence: They are all insoluble in water and are hydrocarbons, consistent with alkanes.
Marking: 1 mark for identifying the series; 1 mark for valid evidence from the table.

(b) The boiling point increases as the number of carbon atoms (and hence the relative molecular mass) increases. [1]
This is because larger molecules have more electrons, leading to stronger London dispersion forces (van der Waals forces) between molecules. [1]
More energy is required to overcome these stronger intermolecular forces, so the boiling point is higher. [1]
Marking: 1 mark for describing the trend; 1 mark for mentioning more electrons / larger molecular size / stronger intermolecular forces; 1 mark for linking to energy required / boiling point.

(c) The boiling point of C₅H₁₂ (pentane) would be higher than −1 °C (approximately 36 °C is acceptable). [1]
This is because pentane has one more CH₂ group than butane, so it has a larger molecular size, more electrons, and stronger London dispersion forces, requiring more energy to boil. [1]
Marking: 1 mark for predicting a boiling point higher than −1 °C (accept any reasonable value above −1 °C, e.g., 0–50 °C); 1 mark for correct reasoning. Award 1 mark if the prediction is wrong but the reasoning is correct.

17. (a) Full structural formula of ethanol: [2]

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

Marking: 1 mark for correct carbon–carbon and carbon–oxygen skeleton; 1 mark for correct number of hydrogen atoms (6 H atoms total). Deduct 1 mark if the O–H bond is missing.

(b) C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ [2]
Marking: 1 mark for correct reactants and products; 1 mark for balancing. Accept "glucose → ethanol + unbalanced CO₂" for 1 mark.

(c) Any two of the following: [1 mark each, total 2]
(i) Temperature of approximately 30–40 °C (warm conditions; not too high, as enzymes would denature).
(ii) Absence of oxygen (anaerobic conditions).
(iii) Use of yeast (which contains the enzyme zymase).
(iv) Aqueous / dilute solution of glucose.

(d) Any one of the following: [1]

• Fermentation uses renewable resources (e.g., sugar cane, corn) whereas ethene is obtained from crude oil, which is a non-renewable resource.
• Fermentation is more environmentally friendly / sustainable.
• Fermentation requires less energy (lower temperature and pressure).

18. (a) Add bromine water to each compound separately. [1]
Butene will decolorise the bromine water (orange → colourless) because it contains a C=C double bond that undergoes an addition reaction with bromine. [1]
Butane will not decolorise the bromine water (orange colour remains) because it is saturated and does not react with bromine water under these conditions. [1]
Marking: 1 mark for reagent; 1 mark for correct observation with butene; 1 mark for correct observation with butane and/or conclusion.

(b) Acidified potassium dichromate(VI) solution (or K₂Cr₂O₇ / H₂SO₄) [1]
Accept: "acidified potassium manganate(VII)" or "K₂Cr₂O₇" alone.

(c) Carboxylic acid [1]

19. (a) CₙH₂ₙ₊₁OH (or CₙH₂ₙ₊₂O) [1]
Marking: Accept either form. Deduct if the formula is incorrect.

(b) C₅H₁₁OH (or C₅H₁₂O) [1]

(c) As the number of carbon atoms increases, the molecular size and number of electrons increase. [1] This results in stronger London dispersion forces between molecules, so more energy is required to overcome these forces, and the boiling point increases. [1]
Marking: 1 mark for linking increasing carbon atoms to larger molecular size / more electrons; 1 mark for stronger intermolecular forces and higher energy required.

(d) Alcohols contain an –OH (hydroxyl) group which can form hydrogen bonds with water molecules. [1] Since methanol and ethanol have small hydrocarbon chains, the –OH group dominates, allowing them to mix freely with water. [1]
Marking: 1 mark for mentioning the –OH group / hydrogen bonding; 1 mark for explaining solubility in terms of interaction with water. Award 1 mark if only partial explanation is given.

20. (a) A polymer is a large molecule (macromolecule) made up of many repeating units (monomers) joined together by covalent bonds. [1]
Marking: 1 mark for "large molecule made of repeating monomer units."

(b) Addition polymerisation: [1]

• Involves monomers with C=C double bonds (usually alkenes) joining together without the loss of any atoms.
• The double bond opens up and monomers add to a growing chain.
• Example: Poly(ethene) / polyethylene, made from ethene monomers. [½]

Condensation polymerisation: [1]

• Involves monomers with two or more functional groups (e.g., –OH, –COOH, –NH₂) joining together with the loss of a small molecule (usually water).
• Example: Nylon (polyamide) or polyester (e.g., Terylene/PET), made from a dicarboxylic acid and a diol (or diamine). [½]

Marking: 1 mark for correct description of addition polymerisation; 1 mark for correct description of condensation polymerisation; ½ + ½ for correct examples. Award partial marks for partially correct descriptions.

(c) Any two of the following: [1 mark each, total 2]
(i) Plastics are non-biodegradable, so they persist in the environment for hundreds of years.
(ii) Plastics in oceans harm marine life (animals ingest or become entangled in plastic).
(iii) Burning plastics releases toxic gases (e.g., hydrogen chloride from PVC, dioxins).
(iv) Plastics take up space in landfills.
(v) Microplastics enter the food chain and affect ecosystems and human health.

(d) Any one of the following: [1]

• Recycle plastics.
• Use biodegradable alternatives.
• Reduce single-use plastic consumption.
• Reuse plastic products.
• Implement government policies (e.g., plastic bag taxes/bans).

— End of Answer Key —

Mark Summary