AQA A-Level Chemistry Paper 2, 2023: Question 6

14 marks · Easy difficulty · Practical Techniques & Data Analysis

Identifying Organic Liquids and Fractional Distillation This question involves identifying reagents and corrections for chemical tests on liquids J, K, L, and M, determining reactions and observations for each test, and explaining the use of fractional distillation with appropriate labels for the apparatus.

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Question

AQA A-Level Chemistry Paper 2, 2023: Question 6
Question text

06 A student plans a series of chemical tests to confirm the identities of four organic

liquids.

This is the student’s method.

To separate test tubes containing samples of each liquid:

Test 1 add potassium dichromate(VI) solution and warm gently

Test 2 add Fehling’s solution and cool in iced water

Test 3 add sodium hydrogencarbonate solution and test any gas produced

with a lighted splint

Test 4 add bromine water and shake at room temperature.

06.1 Identify the missing reagent needed in Test 1.

[1 mark]

06.2 In addition to the missing reagent in Test 1, there is a mistake in the method for two

of the other tests.

State the two mistakes.

Suggest how each of the mistakes should be corrected.

[2 marks]

Mistake 1

Suggestion

Mistake 2

*16* Suggestion

06.3 The missing reagent is added and the mistakes are corrected.

Identify the liquid(s), J, K, L and M, that would react in each test.

State the expected observation for each reaction.

[8 marks]

Liquid(s) that react in Test 1

Expected observation

Liquid(s) that react in Test 2

Expected observation

Liquid(s) that react in Test 3

Expected observation

Liquid(s) that react in Test 4

Expected observation

06.4 Figure 10 shows the apparatus that is used to separate a mixture of

liquids K and M using fractional distillation.

Suggest labels that should be added to positions S, T and U in Figure 10.

Explain why fractional distillation is preferred to simple distillation to separate

liquids K and M.

[3 marks]

Figure 10

Label S

Label T

Label U

Explanation

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 2, 2023: Question 6

Question Answers Additional Comments/Guidelines Mark

Use H2SO4 Allow HCl / H3PO4 1

06.1

Ignore conc / dilute (AO1)

M1 Cool test 2

warm (water bath) Allow heat / hot

06.2

(2 x AO3)

M2 Gas is tested with lighted splint in test 3

Bubble into limewater Allow no test on gas needed

M1 J and M

M2 Test 1 (Orange solution goes) green

M3 M

M4 Test 2 (Blue solution gives a brick) red precipitate Allow (Brown-red/orange/orange-red)

06.3

M5 J and L (8 x AO3)

M6 Test 3 (Colourless gas that turns) limewater cloudy Allow M6 Test 3 fizz / effervescence

M7 K

M8 Test 4 (Orange solution goes) colourless Allow (Brown/Brown-red/yellow/yellow-orange)

Allow decolorises bromine

M1 S – Fractionating column M1 Allow beads

06.4 M2 Both T – Water out AND U – Water in

(3 x AO3)

M3 Liquids K and M are likely to have similar boiling points

How to answer it

Step-by-Step Commentary for Chemical Tests

Question (a): Missing Reagent in Test 1

Step 1: Identify the Reagent

The missing reagent for Test 1 is H2SO4 (sulfuric acid).

Alternative Answers: HCl or H3PO4 (accept either concentration).

Why: Acid is required to acidify potassium dichromate(VI) to act as an oxidising agent.

Answer: H2SO4

Question (b): Mistakes and Corrections

Mistake 1: Test 2 Method

Mistake: Fehling's solution is cooled in iced water .

Correction: The solution should be warmed in a water bath to allow the reaction to proceed.

Mistake 2: Test 3 Gas Testing

Mistake: The gas is tested using a lighted splint .

Correction: Bubble the gas into limewater to test for CO2.

Tip: Use appropriate tests for specific gases: CO2 turns limewater cloudy.

Question (c): Identifying Liquids and Observations

Test 1: Potassium Dichromate(VI)

Liquids that react: J and M

Observation: The orange solution turns green .

Test 2: Fehling's Solution

Liquid that reacts: M

Observation: A blue solution forms a brick-red precipitate .

Test 3: Sodium Hydrogen Carbonate

Liquids that react: J and L

Observation: Effervescence occurs, producing a colourless gas that turns limewater cloudy.

Test 4: Bromine Water

Liquid that reacts: K

Observation: The orange bromine water turns colourless .

Tip: Bromine water decolourisation is a test for unsaturation (e.g., alkenes).

Question (d): Fractional Distillation

Step 1: Label the Apparatus

  • Label S: Fractionating column
  • Label T: Water out
  • Label U: Water in

Step 2: Why Fractional Distillation is Preferred

Reason: Liquids K and M have similar boiling points.

Fractional distillation provides a greater separation efficiency due to the repeated condensation and evaporation within the column.

Topics

Organic Chemistry · 3.3.6 Organic Analysis

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 2, 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.