A-Level Chemistry, Specimen Paper question and answers
update 2025
Describe how the student could use chemical tests to confirm that the liquid contained ethanal and did
not contain ethanoic acid. - ANSWERReagent to confirm the presence of ethanal:
Add Tollens' reagent
OR
Add Fehling's solution;
Warm;
Result with Tollen's reagent: Silver mirror
OR
Result with Fehling's solution: Red precipitate;
Reagent to confirm the absence of ethanoic acid:
Add sodium hydrogencarbonate;
Result: no effervescence observed, hence no acid present;
CH3COOH + CH3CH2OH <-> CH3COOCH2CH3 + H2O
A mixture of 8.00 × 10-2 mol of ethanoic acid and 1.20 × 10-1 mol of ethanol is allowed to reach
equilibrium at 20 °C.
• The equilibrium mixture is placed in a graduated flask and the volume made up to 250 cm3 with
distilled water.
• A 10.0 cm3 sample of this equilibrium mixture is titrated with sodium hydroxide added from a burette.
• The ethanoic acid in this sample reacts with 3.20 cm3 of 2.00 × 10-1 mol dm-3 sodium hydroxide
solution.
Describe what happens when a reaction mixture is refluxed and why it is necessary, in this case, for
complete oxidation to ethanoic acid. - ANSWERA mixture of liquids is heated to boiling point for a
, prolonged time; Vapour is formed which escapes from the liquid mixture, is changed back into liquid and
returned to the liquid mixture;
Any ethanal and ethanol that initially evaporates can then be oxidised;
Write a half-equation for the overall oxidation of ethanol into ethanoic acid. - ANSWERCH3CH2OH +
H2O -> CH3COOH + 4H+ + 4e-
Boiling Points: Ethanol = 78, Ethanal = 21, Ethanoic Acid = 118.
Use these data to describe how you would obtain a sample of ethanal from a mixture of these three
compounds. Include in your answer a description of the apparatus you would use and how you would
minimise the loss of ethanal. - ANSWERMixture heated in a suitable flask / container;
With still head containing a thermometer;
Water cooled condenser connected to the still head and suitable cooled collecting vessel;
Collect sample at the boiling point of ethanal;
Cooled collection vessel necessary to reduce evaporation of ethanal;
Use your knowledge of structure and bonding to explain why it is possible to separate ethanal in this
way. - ANSWERHydrogen bonding in ethanol and ethanoic acid or no hydrogen bonding in ethanal;
Intermolecular forces / dipole-dipole are weaker than hydrogen bonding;
Calculate the value for Kc for the reaction of ethanoic acid and ethanol at 20 °C. Give your answer to the
appropriate number of significant figures - ANSWERStage 1: Moles of acid at equilibrium Moles of
sodium hydroxide in each titration = (3.20 × 2.00 × 10-1 ) / 1000 = 6.40 × 10-4 Sample = 10 cm3 so moles
of acid in 250 cm3 of equilibrium mixture = 25 × 6.40 × 10-4 = 1.60 × 10-2
Stage 2: Moles of ester and water formed Moles of acid reacted = 8.00 × 10-2 - 1.60 × 10-2 = 6.40 × 10-2
= moles ester and water formed
update 2025
Describe how the student could use chemical tests to confirm that the liquid contained ethanal and did
not contain ethanoic acid. - ANSWERReagent to confirm the presence of ethanal:
Add Tollens' reagent
OR
Add Fehling's solution;
Warm;
Result with Tollen's reagent: Silver mirror
OR
Result with Fehling's solution: Red precipitate;
Reagent to confirm the absence of ethanoic acid:
Add sodium hydrogencarbonate;
Result: no effervescence observed, hence no acid present;
CH3COOH + CH3CH2OH <-> CH3COOCH2CH3 + H2O
A mixture of 8.00 × 10-2 mol of ethanoic acid and 1.20 × 10-1 mol of ethanol is allowed to reach
equilibrium at 20 °C.
• The equilibrium mixture is placed in a graduated flask and the volume made up to 250 cm3 with
distilled water.
• A 10.0 cm3 sample of this equilibrium mixture is titrated with sodium hydroxide added from a burette.
• The ethanoic acid in this sample reacts with 3.20 cm3 of 2.00 × 10-1 mol dm-3 sodium hydroxide
solution.
Describe what happens when a reaction mixture is refluxed and why it is necessary, in this case, for
complete oxidation to ethanoic acid. - ANSWERA mixture of liquids is heated to boiling point for a
, prolonged time; Vapour is formed which escapes from the liquid mixture, is changed back into liquid and
returned to the liquid mixture;
Any ethanal and ethanol that initially evaporates can then be oxidised;
Write a half-equation for the overall oxidation of ethanol into ethanoic acid. - ANSWERCH3CH2OH +
H2O -> CH3COOH + 4H+ + 4e-
Boiling Points: Ethanol = 78, Ethanal = 21, Ethanoic Acid = 118.
Use these data to describe how you would obtain a sample of ethanal from a mixture of these three
compounds. Include in your answer a description of the apparatus you would use and how you would
minimise the loss of ethanal. - ANSWERMixture heated in a suitable flask / container;
With still head containing a thermometer;
Water cooled condenser connected to the still head and suitable cooled collecting vessel;
Collect sample at the boiling point of ethanal;
Cooled collection vessel necessary to reduce evaporation of ethanal;
Use your knowledge of structure and bonding to explain why it is possible to separate ethanal in this
way. - ANSWERHydrogen bonding in ethanol and ethanoic acid or no hydrogen bonding in ethanal;
Intermolecular forces / dipole-dipole are weaker than hydrogen bonding;
Calculate the value for Kc for the reaction of ethanoic acid and ethanol at 20 °C. Give your answer to the
appropriate number of significant figures - ANSWERStage 1: Moles of acid at equilibrium Moles of
sodium hydroxide in each titration = (3.20 × 2.00 × 10-1 ) / 1000 = 6.40 × 10-4 Sample = 10 cm3 so moles
of acid in 250 cm3 of equilibrium mixture = 25 × 6.40 × 10-4 = 1.60 × 10-2
Stage 2: Moles of ester and water formed Moles of acid reacted = 8.00 × 10-2 - 1.60 × 10-2 = 6.40 × 10-2
= moles ester and water formed
