Benzic acid can be prepared from ethyl benzoate - AQA - A-Level Chemistry - Question 3 - 2017 - Paper 3
Question 3
Benzic acid can be prepared from ethyl benzoate.
Ethyl benzoate is first hydrolysed in alkaline conditions as shown:
Add 5.0 cm³ of ethyl benzoate (density = 1.... show full transcript
Worked Solution & Example Answer:Benzic acid can be prepared from ethyl benzoate - AQA - A-Level Chemistry - Question 3 - 2017 - Paper 3
Step 1
Suggest how the anti-bumping granules prevent bumping during reflux.
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Answer
The anti-bumping granules provide a large surface area for small bubbles to form, preventing the development of very large bubbles which can lead to bumping.
Step 2
Show, by calculation, that an excess of sodium hydroxide is used in this reaction.
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Answer
The mass of ethyl benzoate used is calculated as follows:
extMassofester=5.0extcm3imes1.05extgcm−3=5.25extg
Using the molar mass of ethyl benzoate (Mr = 150 g/mol), the moles of ethyl benzoate can be calculated:
Since 0.060 mol of NaOH is available against 0.0350 mol of ethyl benzoate, there is an excess of sodium hydroxide.
Step 3
Suggest why an excess of sodium hydroxide is used.
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An excess of sodium hydroxide ensures that all of the ethyl benzoate reacts completely, preventing any unreacted ester from remaining in the mixture.
Step 4
Suggest why an electric heater is used rather than a Bunsen burner in this hydrolysis.
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An electric heater is used to provide a more even and controlled heating, which reduces the risk of igniting flammable vapors that may be present.
Step 5
State why reflux is used in this hydrolysis.
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Reflux is used to maintain the reaction mixture at its boiling point for an extended period without losing any volatile components, ensuring thorough hydrolysis of the ester.
Step 6
Write an equation for the reaction between sodium benzoate and hydrochloric acid.
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The equation for the reaction is:
C6H5COONa+HCl→C6H5COOH+NaCl
Step 7
Suggest why sodium benzoate is soluble in cold water but benzoic acid is insoluble in cold water.
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Sodium benzoate is soluble in cold water due to its ionic nature and the ability to form interactions with water molecules, while benzoic acid has non-polar characteristics and fewer favorable interactions with water, leading to its insolubility.
Step 8
Describe the method the student should use to purify the benzoic acid.
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To purify the benzoic acid, the student should:
Dissolve the crude benzoic acid in a minimum volume of hot water to ensure it completely dissolves.
Allow the solution to cool slowly, leading to the crystallization of pure benzoic acid.
Filter the crystals using a Buchner funnel and wash them with cold water to remove impurities.
Dry the purified benzoic acid using filter paper or in a desiccator.
Step 9
Calculate the percentage yield of benzoic acid.
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Answer
Using the provided data, the moles of ethyl benzoate used in the reaction were 0.0400 mol. The theoretical yield of benzoic acid can be calculated based on the stoichiometry (1:1 ratio) of the reaction. Each mole of ethyl benzoate yields one mole of benzoic acid.
