AQA A-Level Chemistry Paper 2, 2022: Question 5

12 marks · Medium difficulty · Practical Techniques & Data Analysis

Preparation of Hexan-2-ol This question involves the preparation of hexan-2-ol from hex-1-ene using phosphoric acid as a catalyst. Key tasks include suggesting an additional safety precaution, explaining the reasons for adding distilled water and anhydrous magnesium sulfate during the procedure, and identifying potential organic impurities in the final distillate. Additionally, students are required to label vacuum filtration apparatus and calculate the mass of hexan-2-ol formed given a specific yield, using the density of hex-1-ene.

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Question

AQA A-Level Chemistry Paper 2, 2022: Question 5
Question text

05 This question is about the preparation of hexan-2-ol.

Hexan-2-ol does not mix with water and has a boiling point of 140 oC

Hexan-2-ol can be prepared from hex-1-ene using this method.

a Measure out 11.0 cm3 of hex-1-ene into a boiling tube in an ice bath.

b Carefully add 5 cm3 of concentrated phosphoric acid to the hex-1-ene.

c After 5 minutes add 10 cm3 of distilled water to the mixture and transfer the

boiling tube contents to a separating funnel.

d Shake the mixture and allow it to settle.

e Discard the lower (aqueous) layer.

f Add a fresh 10 cm3 sample of distilled water and repeat steps d and e.

g Transfer the remaining liquid to a beaker.

h Add 2 g of anhydrous magnesium sulfate and allow to stand for 5 minutes.

i Filter the mixture under reduced pressure.

j Distil the filtrate and collect the distillate that boils in the range 130–160 oC

05.1 It is important to wear eye protection and a lab coat when completing this experiment.

Suggest, with a reason, one other appropriate safety precaution for this experiment.

[2 marks]

Precaution

Reason

05.2 Give a reason for adding the distilled water in steps c and f.

[1 mark]

05.3 Give a reason for adding anhydrous magnesium sulfate in step h.

[1 mark]

05.4 Complete and label the diagram of the apparatus used to filter the mixture under

reduced pressure in step i.

[2 marks]

05.5 Identify the most likely organic impurity, other than hex-1-ene, in the distillate collected

in step j.

Suggest one reason why it could be difficult to remove this impurity.

[2 marks]

Impurity

Reason

05.6 Calculate the mass, in g, of hexan-2-ol formed from 11.0 cm3 of hex-1-ene if the

yield is 31.0%

Give your answer to 1 decimal place.

*16* Density of hex-1-ene = 0.678 g cm−3

[4 marks]

Mass g

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 2, 2022: Question 5

Question Answers Additional Comments/Guidelines Mark

Wear gloves Allow wash spillages with lots of water 1

Conc phosphoric acid is corrosive 1

OR

Use a fume cupboard Allow work in a well-ventilated lab space (2 x AO3)

Volatile organic compounds are harmful / toxic

05.1 OR

Keep away from naked flames Other valid suggestions eg heating mantle or

Organic compounds are flammable electric heater

OR Not water bath

Periodically release pressure inside separating funnel

Prevent build-up of pressure

To remove (water) soluble impurities Allow to remove (excess) acid 1

05.2

(AO2)

To remove water / absorb water / dry the liquid Allow drying agent 1

05.3

(AO2)

Deduct a mark(s) for error(s) / omission(s) 2

Minimum (2 x AO2)

22 • Cross sectional (ie funnel top and end

shown open)

05.4 • Bung or collar drawn

• (Buchner) Funnel – approximate shape

WITH label

• Filter paper – WITH label

Impurity: hexan-1-ol If hexan-3-ol allow ecf for M2 M1

05.5 Reason: It is likely to have a similar boiling point M2

(2 x AO2)

Mass hex-1-ene = 11.0 × 0.678 (or = 7.46 g) Allow consequential marks for M2,M3,M4 M1

7.46 M2

n hex-1-ene = (or = 0.0888)

84.0

05.6

Mass of product = 0.0888 × 0.31 x 102 M3

Allow answers 2.8 or 2.9 only M4

Mass product = 2.8 g

(4 x AO2)

How to answer it

Learning Guide: Preparation of Hexan-2-ol

Part (a)

What to do: Suggest an additional safety precaution beyond wearing eye protection and a lab coat. Common acceptable answers include:

  • Wear gloves to protect against corrosive phosphoric acid.
  • Use a fume cupboard to avoid exposure to volatile, harmful organic vapours.
  • Avoid naked flames since organic compounds are flammable.

Common errors:

  • Failing to specify why the precaution is necessary.
Teacher Tip: Always link your safety precaution to the specific risks in the experiment.

Part (b)

What to do: Explain why distilled water is added during steps c and f. The correct answer is to remove water-soluble impurities or excess acid.

Common errors:

  • Failing to mention impurities or excess acid.
Common Misconception: Some students incorrectly stated that water was added to dissolve the product. Hexan-2-ol does not mix with water.

Part (c)

What to do: State why anhydrous magnesium sulfate is added during step h. The correct answer is to dry the liquid product by removing water.

Common errors:

  • Failing to mention drying or removing water.

Part (d)

What to do: Draw and label the apparatus used for vacuum filtration, ensuring:

  • Cross-sectional view of a Buchner funnel.
  • A collar or bung between the funnel and the flask.
  • Labelled filter paper and vacuum line.

Common errors:

  • Apparatus not shown in cross-section.
  • Missing labels such as "filter paper" or "vacuum line".

Part (e)

What to do: Identify the organic impurity in the distillate and explain why it is hard to remove:

  • Impurity: Hexan-1-ol (or hexan-3-ol).
  • Reason: It has a boiling point similar to hexan-2-ol.

Common errors:

  • Failing to identify hexan-1-ol as the impurity.
  • Not linking difficulty in removal to boiling point similarity.

Part (f)

What to do: Perform a calculation to determine the mass of hexan-2-ol formed. Ensure the answer is given to 1 decimal place:

Example:
Mass of hex-1-ene = 11.0 × 0.678 = 7.46 g
Moles of hex-1-ene = 7.46 / 84.0 = 0.0888 mol
Mass of product = 0.0888 × 0.31 × 102 = 2.8 g (to 1 d.p.)

Common errors:

  • Not rounding the final answer to 1 decimal place.
  • Incorrectly using the yield percentage or molar masses.

Final Notes

When answering experimental questions, always pay close attention to instructions such as the number of decimal places or specific safety requirements. Link your reasoning to the properties of the chemicals or apparatus used.

Topics

Organic Chemistry · Required Practicals · 3.3.5 Alcohols · Required Practical 5: Distillation of a product from a reaction

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 2, 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.