AQA A-Level Chemistry Paper 3, 2023: Question 1

17 marks · Easy difficulty · Practical Techniques & Data Analysis

Preparation of Ethyl Ethanoate This question focuses on esterification, involving the role of concentrated sulphuric acid, identifying apparatus mistakes, explaining the use of sodium carbonate and anhydrous calcium chloride, determining the limiting reagent, calculating percentage yield, and identifying reasons for yield loss.

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Question

Question text

01 Ethyl ethanoate can be made by reacting ethanol with ethanoic acid in the presence

of concentrated sulfuric acid.

Method

1. A mixture of ethanol, ethanoic acid, and concentrated sulfuric acid, with

anti-bumping granules, is heated under reflux for 10 minutes.

2. The apparatus is rearranged for distillation.

3. The mixture is heated to collect the liquid that distils between 70 and 85 °C

4. The distillate is placed in a separating funnel. Aqueous sodium carbonate is

added, and a stopper is placed in the funnel. The mixture is shaken, releasing

pressure as necessary.

5. The lower aqueous layer is removed and the upper organic layer is placed in a

small conical flask.

6. Anhydrous calcium chloride is added to the sample in the conical flask. The

flask is shaken well and left for a few minutes.

7. The liquid from the flask is redistilled and the distillate is collected between

74 and 79 °C

01.1 State the role of concentrated sulfuric acid in this reaction.

[1 mark]

01.2 The reaction mixture is flammable.

Suggest how the reaction mixture should be heated in step 1.

[1 mark]

01.3 Figure 1 shows how a student set up the apparatus for reflux in step 1.

You should assume that the apparatus is clamped correctly.

Figure 1

Identify two mistakes the student made in setting up the apparatus.

State the problem caused by each mistake.

[4 marks]

Mistake 1

Problem caused

Mistake 2

Problem caused

01.4 State why sodium carbonate is added to the distillate in step 4.

*03* Explain why there is a build-up of pressure in the separating funnel.

[2 marks]

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

Suggest why ethyl ethanoate forms the upper layer.

[2 marks]

Reason

Suggestion

01.6 State why anhydrous calcium chloride is added in step 6.

[1 mark]

01.7 A student uses the method to prepare some ethyl ethanoate.

The student adds 10.0 cm3 of ethanol (M = 46.0) to 5.25 g of

ethanoic acid (Mr = 60.0) and obtains 5.47 g of ethyl ethanoate (Mr = 88.0).

For ethanol, density = 0.790 g cm–3

Determine the limiting reagent.

Calculate the percentage yield of ethyl ethanoate.

[5 marks]

Limiting reagent

Percentage yield

01.8 Suggest a reason why the percentage yield is not 100%.

[1 mark]

Mark scheme

Show the mark scheme

Question Answers Additional comments/Guidelines Mark

1.1 catalyst ALLOW reduces Ea 1

IGNORE speeds up reaction

IGNORE provides alternative path

IGNORE proton donor

IGNORE dehydrating agent

1.2 electric heater/heat mantle or (hot) water bath IGNORE not with a Bunsen/naked flame / gently 1

ALLOW hot water

ALLOW heating/hot plate/ sand bath / oil bath

ALLOW reference to flame/Bunsen if in context

of heating a water bath

NOT any indication of direct heat from Bunsen

– A-LEVEL CHEMISTRY – –

1.3 M1 there is a bung/stopper (in the end of the condenser) owtte

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

IGNORE glass shatters / explodes

M3 water goes the wrong way through the condenser water in at top / out at bottom

IGNORE condenser is wrong way round

M4 water does not fill the condenser / condenser is not cool ALLOW less condensing /

enough vapour/gas/reactants/products will not condense 4

/ not as effective at cooling/condensing

vapour/reactants/products escapes

IGNORE uneven cooling

NOT mixture in flask not cooled

ALLOW M1/M3 neck of flask not sealed owtte

M2/M4 vapour/reactants/products can escape

IGNORE references to clamps

– A-LEVEL CHEMISTRY – –

1.4 M1 to neutralise/react with/remove the acid ALLOW carboxylic/ethanoic and/or sulfuric 2

IGNORE react with/neutralise the

distillate/mixture

IGNORE to act as a base

NOT if incorrect acid named

M2 carbon dioxide / gas is produced

ALLOW effervescence / bubbles / fizzes

IGNORE water vapour produced

12 NOT if incorrect gas named

IGNORE pressure build up (as in Q)

1.5 M1 ethyl ethanoate/it is immiscible with / insoluble in water ALLOW water/solution and ethyl ethanoate/it do

not mix,

OR aqueous and organic layers do not mix

ALLOW ethyl ethanoate/it is hydrophobic

IGNORE references to polarity / intermolecular

forces

M2 ethyl ethanoate/it is less dense / has lower density (than

water) IGNORE different/low density

NOT lighter

ALLOW answers for either mark from either part

– A-LEVEL CHEMISTRY – –

1.6 to remove/absorb water / as a drying agent ALLOW reacts with water 1

ALLOW to dry the product/it 13

IGNORE dehydrates

NOT reference to crystals forming

NOT to dry the reactants

NOT to remove soluble impurities

– A-LEVEL CHEMISTRY – –

1.7 M1 mass of ethanol = 10 x 0.790 (= 7.90 g) Allow ECF at each stage

M1 scores from 0.172 mol of ethanol

7.90 M2 need to see numbers or sums for both

M2 amount of ethanol = (= 0.172 mol) AND

46.0 substances

5.25

amount of ethanoic acid = (= 0.0875 mol) M2 10/46 can only be ECF if 10 is identified as a

60.0

mass

M3 ECF from M2 if both amounts clearly shown

M3 (limiting reagent is) ethanoic acid

and nethanol<nethanoic acid

M4 independent of M3

M4 (max amount of ethyl ethanoate = 0.0875 mol) 5

Alternative M4 & 5

max mass of ethyl ethanoate = 88.0 x 0.0875 (= 7.70 g)

M4 Amount of ethyl ethanoate formed

5.47

M5 % yield = x 100 = 71.0% (70.6 to 71.1 to min 2sf) 5.47

M4 = (=0.0622)

88.0

M5 % yield = x 100 = 71.0%

0.0875

Correct answer scores M4 and M5 but mark

14 M1/2/3 separately

M5 must show an attempt at mass or moles of

ester formed divided by mass or moles of ester

expected

– A-LEVEL CHEMISTRY – –

1.8 reaction is an equilibrium/reversible ALLOW losses during 1

distillation/isolation/purification/transfer /

incomplete distillation / side reactions / by-

products

ALLOW incomplete reaction

ALLOW impurities/contamination/water present /

not dry

IGNORE water is also produced (during the

reaction)

How to answer it

Step-by-Step Guide: Ethyl Ethanoate Preparation

Question (a): Role of Sulfuric Acid

Step 1: Identify the Role

Concentrated sulfuric acid acts as a catalyst .

Alternative Answers: Reduces E_a or provides an alternative pathway for the reaction.

Answer: Catalyst

Question (b): Heating the Reaction Mixture

Step 1: Use of an Appropriate Heating Method

The reaction mixture is flammable, so it must not be heated with a naked flame.

Correct Method: Use an electric heater , heat mantle, or hot water bath.

Question (c): Apparatus Mistakes

Step 1: Mistakes in Setup

Mistake 1: A bung/stopper is present in the condenser.
Problem: Pressure could build up, causing the stopper to be forced out.
Mistake 2: Water is flowing the wrong way through the condenser.
Problem: Inefficient cooling and vapours may escape.

Tip: Always ensure water flows from the bottom of the condenser to the top.

Question (d): Use of Sodium Carbonate

Step 1: Neutralise the Acid

Sodium carbonate is added to neutralise any residual acid (carboxylic or sulfuric acid).

Step 2: Build-up of Pressure

The reaction produces CO₂ gas , which causes a pressure build-up in the separating funnel.

Question (e): Two Layers in the Separating Funnel

Step 1: Reason for Layers

Ethyl ethanoate is immiscible with water, so the two layers form.

Step 2: Position of Ethyl Ethanoate

Ethyl ethanoate forms the upper layer because it is less dense than water.

Question (f): Role of Anhydrous Calcium Chloride

Step 1: Drying the Organic Product

Anhydrous calcium chloride is used to remove water from the product as a drying agent.

Question (g): Limiting Reagent and Percentage Yield

Step 1: Calculate Moles of Ethanol

Mass: 10.0 cm³ × 0.790 g cm^-3 = 7.90 g

Moles: 7.90 ÷ 46.0 = 0.172 mol

Step 2: Calculate Moles of Ethanoic Acid

Mass: 5.25 g

Moles: 5.25 ÷ 60.0 = 0.0875 mol

Step 3: Identify the Limiting Reagent

Answer: Ethanoic acid is the limiting reagent because it is present in fewer moles.

Step 4: Calculate Percentage Yield

Theoretical Yield: 88.0 × 0.0875 = 7.70 g

Percentage Yield: (5.47 ÷ 7.70) × 100 = 71.0%

Question (h): Percentage Yield Less than 100%

Step 1: Reasons for Losses

Possible reasons include:

The reaction is reversible.
Losses during distillation or purification.
Side reactions or incomplete reaction.

Answer: Reaction is reversible .

Topics

Organic Chemistry · Physical Chemistry · Required Practicals · 3.3.9 Carboxylic Acids and Derivatives · 3.1.2 Amount of Substance · Required Practical 7: Measuring the rate of reaction by an initial rate method

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.