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Section 4: Organic Chemistry Exam Practice

Edexcel IGCSE Chemistry • Exam-style Practice Questions • Structured Questions

Exam Practice

Section 4: Organic Chemistry Practice

Test your understanding of Organic Chemistry with exam-style questions. Attempt the multiple-choice section, then write your answers to the structured questions and compare them to the mark scheme.

Edexcel IGCSE Hub › Section 4 › Exam Practice

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📋 Structured Questions

These questions test key concepts from Section 4. Attempt each question on paper, then click "Show Mark Scheme" to check your answer.

Question 1: Fractional Distillation of Crude Oil

4 marks

(a) Describe how crude oil is separated into different fractions in a fractionating column. [4]

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  • Crude oil is heated and vaporised before entering the column [1]
  • The fractionating column has a temperature gradient: it is hot at the bottom and cold at the top [1]
  • Vapours rise up the column, cool, and condense back into liquids [1]
  • Different fractions condense and are tapped off at different heights based on their respective boiling points (shorter-chain hydrocarbons with lower boiling points condense near the top; longer-chain hydrocarbons with higher boiling points condense near the bottom) [1]
Examiner tip: Be sure to explicitly mention the temperature gradient in the column (hot at the bottom, cold at the top) and explain that separation is based on differences in boiling points.

Question 2: Alkanes vs Alkenes

4 marks

(a) Write a balanced chemical equation for the addition reaction between ethene (C2H4) and bromine (Br2). [1]

(b) Describe a chemical test to distinguish between ethene and ethane. Include the reagent used and the observations for both substances. [3]

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(a)

  • C2H4 + Br2 → C2H4Br2 (or CH2CH2 + Br2 → CH2BrCH2Br) [1]

(b)

  • Reagent: Add bromine water [1]
  • Observation with ethene: The orange solution is decolourised / turns colourless [1]
  • Observation with ethane: The solution remains orange / no change [1]
Examiner tip: For ethene, write that it turns "colourless", not "clear". A solution can be orange and clear; "colourless" means the colour has disappeared. Make sure you state that ethane does not change color under normal laboratory conditions.

Question 3: Cracking Hydrocarbons

3 marks

(a) A long-chain alkane with molecular formula C12H26 is cracked to produce ethene (C2H4) and one other hydrocarbon. Write a balanced chemical equation for this cracking reaction. [1]

(b) Explain why cracking is an important process in the petroleum industry. [2]

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(a)

  • C12H26 → C2H4 + C10H22 [1]

(b)

  • It breaks down surplus, less useful long-chain hydrocarbons into shorter-chain hydrocarbons to match supply with market demand [1]
  • It produces short-chain alkanes which are highly in demand as fuels, and alkenes which are used as raw materials to make polymers/plastics [1]
Examiner tip: In part (a), count your carbon and hydrogen atoms carefully to make sure the equation balances. For part (b), remember to mention both the supply/demand mismatch of crude oil fractions and the specific utility of the products (fuels and ethene for polymers).

Question 4: Manufacturing Ethanol

4 marks

(a) Compare the two industrial methods used to manufacture ethanol: the fermentation of glucose and the hydration of ethene. State one advantage of each method. [4]

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  • Fermentation description: Uses glucose from crops, yeast catalyst, anaerobic conditions at around 30°C [1]
  • Fermentation advantage: Uses renewable resources (sugar cane) / requires low energy / cheap equipment [1]
  • Hydration description: Reacts ethene with steam using a phosphoric acid catalyst at 300°C and 60-70 atmospheres pressure [1]
  • Hydration advantage: Fast reaction rate / produces pure ethanol / continuous process (more efficient) [1]
Examiner tip: When comparing, make sure you address both raw materials (renewable vs non-renewable) and reaction type (batch vs continuous). Hydration yields pure ethanol, whereas fermentation yields a dilute mixture that must be distilled.

Question 5: Preparation of an Ester

4 marks

(a) Explain how ethyl ethanoate can be prepared in the laboratory from ethanol and ethanoic acid. Include the catalyst used and the balanced chemical equation for the reaction. [4]

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  • Catalyst: Concentrated sulfuric acid (H2SO4) [1]
  • Method: Heat the mixture of ethanoic acid, ethanol, and the catalyst in a water bath / under reflux [1]
  • Equation: CH3COOH + CH3CH2OH ⇌ CH3COOCH2CH3 + H2O [2]
    (Award [1] for correct reactants, award [1] for correct products.)
Examiner tip: Concentrated sulfuric acid acts as both a catalyst and a dehydrating agent to shift the equilibrium position to the right. Note that water is a product of this reaction, so always include H2O in your equation. Since organic mixtures are flammable, heat them using a water bath rather than a direct Bunsen burner flame.

Question 6: Addition vs Condensation Polymers

6 marks

(a) Compare addition polymerisation and condensation polymerisation. In your answer, explain how the structures of the monomers differ and how the reactions differ in terms of the products formed. Use poly(ethene) and a polyester as examples. [6]

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Indicative content:

  • Monomer Structures:
    • Addition polymerisation: The monomers contain a carbon-carbon double bond (C=C), making them unsaturated. Example: ethene is the monomer for poly(ethene).
    • Condensation polymerisation: The monomers contain two different functional groups (e.g. a diol with two -OH groups and a dicarboxylic acid with two -COOH groups).
  • Reaction & Products:
    • Addition polymerisation: Monomers simply join together, and the only product formed is the polymer itself. No other molecules are produced.
    • Condensation polymerisation: The monomers react and join together, but for each link formed, a small molecule (such as water or hydrogen chloride) is released as a byproduct.
  • Examples:
    • Poly(ethene) is formed from ethene monomers by opening the C=C double bonds to form single C-C chains.
    • Polyester is formed when a dicarboxylic acid monomer reacts with a diol monomer, releasing water molecules as ester linkages are created.

Marking guidance:

  • 5 to 6 marks: Detailed comparisons of both monomer structures and reaction products are explained correctly. Both poly(ethene) and polyesters are used as appropriate examples.
  • 3 to 4 marks: Compares either monomer structures or reaction products, with at least one example discussed.
  • 1 to 2 marks: Simple statements identifying ethene as an addition monomer or that condensation polymerisation releases water.
Examiner tip: Ensure you state the exact small molecule produced during polyester formation (which is water, H2O). When discussing addition polymerisation, emphasize that the polymer is the only product formed.