2023 Paper 3

• Mistake 1 - there is a bung/stopper in the end of the condenser

  M2 idea of pressure build up - stopper could be forced out

  Mistake 2 - water goes the wrong way through the condenser

  M4 - water does not fill the condenser / condenser is not cool

  enough

  OR products will not condense

  
• The reaction mixture is flammable - Suggest how the reaction mixture should be heated in Step 1

  • electric heater/heat mantle or (hot) water bath
  • ALLOW hot water
• Ethanol + ethanoic acid -> ethyl ethanoate + water

  Give a reason why two layers form in the separating funnel.

  Suggest why ethyl ethanoate forms the upper layer

  • ethyl ethanoate/it is immiscible with / insoluble in water
  • OR water/solution and ethyl ethanoate/it do not mix
  • Ethyl ethanoate is LESS DENSE / has lower density than water
• STATE WHY sodium carbonate is added to distillate.

  Explain why there is a build-up of pressure in separating funnel

  • 01.4 was a question where a lack of precision in answers
  • A clear reference to the fact that the sodium carbonate was added to neutralise ACID! was needed
  • together with recognition that this reaction produces carbon dioxide gas.
• Give a reason why two layers form in separating funnel

  Suggest why ethyl ethanoate forms the upper layer

  • Answers to 01.5 also often lacked clarity and precision.
  • The key factor is that the organic and aqueous materials are immiscible.
  • The fact that the ethyl ethanoate forms the upper layer is then explained in terms that it is LESS DENSE (NOT lighter) than the aqueous layer.
• Suggest a reason why percentage yield is not 100%

  • reaction is an equilibrium/reversible (ethanoic acid and ethanol)
  • losses during distillation/isolation/purification/transfer /
  • incomplete distillation / side reactions / by- products
  • incomplete reaction
  • Impurities / contamination / water present / not dry in product
• DEHYDRATION OF ALCOHOL 3.5.3 Elimination

  • Most of question 2 required students to be familiar with the dehydration of alcohols to form alkenes
  • and therefore with the idea that the reaction involves the loss of OH from one C atom and the loss of an H from an ADJACENT Carbon atom.
  • Familiarity with the concept of isomerism was also required.
• Drawing an alcohol that is NOT dehydrated when heated with concentrated sulfuric acid [1 mark]

  • NOT tertiary alcohol - this suggested a confusion about which alcohol cannot be oxidised rather than which alcohol cannot be dehydrated
  • CH2OH group next to it is a carbon bonded to 3 methyl groups

  
• • 03.2 required students to deduce the structure of proline by working out what would remain from hydrolysing the peptide groups either side of proline in the partial structure given.
  • The commonest error was, as expected, to show an NH2 group instead of NH.
  • Nitrogen can have a maximum of 3 bonds - N bonded to H, and 2 carbons

  
• A student hydrolyses a sample of endomorphin box -2 to break it down into its constituent amino acids.

  The student analyses the resulting mixture by thin-layer chromatography, TLC.

  0 3 . 3 State a reagent and the conditions needed for the hydrolysis.

  • (aqueous) HCl /hydrochloric acid (Ignore concentrated)
  • Conditions = Reflux OR Heat
• • Missing piece of apparatus - Lid
  • In 03.4, the key idea of this sort of chromatography (which is the need for the atmosphere in the tank to be saturated with vapour of the mobile phase) is clearly not well known by students,
  • to reduce evaporation from the thin layer plate
  • so atmosphere in beaker is saturated with solvent vapour owtte
  • •to reduce evaporation from the plate
  • prevents escape of vapour (from beaker) / evaporation of solvent (from beaker)

  
• • In 03.3, the need for reflux with 6 mol dm–3 HCl is not well known by students – but nor is it specified in the specification.
  • the key idea of this sort of (TLC) chromatography which is the need for the atmosphere in the tank to be saturated with vapour of the mobile phase is clearly not well known by students,
• State why amino acids separate on the TLC plate.

  • Separation depends on the balance between solubility in the moving phase AND retention by the stationary phase.
  • Direct test of a statement in the specification
• Name a suitable developing agent - State why it is needed

  • Sprayed on the TLC plate - so therefore spraying agent is NINHYDRIN NOT UV light
  • Amino acids are colourless / to make amino acids visible
• This question was also a prime example of the need for greater precision in the language used for an explanation –

  • many students referred to differences in electronegativity without explicitly stating which atoms the difference was between
  • or which way round the difference was (ie. which atom had the greater electronegativity).

  
• • Electronegativity was also often associated with a group of atoms or even a whole molecule rather than as a property of each INDIVIDUAL ATOM.
  • The shapes of all three molecules then needed to be identified (2 bent and one pyramidal) together with a reference to the fact that they are all asymmetrical.
• • Students need to remember that, for a molecule to have a dipole moment (ie for it to be polar)
  • there is the need both for polar bonds to be present and for the molecule to be asymmetrical
  • (eg the C=O bonds in CO2 are polar as O is more electronegative than C, but the molecule overall is not polar due to its symmetrical, linear shape).