Chemistry

1. 1.

   Describe a test distinguishing ethene from ethane at room temperature without ultraviolet light, and state the observation for each.

   [3 marks] · no calculator

   Answer explanation

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

   1. Ethene's double bond undergoes addition with bromine. Ethane lacks that bond and needs different radical-substitution conditions, which the prompt deliberately excludes.

   Marking points

   • Add bromine water and shake.
   • Ethene decolourises it.
   • Ethane does not rapidly decolourise it under these conditions.

   Examiner tip: Keep the specified absence of UV in your comparison.

2. 2.

   State a reagent and condition for converting bromoethane into ethanol, and name the reaction type.

   [3 marks] · no calculator

   Answer explanation

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

   1. Hydroxide replaces bromide at the carbon bearing the halogen. An aqueous solvent favours the substitution route; ethanolic base is associated with elimination instead.

   Marking points

   • Use aqueous hydroxide, such as aqueous NaOH.
   • Heat under reflux.
   • The reaction is nucleophilic substitution.

   Examiner tip: The solvent is part of the condition, not an optional detail.

3. 3.

   Explain electrophilic addition of HBr to ethene, identifying the electron source, intermediate and final attacking species.

   [3 marks] · no calculator

   Answer explanation

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

   1. The electron-rich double bond reacts first with the electrophilic hydrogen. Bromide then supplies a lone pair to the positively charged carbon; the two stages consume the pi bond.

   Marking points

   • The pi bond donates an electron pair to H of polarised HBr.
   • H-Br breaks heterolytically, producing Br^- and a carbocation.
   • Br^- attacks the carbocation to form bromoethane.

   Examiner tip: Curly arrows start at an electron pair, never at the positive charge.

4. 4.

   Plan conversion of propan-1-ol to propanoic acid. Specify reagent and heating arrangement, and explain why distillation during oxidation would be unsuitable.

   [3 marks] · no calculator

   Answer explanation

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

   1. A primary alcohol oxidises through an aldehyde. Reflux returns vapour to the flask, giving retained propanal time to undergo the second oxidation rather than collecting it prematurely.

   Marking points

   • Use excess acidified dichromate(VI) or another suitable strong oxidant.
   • Heat under reflux to retain the organic intermediate.
   • Distillation can remove propanal before further oxidation to the acid.

   Examiner tip: Distinguish retaining an aldehyde for oxidation from distilling it to isolate it.

5. 5.

   A tertiary halogenoalkane hydrolyses through a carbocation; a primary analogue reacts in one concerted step. Compare how doubling hydroxide concentration affects each rate, under the simple SN1 and SN2 models.

   [4 marks] · no calculator

   Answer explanation

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

   1. Identify participants in the slow event rather than in the overall equation. Hydroxide traps the SN1 intermediate after the slow step, but participates directly in the SN2 transition state.

   Marking points

   • SN1's slow step is unimolecular ionisation of the substrate.
   • Its rate is independent of hydroxide in this model, so is unchanged.
   • SN2's rate-determining event involves substrate and hydroxide together.
   • Doubling hydroxide doubles the SN2 rate at fixed substrate concentration.

   Examiner tip: These are idealised mechanistic predictions, not universal solvent-independent rules.

6. 6.

   An ester is prepared from ethanol and ethanoic acid using an acid catalyst. Explain reflux, why conversion is incomplete, and how removing a product differs from adding more catalyst.

   [4 marks] · no calculator

   Answer explanation

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

   1. Separate an operational technique, a reversible chemical limitation and a way to perturb equilibrium. Catalysis affects forward and reverse pathways, whereas product removal changes composition.

   Marking points

   • Reflux permits heating without sustained loss of volatile reactants.
   • Esterification is reversible and reaches equilibrium.
   • Removing water or ester shifts equilibrium towards products.
   • More catalyst speeds approach but does not change the equilibrium constant/yield at fixed conditions.

   Examiner tip: A condenser returns vapour; it does not remove water from the equilibrium mixture by itself.

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