Core Practical 4.43C

Core Practical 4.43C: Preparation of an Ester

Revision guide containing method, variables, safety, sample calculations, and model exam answers.

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Scientific Principles & Theory

Scientific Background: Esters are organic compounds with the functional group -COO-. They are formed by the condensation reaction (esterification) between an alcohol and a carboxylic acid, heated in the presence of a concentrated sulfuric acid catalyst:

Ethanol + Ethanoic Acid ⇌ Ethyl Ethanoate + Water
CH₃CH₂OH(l) + CH₃COOH(l) ⇌ CH₃COOCH₂CH₃(l) + H₂O(l)

The reaction is reversible and reaches dynamic equilibrium. To obtain a high yield, the ester is distilled off as it forms. The product must then be purified to remove unreacted reactants and the acid catalyst.

Experimental Variables

Independent Variable

The reaction conditions (reflux vs simple distillation).

Dependent Variable

The volume/yield and purity of the isolated ethyl ethanoate.

Control Variables

Volumes of starting reactants (ethanol, ethanoic acid), volume of concentrated sulfuric acid catalyst.

⚠️ Lab Risk Assessment

Hazard Associated Risk Control Measure
Concentrated sulfuric acid (catalyst) Severely corrosive; causes severe skin burns and eye damage Wear safety goggles and gloves; handle with extreme care; use a dropper pipet under a fume hood.
Ethanol / Ethyl ethanoate Highly flammable and volatile liquids Use an electric heating mantle or water bath; never heat the reaction mixture with an open Bunsen burner flame.

Apparatus & Procedure

Required Apparatus

  • Round-bottom flask (50 cm³)
  • Liebig condenser with tubing
  • Fractional distillation column
  • Separating funnel (100 cm³)
  • Electric heating mantle
  • Thermometer (0 to 100 °C)
  • Beakers and conical flasks
  • Dropping pipette
  • Glacial ethanoic acid
  • Ethanol
  • Concentrated sulfuric acid
  • Sodium carbonate solution (30% - to neutralise acid impurities)
  • Anhydrous calcium chloride (drying agent)

Step-by-Step Procedure

  1. Add 10 cm³ of ethanol and 10 cm³ of glacial ethanoic acid to a round-bottom flask.
  2. Carefully add about 1 cm³ of concentrated sulfuric acid dropwise while swirling the flask.
  3. Add a few anti-bumping granules to the flask to ensure smooth boiling.
  4. Set up the apparatus for reflux: fit the Liebig condenser vertically into the neck of the round-bottom flask. Connect the cooling water to flow in at the bottom and out at the top.
  5. Heat the flask gently in an electric heating mantle under reflux for 20 minutes. Reflux allows the mixture to be heated without losing volatile reactants.
  6. Rearrange the apparatus for distillation. Heat the mixture and collect the distillate that boils off between 70 °C and 85 °C (ethyl ethanoate boils at 77 °C).
  7. Transfer the impure distillate to a separating funnel. Add an equal volume of sodium carbonate solution to react with any unreacted ethanoic acid and sulfuric acid.
  8. Stopper the funnel, invert it, and shake gently, opening the tap periodically to release carbon dioxide gas pressure.
  9. Allow the layers to separate. The ethyl ethanoate (ester) is less dense than water and forms the upper organic layer. Run off and discard the lower aqueous layer.
  10. Wash the organic layer with saturated calcium chloride solution to remove any remaining ethanol. Run off the aqueous layer.
  11. Run the ester into a clean conical flask and add a few spatulas of anhydrous calcium chloride (drying agent). Swirl and leave for 10 minutes until the liquid becomes clear.
  12. Decant or filter the clear ester into a clean, pre-weighed sample bottle and record its mass.
Separating funnel purification of an ester Pear-shaped separating funnel with tap and stopper showing two immiscible layers: the upper organic ester layer and the denser lower aqueous layer, which is run off and discarded. Stopper Upper organic layer ethyl ethanoate (ester), less dense than water Lower aqueous layer sodium carbonate solution and impurities Tap open to run off the lower layer Beaker aqueous layer, discarded Separating funnel

Fig 1. Laboratory experimental setup for Core Practical 4.43C.

Sample Data & Calculations

This representative dataset illustrates the values typically obtained when carrying out this experiment in the laboratory:

Parameter Value
Volume of ethanol used 10.0 cm³ (8.0 g, 0.174 mol)
Volume of glacial ethanoic acid used 10.0 cm³ (10.5 g, 0.175 mol)
Mass of pure ethyl ethanoate obtained 9.20 g

Data Processing & Analysis

  1. Reaction: Ethanol + Ethanoic Acid → Ethyl Ethanoate + Water
  2. Reacting Ratio: 1 : 1, so ethanol is the limiting reactant (0.174 mol)
  3. Theoretical Yield = Moles * Mr of Ethyl Ethanoate = 0.174 mol * 88.0 g/mol = 15.31 g
  4. Percentage Yield = (Actual Yield / Theoretical Yield) * 100 = (9.20 g / 15.31 g) * 100 = 60.1%

Conclusion & Evaluation

Chemical Explanation: Esters are formed by a condensation esterification reaction between a carboxylic acid and an alcohol, yielding an ester and water. This is a slow, reversible reaction catalysed by concentrated sulfuric acid. The acid acts as both a catalyst to speed up the rate and a dehydrating agent to shift the position of equilibrium to favour the products.

Experimental Error Analysis

Error Type & Source Effect on Final Result Mitigation Strategy
Systematic Error
Esterification reaction is reversible and reaches equilibrium		 Reactants are never 100% converted to products, reducing the maximum yield. Use excess acid or remove the product ester continuously by distillation to shift the equilibrium position to the right (Le Chatelier's Principle).
Random Error
Loss of product during separating funnel transfers		 Some ester remains stuck to the glassware walls or escapes with the water layer, reducing yield. Wash the separating funnel with a tiny volume of pure solvent to recover residue, and perform separations carefully.

Exam Practice

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Exam-style design question

6 marks
[6 marks]

Describe how you would prepare, isolate, and purify a sample of the ester ethyl ethanoate starting from ethanol and ethanoic acid. Include details of safety precautions and explain the role of each purification step.

View Model Answer & Mark Scheme

Marking guidance: award 1 mark for each correct, clearly explained step in the model answer below, up to 6 marks.

Model Answer (6/6 Marks):

  1. Preparation & Reflux: Mix 10 cm³ of ethanol, 10 cm³ of ethanoic acid, and 1 cm³ of concentrated sulfuric acid (catalyst) in a flask. Heat under reflux for 20 minutes to prevent the loss of volatile reactants. Use an electric mantle for safety (flammable compounds).
  2. Distillation: Distill the mixture and collect the fraction boiling between 74 °C and 80 °C to separate the ester from non-volatile compounds.
  3. Neutralisation: Shake the distillate with sodium carbonate solution in a separating funnel. This neutralises any unreacted acid impurities. Periodically open the tap to release CO₂ pressure.
  4. Separation: Allow layers to separate. Discard the lower aqueous layer, keeping the upper ester layer.
  5. Drying: Add anhydrous calcium chloride to the ester in a flask. Swirl and leave until the liquid turns clear, which removes trace water.
  6. Redistillation: Perform a final distillation to collect pure ethyl ethanoate at exactly 77 °C.
Examiner Tip:

State the safety precaution of using a heating mantle instead of a Bunsen burner because alcohols and esters are highly flammable. Explain that shaking with sodium carbonate removes acid impurities.

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