Chemistry

1. 1.

   Propane, C₃H₈, is a member of the alkane homologous series. State the general formula of the alkanes and use it to write the molecular formula of the alkane with 6 carbon atoms.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Work through this mathematical step: States the general formula CₙH₂ₙ₊₂. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   3. Work through this mathematical step: Substitutes n = 6. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   4. Work through this mathematical step: Writes the molecular formula C₆H₁₄. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Substitute the carbon number directly into the general formula rather than counting atoms from a drawn structure.

   Marking points

   • States the general formula CₙH₂ₙ₊₂.
   • Substitutes n = 6.
   • Writes the molecular formula C₆H₁₄.

   Examiner tip: Substitute the carbon number directly into the general formula rather than counting atoms from a drawn structure.

2. 2.

   Ethene, C₂H₄, reacts with bromine water. Describe the observation and state the type of reaction that occurs, explaining how it differs from the reaction of an alkane with bromine water.

   [4 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: States that the orange/brown bromine water is decolourised (turns colourless). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: States that this is an addition reaction across the carbon-carbon double bond. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Develop this part of the answer: States that alkanes do not readily decolourise bromine water because they lack a double bond. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   5. Develop this part of the answer: Identifies the double bond (unsaturation) as the key structural difference responsible for the different reactivity. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Decolourisation of bromine water is the standard test for unsaturation (a C=C double bond).

   Marking points

   • States that the orange/brown bromine water is decolourised (turns colourless).
   • States that this is an addition reaction across the carbon-carbon double bond.
   • States that alkanes do not readily decolourise bromine water because they lack a double bond.
   • Identifies the double bond (unsaturation) as the key structural difference responsible for the different reactivity.

   Examiner tip: Decolourisation of bromine water is the standard test for unsaturation (a C=C double bond).

3. 3.

   Ethanol, C₂H₅OH, can be produced by fermentation of glucose. Write the word equation for this reaction and state the conditions required.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: Writes glucose → ethanol + carbon dioxide. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: States that yeast (containing the enzyme zymase) is required as the catalyst. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Develop this part of the answer: States that warm temperature (around 25–35°C) and anaerobic (absence of oxygen/air) conditions are required. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Fermentation must occur without oxygen — if air/oxygen is present, the ethanol produced would instead be oxidised to ethanoic acid (vinegar).

   Marking points

   • Writes glucose → ethanol + carbon dioxide.
   • States that yeast (containing the enzyme zymase) is required as the catalyst.
   • States that warm temperature (around 25–35°C) and anaerobic (absence of oxygen/air) conditions are required.

   Examiner tip: Fermentation must occur without oxygen — if air/oxygen is present, the ethanol produced would instead be oxidised to ethanoic acid (vinegar).

4. 4.

   Describe the addition polymerisation of ethene to form poly(ethene), including a diagram description of the repeat unit.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Work through this mathematical step: States that many ethene molecules (monomers), each containing a C=C double bond, join together. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   3. Develop this part of the answer: States that the double bonds open up, allowing the monomers to join end to end in long chains, forming poly(ethene). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Develop this part of the answer: Describes the repeat unit as −CH₂−CH₂−, with the double bond replaced by a single bond, repeated many times in the chain. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Addition polymerisation requires a monomer containing a C=C double bond, which opens to allow monomers to link without any atoms being lost.

   Marking points

   • States that many ethene molecules (monomers), each containing a C=C double bond, join together.
   • States that the double bonds open up, allowing the monomers to join end to end in long chains, forming poly(ethene).
   • Describes the repeat unit as −CH₂−CH₂−, with the double bond replaced by a single bond, repeated many times in the chain.

   Examiner tip: Addition polymerisation requires a monomer containing a C=C double bond, which opens to allow monomers to link without any atoms being lost.

5. 5.

   Distinguish between a saturated and an unsaturated hydrocarbon, and describe a simple chemical test to distinguish between them.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: States that a saturated hydrocarbon contains only single carbon-carbon bonds, e.g. alkanes. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: States that an unsaturated hydrocarbon contains at least one carbon-carbon double (or triple) bond, e.g. alkenes. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Develop this part of the answer: Describes adding bromine water: it stays orange/brown with a saturated hydrocarbon, but decolourises (turns colourless) with an unsaturated one. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The bromine water test distinguishes saturation directly: no colour change means saturated, decolourisation means unsaturated.

   Marking points

   • States that a saturated hydrocarbon contains only single carbon-carbon bonds, e.g. alkanes.
   • States that an unsaturated hydrocarbon contains at least one carbon-carbon double (or triple) bond, e.g. alkenes.
   • Describes adding bromine water: it stays orange/brown with a saturated hydrocarbon, but decolourises (turns colourless) with an unsaturated one.

   Examiner tip: The bromine water test distinguishes saturation directly: no colour change means saturated, decolourisation means unsaturated.

6. 6.

   Ethanoic acid, CH₃COOH, is a member of the carboxylic acid homologous series. State the general properties expected of a carboxylic acid, referring to its reaction with a carbonate.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: States that carboxylic acids display typical acid properties, e.g. turning blue litmus red. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: States that a carboxylic acid reacts with a carbonate to produce a salt, water and carbon dioxide gas. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Develop this part of the answer: States that effervescence (bubbling, from carbon dioxide gas) is observed during this reaction. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Carboxylic acids react with carbonates in the same general way as any other acid — this reaction is a standard test to confirm the presence of the carboxylic acid functional group.

   Marking points

   • States that carboxylic acids display typical acid properties, e.g. turning blue litmus red.
   • States that a carboxylic acid reacts with a carbonate to produce a salt, water and carbon dioxide gas.
   • States that effervescence (bubbling, from carbon dioxide gas) is observed during this reaction.

   Examiner tip: Carboxylic acids react with carbonates in the same general way as any other acid — this reaction is a standard test to confirm the presence of the carboxylic acid functional group.

7. 7.

   State the name and structural feature of the functional group present in alcohols, and use it to explain why ethanol is soluble in water.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: States that alcohols contain the hydroxyl functional group, −OH. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: States that the −OH group is polar and can form hydrogen bonds with water molecules. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Develop this part of the answer: States that this hydrogen bonding between ethanol and water molecules allows ethanol to mix freely (dissolve) with water, especially for small alcohol molecules. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: As the hydrocarbon chain of an alcohol gets longer, the non-polar hydrocarbon part dominates and solubility in water decreases, despite the polar −OH group being present.

   Marking points

   • States that alcohols contain the hydroxyl functional group, −OH.
   • States that the −OH group is polar and can form hydrogen bonds with water molecules.
   • States that this hydrogen bonding between ethanol and water molecules allows ethanol to mix freely (dissolve) with water, especially for small alcohol molecules.

   Examiner tip: As the hydrocarbon chain of an alcohol gets longer, the non-polar hydrocarbon part dominates and solubility in water decreases, despite the polar −OH group being present.

8. 8.

   Butane, C₄H₁₀, undergoes complete combustion in excess oxygen. Write the balanced symbol equation for this reaction.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Work through this mathematical step: Writes the unbalanced equation C₄H₁₀ + O₂ → CO₂ + H₂O. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   3. Develop this part of the answer: Balances carbon and hydrogen atoms: C₄H₁₀ + O₂ → 4CO₂ + 5H₂O. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Work through this mathematical step: Balances oxygen atoms by using the coefficient 13/2 (or doubling the whole equation): C₄H₁₀ + 13/2 O₂ → 4CO₂ + 5H₂O. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Balance carbon and hydrogen first (from the hydrocarbon formula), and balance oxygen last, since it often requires a fractional or larger coefficient.

   Marking points

   • Writes the unbalanced equation C₄H₁₀ + O₂ → CO₂ + H₂O.
   • Balances carbon and hydrogen atoms: C₄H₁₀ + O₂ → 4CO₂ + 5H₂O.
   • Balances oxygen atoms by using the coefficient 13/2 (or doubling the whole equation): C₄H₁₀ + 13/2 O₂ → 4CO₂ + 5H₂O.

   Examiner tip: Balance carbon and hydrogen first (from the hydrocarbon formula), and balance oxygen last, since it often requires a fractional or larger coefficient.

9. 9.

   Distinguish between a monomer and a polymer, using poly(ethene) as an example.

   [2 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: States that a monomer is a small molecule that can join with many other identical (or similar) molecules, e.g. ethene. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: States that a polymer is a very long chain molecule formed from many monomer units joined together, e.g. poly(ethene). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The prefix 'poly-' in a polymer's name directly indicates it is made of many repeating monomer units — poly(ethene) is made from many ethene monomers.

   Marking points

   • States that a monomer is a small molecule that can join with many other identical (or similar) molecules, e.g. ethene.
   • States that a polymer is a very long chain molecule formed from many monomer units joined together, e.g. poly(ethene).

   Examiner tip: The prefix 'poly-' in a polymer's name directly indicates it is made of many repeating monomer units — poly(ethene) is made from many ethene monomers.

10. 10.

    Methane, CH₄, reacts with chlorine, Cl₂, in the presence of ultraviolet light in a substitution reaction. State the type of reaction and write a word equation for the first substitution step.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Work through this mathematical step: States that this is a substitution reaction. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Develop this part of the answer: States that ultraviolet light is required to initiate the reaction. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Work through this mathematical step: Writes the word equation: methane + chlorine → chloromethane + hydrogen chloride. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: In a substitution reaction, one hydrogen atom in the alkane is replaced by one chlorine atom — the reaction can continue further if excess chlorine is present.

    Marking points

    • States that this is a substitution reaction.
    • States that ultraviolet light is required to initiate the reaction.
    • Writes the word equation: methane + chlorine → chloromethane + hydrogen chloride.

    Examiner tip: In a substitution reaction, one hydrogen atom in the alkane is replaced by one chlorine atom — the reaction can continue further if excess chlorine is present.

11. 11.

    Long-chain alkanes obtained from crude oil are of limited use and are in low demand, while shorter-chain alkanes and alkenes are in high demand. Describe the process of cracking, and state the two types of product it forms.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that cracking involves heating long-chain hydrocarbons strongly, often in the presence of a catalyst. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that this breaks (decomposes) long-chain molecules into smaller molecules. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that the products are a mixture of shorter-chain alkanes and alkenes. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Develop this part of the answer: States that cracking is used because it converts less useful, low-demand long-chain hydrocarbons into more useful, high-demand shorter-chain hydrocarbons and alkenes needed for the plastics industry. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Cracking is an example of thermal decomposition — a single type of long-chain molecule breaks apart to give a mixture of different, more useful, shorter products.

    Marking points

    • States that cracking involves heating long-chain hydrocarbons strongly, often in the presence of a catalyst.
    • States that this breaks (decomposes) long-chain molecules into smaller molecules.
    • States that the products are a mixture of shorter-chain alkanes and alkenes.
    • States that cracking is used because it converts less useful, low-demand long-chain hydrocarbons into more useful, high-demand shorter-chain hydrocarbons and alkenes needed for the plastics industry.

    Examiner tip: Cracking is an example of thermal decomposition — a single type of long-chain molecule breaks apart to give a mixture of different, more useful, shorter products.

12. 12.

    Crude oil is separated into fractions by fractional distillation. State the property used to separate the fractions, and describe how the boiling point, viscosity and flammability of the fractions change as chain length increases.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that crude oil is separated according to boiling point. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that boiling point increases as chain length (number of carbon atoms) increases. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that viscosity increases as chain length increases (fractions become thicker and flow less easily). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Develop this part of the answer: States that flammability decreases as chain length increases (longer-chain fractions are harder to ignite). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: All three trends (boiling point, viscosity up; flammability down) change together and consistently as chain length increases — remembering the boiling-point trend lets you deduce the other two.

    Marking points

    • States that crude oil is separated according to boiling point.
    • States that boiling point increases as chain length (number of carbon atoms) increases.
    • States that viscosity increases as chain length increases (fractions become thicker and flow less easily).
    • States that flammability decreases as chain length increases (longer-chain fractions are harder to ignite).

    Examiner tip: All three trends (boiling point, viscosity up; flammability down) change together and consistently as chain length increases — remembering the boiling-point trend lets you deduce the other two.

13. 13.

    State three features that define a homologous series, using the alkanes as an example.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Work through this mathematical step: States that members of a homologous series have the same general formula. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Develop this part of the answer: States that they show a gradual trend in physical properties, such as boiling point, as relative molecular mass increases. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that they have similar chemical properties, and each successive member differs from the last by a CH₂ unit. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: A homologous series behaves like a 'family' of compounds: same general formula, same chemical behaviour, and a smooth, predictable trend in physical properties as the chain gets longer.

    Marking points

    • States that members of a homologous series have the same general formula.
    • States that they show a gradual trend in physical properties, such as boiling point, as relative molecular mass increases.
    • States that they have similar chemical properties, and each successive member differs from the last by a CH₂ unit.

    Examiner tip: A homologous series behaves like a 'family' of compounds: same general formula, same chemical behaviour, and a smooth, predictable trend in physical properties as the chain gets longer.

14. 14.

    Butane, C₄H₁₀, has two structural isomers. State what is meant by the term structural isomers, and name the two isomers of butane.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Work through this mathematical step: States that structural isomers are compounds with the same molecular formula but different structural arrangements (different structural formulae). Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Develop this part of the answer: Names one isomer as butane (the straight-chain isomer). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: Names the other isomer as methylpropane (2-methylpropane, the branched-chain isomer). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Structural isomers share the exact same molecular formula and often similar chemical properties, but can have noticeably different physical properties, such as boiling point, due to their different shapes.

    Marking points

    • States that structural isomers are compounds with the same molecular formula but different structural arrangements (different structural formulae).
    • Names one isomer as butane (the straight-chain isomer).
    • Names the other isomer as methylpropane (2-methylpropane, the branched-chain isomer).

    Examiner tip: Structural isomers share the exact same molecular formula and often similar chemical properties, but can have noticeably different physical properties, such as boiling point, due to their different shapes.

15. 15.

    Ethanoic acid reacts with ethanol in the presence of a concentrated sulfuric acid catalyst to form an ester. Write the word equation for this reaction, name the ester formed, and state the type of reaction that occurs.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Work through this mathematical step: Writes the word equation: ethanoic acid + ethanol → ethyl ethanoate + water. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Develop this part of the answer: Names the ester formed as ethyl ethanoate. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that this reaction is called esterification, a type of condensation reaction. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Develop this part of the answer: States that the concentrated sulfuric acid acts as a catalyst for the reaction. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Ester names always follow the pattern 'alkyl alkanoate': the alcohol contributes the 'alkyl' part and the carboxylic acid contributes the 'alkanoate' part.

    Marking points

    • Writes the word equation: ethanoic acid + ethanol → ethyl ethanoate + water.
    • Names the ester formed as ethyl ethanoate.
    • States that this reaction is called esterification, a type of condensation reaction.
    • States that the concentrated sulfuric acid acts as a catalyst for the reaction.

    Examiner tip: Ester names always follow the pattern 'alkyl alkanoate': the alcohol contributes the 'alkyl' part and the carboxylic acid contributes the 'alkanoate' part.

16. 16.

    Distinguish between complete and incomplete combustion of a hydrocarbon fuel, and explain why incomplete combustion is dangerous.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that complete combustion occurs in a plentiful (excess) supply of oxygen, producing carbon dioxide and water only. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that incomplete combustion occurs in a limited (insufficient) supply of oxygen, producing carbon monoxide and/or carbon (soot) as well as water. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that carbon monoxide is toxic (poisonous). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Develop this part of the answer: Explains that carbon monoxide binds to haemoglobin in red blood cells, preventing them from carrying oxygen around the body, and that it is colourless and odourless, making it difficult to detect. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Carbon monoxide's danger comes specifically from being undetectable by human senses while binding to haemoglobin far more strongly than oxygen — this is why carbon monoxide detectors are essential wherever fuel is burned indoors.

    Marking points

    • States that complete combustion occurs in a plentiful (excess) supply of oxygen, producing carbon dioxide and water only.
    • States that incomplete combustion occurs in a limited (insufficient) supply of oxygen, producing carbon monoxide and/or carbon (soot) as well as water.
    • States that carbon monoxide is toxic (poisonous).
    • Explains that carbon monoxide binds to haemoglobin in red blood cells, preventing them from carrying oxygen around the body, and that it is colourless and odourless, making it difficult to detect.

    Examiner tip: Carbon monoxide's danger comes specifically from being undetectable by human senses while binding to haemoglobin far more strongly than oxygen — this is why carbon monoxide detectors are essential wherever fuel is burned indoors.

17. 17.

    Vegetable oils, which contain carbon-carbon double bonds, are reacted with hydrogen gas in the presence of a nickel catalyst to produce margarine. State the type of reaction that occurs, and explain what happens to the double bonds during this process.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that this is an addition reaction (hydrogenation). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that hydrogen adds across the carbon-carbon double bonds, converting them to single bonds. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that this converts the unsaturated liquid oil into a more saturated product with a higher melting point, which is solid (margarine) at room temperature. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Hydrogenation reduces the number of carbon-carbon double bonds in the oil, and it is this reduced unsaturation that raises the melting point enough to turn a liquid oil into a solid fat.

    Marking points

    • States that this is an addition reaction (hydrogenation).
    • States that hydrogen adds across the carbon-carbon double bonds, converting them to single bonds.
    • States that this converts the unsaturated liquid oil into a more saturated product with a higher melting point, which is solid (margarine) at room temperature.

    Examiner tip: Hydrogenation reduces the number of carbon-carbon double bonds in the oil, and it is this reduced unsaturation that raises the melting point enough to turn a liquid oil into a solid fat.

18. 18.

    Ethene reacts with steam in the presence of a phosphoric acid catalyst to produce ethanol industrially. Write the word equation for this reaction, and state the type of reaction that occurs.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Work through this mathematical step: Writes the word equation: ethene + steam → ethanol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Develop this part of the answer: States that this is an addition reaction (hydration), since steam (water) adds across the carbon-carbon double bond. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that phosphoric acid acts as a catalyst for the reaction. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: This industrial (hydration) route to ethanol is much faster than fermentation but requires ethene derived from crude oil, whereas fermentation uses renewable plant sugars but is slower and produces a dilute solution.

    Marking points

    • Writes the word equation: ethene + steam → ethanol.
    • States that this is an addition reaction (hydration), since steam (water) adds across the carbon-carbon double bond.
    • States that phosphoric acid acts as a catalyst for the reaction.

    Examiner tip: This industrial (hydration) route to ethanol is much faster than fermentation but requires ethene derived from crude oil, whereas fermentation uses renewable plant sugars but is slower and produces a dilute solution.

19. 19.

    But-1-ene and but-2-ene are both alkenes with the molecular formula C₄H₈. Explain what the numbers '1' and '2' in these names indicate.

    [2 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that the number indicates the position of the carbon-carbon double bond within the carbon chain. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that but-1-ene has the double bond starting at the first carbon atom, while but-2-ene has the double bond starting at the second carbon atom, making them different structural isomers of each other. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The locant number in an alkene's name is always chosen to be as low as possible, counting from whichever end of the chain gives the double bond the lower position number.

    Marking points

    • States that the number indicates the position of the carbon-carbon double bond within the carbon chain.
    • States that but-1-ene has the double bond starting at the first carbon atom, while but-2-ene has the double bond starting at the second carbon atom, making them different structural isomers of each other.

    Examiner tip: The locant number in an alkene's name is always chosen to be as low as possible, counting from whichever end of the chain gives the double bond the lower position number.

20. 20.

    State one difference between addition polymerisation and condensation polymerisation, in terms of the number of monomer types involved and whether a small molecule is lost.

    [2 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that addition polymerisation uses only one type of monomer, and no other product is formed besides the polymer. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that condensation polymerisation usually involves two different types of monomer, and a small molecule (such as water) is lost (eliminated) as each new bond forms between monomers. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Checking whether a small molecule such as water is released as a by-product is the quickest way to tell condensation polymerisation apart from addition polymerisation.

    Marking points

    • States that addition polymerisation uses only one type of monomer, and no other product is formed besides the polymer.
    • States that condensation polymerisation usually involves two different types of monomer, and a small molecule (such as water) is lost (eliminated) as each new bond forms between monomers.

    Examiner tip: Checking whether a small molecule such as water is released as a by-product is the quickest way to tell condensation polymerisation apart from addition polymerisation.

21. 21.

    Discuss one environmental advantage and one disadvantage of using ethanol produced by fermentation of plant sugars as a biofuel, compared with using petrol derived from crude oil.

    [2 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Build a supported judgement: identify the claim, use the question's evidence, consider a relevant limitation or alternative, and make the conclusion depend on that evidence. There may be more than one defensible answer.
    2. Develop this part of the answer: States a valid advantage, e.g. the carbon dioxide released when ethanol is burned is approximately balanced by the carbon dioxide absorbed by the growing plants during photosynthesis, making it more carbon-neutral than a fossil fuel. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States a valid disadvantage, e.g. growing crops for biofuel production uses land and water that could otherwise be used to grow food, potentially raising food prices or reducing food supply. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Ethanol as a biofuel is described as 'more carbon-neutral', not zero-carbon, since growing, harvesting and processing the crop still typically requires energy from fossil fuels.

    Marking points

    • States a valid advantage, e.g. the carbon dioxide released when ethanol is burned is approximately balanced by the carbon dioxide absorbed by the growing plants during photosynthesis, making it more carbon-neutral than a fossil fuel.
    • States a valid disadvantage, e.g. growing crops for biofuel production uses land and water that could otherwise be used to grow food, potentially raising food prices or reducing food supply.

    Examiner tip: Ethanol as a biofuel is described as 'more carbon-neutral', not zero-carbon, since growing, harvesting and processing the crop still typically requires energy from fossil fuels.

22. 22.

    Ethanol can be converted into ethanoic acid (vinegar) by oxidation. State a condition or reagent under which this oxidation can occur, and write the word equation for the reaction.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that ethanol can be oxidised by microbial (bacterial) action in the presence of air/oxygen, or by using an oxidising agent such as potassium manganate(VII) or potassium dichromate(VI). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Work through this mathematical step: Writes the word equation: ethanol + oxygen → ethanoic acid + water. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Develop this part of the answer: States that this is why wine or beer left exposed to air can turn sour (acidic) over time, as airborne bacteria oxidise the ethanol it contains. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: This oxidation is the reverse concern of fermentation: fermentation must be kept anaerobic to make ethanol, while exposing that ethanol to air afterwards is exactly what turns it into vinegar.

    Marking points

    • States that ethanol can be oxidised by microbial (bacterial) action in the presence of air/oxygen, or by using an oxidising agent such as potassium manganate(VII) or potassium dichromate(VI).
    • Writes the word equation: ethanol + oxygen → ethanoic acid + water.
    • States that this is why wine or beer left exposed to air can turn sour (acidic) over time, as airborne bacteria oxidise the ethanol it contains.

    Examiner tip: This oxidation is the reverse concern of fermentation: fermentation must be kept anaerobic to make ethanol, while exposing that ethanol to air afterwards is exactly what turns it into vinegar.

23. 23.

    State three pollutants that can be produced when fossil fuels are burned, and state one environmental or health problem caused by each.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that carbon dioxide contributes to global warming (the enhanced greenhouse effect). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that sulfur dioxide, produced from sulfur impurities in the fuel, causes acid rain. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that oxides of nitrogen, formed at high combustion temperatures, contribute to acid rain and can cause photochemical smog or respiratory problems. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Sulfur dioxide and oxides of nitrogen both cause acid rain, but they arise differently: sulfur dioxide comes from impurities already in the fuel, while oxides of nitrogen form directly from atmospheric nitrogen and oxygen at high temperature.

    Marking points

    • States that carbon dioxide contributes to global warming (the enhanced greenhouse effect).
    • States that sulfur dioxide, produced from sulfur impurities in the fuel, causes acid rain.
    • States that oxides of nitrogen, formed at high combustion temperatures, contribute to acid rain and can cause photochemical smog or respiratory problems.

    Examiner tip: Sulfur dioxide and oxides of nitrogen both cause acid rain, but they arise differently: sulfur dioxide comes from impurities already in the fuel, while oxides of nitrogen form directly from atmospheric nitrogen and oxygen at high temperature.

24. 24.

    State the general formula of the carboxylic acid homologous series, and use it to deduce the molecular formula of the carboxylic acid with 4 carbon atoms in total.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Work through this mathematical step: States the general formula CₙH₂ₙ₊₁COOH. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: States that for 4 carbon atoms in total, n = 3, since one carbon atom is already part of the COOH group. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Work through this mathematical step: Obtains the molecular formula C₃H₇COOH, i.e. C₄H₈O₂ (butanoic acid). Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The COOH group always accounts for one of the acid's total carbon atoms, so n in the general formula is always one less than the acid's total number of carbon atoms.

    Marking points

    • States the general formula CₙH₂ₙ₊₁COOH.
    • States that for 4 carbon atoms in total, n = 3, since one carbon atom is already part of the COOH group.
    • Obtains the molecular formula C₃H₇COOH, i.e. C₄H₈O₂ (butanoic acid).

    Examiner tip: The COOH group always accounts for one of the acid's total carbon atoms, so n in the general formula is always one less than the acid's total number of carbon atoms.

25. 25.

    State the difference between a molecular formula and a structural formula, using butane, C₄H₁₀, as an example.

    [2 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Work through this mathematical step: States that a molecular formula shows only the number of each type of atom present in a molecule, e.g. C₄H₁₀. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: States that a structural formula shows how the atoms are arranged and bonded to each other, e.g. CH₃CH₂CH₂CH₃, revealing the actual structure of the molecule. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: A molecular formula alone cannot distinguish between structural isomers (e.g. butane and methylpropane share the formula C₄H₁₀), which is exactly why a structural formula is needed.

    Marking points

    • States that a molecular formula shows only the number of each type of atom present in a molecule, e.g. C₄H₁₀.
    • States that a structural formula shows how the atoms are arranged and bonded to each other, e.g. CH₃CH₂CH₂CH₃, revealing the actual structure of the molecule.

    Examiner tip: A molecular formula alone cannot distinguish between structural isomers (e.g. butane and methylpropane share the formula C₄H₁₀), which is exactly why a structural formula is needed.

Original, unofficial practice questions from SubjectScout — not official past-paper questions.