Practical: Preparing and Purifying an Ester Revision Notes | A Level AQA Chemistry

1. Why Esters Matter — and What This Practical Tests

Esters are everywhere: in fruit flavourings, perfumes, solvents, and plasticisers. The ability to prepare and purify one in the laboratory sits at the heart of AQA Required Practical 10, and it tests far more than just following a recipe.

This practical demands that you understand why each step is done — from the choice of catalyst to the sequence of washing stages. Examiners regularly ask you to justify procedural decisions, evaluate sources of error, and suggest improvements, so conceptual understanding is non-negotiable.

The reaction itself is straightforward: a carboxylic acid reacts with an alcohol in the presence of a concentrated acid catalyst to form an ester and water. The challenge lies in driving an equilibrium reaction to a useful yield and then isolating a pure product from a messy mixture of unreacted starting materials, catalyst, and water.

2. The Chemistry Behind the Reaction

Esterification is a reversible reaction. Using ethanoic acid and ethanol as the standard example, the equation is:

Because the reaction is reversible, the equilibrium position determines the yield. For the esterification of ethanoic acid with ethanol, the equilibrium constant $K_c \approx 4$ at room temperature, meaning the yield of ester is limited to approximately 65% even at equilibrium — far from complete conversion, and insufficient for a practical synthesis without further intervention.

Two strategies are used to improve yield: using one reagent in excess (shifting the equilibrium towards products by Le Chatelier's principle) and removing a product as it forms. In the laboratory preparation, reflux keeps all volatile components in the flask while the reaction proceeds, and the water produced can later be removed during purification.

3. The Preparation — Reflux Stage

The first stage of the practical is heating the reaction mixture under reflux. This allows the mixture to be heated to boiling point for an extended period without losing volatile reagents such as ethanol (boiling point 78 °C) or the ester product (boiling point 77 °C for ethyl ethanoate) to the atmosphere.

4. Purification — Washing, Drying, and Distillation

After reflux, the crude product is a complex mixture. Purification proceeds through three distinct stages: washing in a separating funnel, drying with an anhydrous salt, and finally distilling to collect the pure ester. Each stage has a specific chemical rationale — understanding that rationale is what examiners test.

4a. Stage 1 — Washing with Sodium Carbonate Solution

The cooled crude mixture is poured into a separating funnel and shaken with saturated sodium carbonate solution. This removes two impurities simultaneously: ethanoic acid is neutralised by the carbonate, and ethanol dissolves preferentially into the aqueous layer due to its miscibility with water.

The carbon dioxide produced causes effervescence inside the separating funnel. This is a critical safety and technique point: the stopper must be released regularly to prevent pressure build-up. Invert the funnel, open the tap briefly, then re-stopper and continue shaking.

After shaking and allowing the layers to separate, the denser aqueous layer (containing sodium ethanoate, ethanol, and dissolved sodium carbonate) is run off from the bottom of the separating funnel and discarded. The less dense organic layer (containing the ester) remains and is retained.

4b. Stage 2 — Washing with Saturated Sodium Chloride Solution (Optional)

A second wash with saturated sodium chloride solution (brine) is sometimes used after the sodium carbonate wash. The high ionic strength of the brine reduces the solubility of the ester in the aqueous phase — a phenomenon called salting out — helping to give a cleaner separation of the organic layer.

This step also removes residual sodium carbonate from the organic layer. After this wash, the aqueous layer is again discarded from the bottom of the separating funnel.

4c. Stage 3 — Drying with Anhydrous Calcium Chloride

The organic layer still contains dissolved water after the washing stages. This is removed by adding an anhydrous drying agent — typically anhydrous calcium chloride — directly to the organic layer in a conical flask. The drying agent absorbs water by forming a hydrate, leaving the ester dry.

Once the liquid appears clear and the drying agent flows freely (no longer clumping), the dried organic layer is filtered or decanted away from the solid drying agent into a clean round-bottomed flask, ready for distillation.

4d. Stage 4 — Distillation to Collect the Pure Ester

The dried organic layer is transferred to a round-bottomed flask fitted with a thermometer and condenser for distillation. The mixture is heated gently, and the fraction that distils at the boiling point of the target ester is collected. For ethyl ethanoate, this is 77 °C.

The thermometer bulb must be positioned at the side arm of the still head — not submerged in the liquid — so that it measures the temperature of the vapour entering the condenser. Fractions collected significantly above or below the expected boiling point are discarded, as they indicate impurities are still present.

Worked example: Purification Sequence for Crude Ethyl Ethanoate

A student has completed the reflux stage and has a crude mixture containing ethyl ethanoate, unreacted ethanoic acid, unreacted ethanol, concentrated sulphuric acid, and water. Describe the correct sequence of purification steps and justify each one.

1. Pour the cooled crude product into a separating funnel. Add an equal volume of saturated sodium carbonate solution. — Sodium carbonate neutralises ethanoic acid (forming sodium ethanoate + CO2 + H2O) and also neutralises any sulphuric acid present. Ethanol partitions into the aqueous layer. Release pressure from the funnel regularly during shaking.
2. Allow layers to settle. Run the lower aqueous layer out of the bottom tap. Retain the upper organic layer. — The aqueous layer contains sodium ethanoate, dissolved ethanol, and sodium sulphate — all impurities. The organic layer contains the ester. Repeat the sodium carbonate wash if necessary.
3. Wash the organic layer with saturated sodium chloride solution. Separate and discard the aqueous layer. — Brine removes residual sodium carbonate and reduces the solubility of the ester in the aqueous phase (salting out), giving a cleaner organic layer.
4. Transfer the organic layer to a conical flask. Add small portions of anhydrous calcium chloride until it flows freely without clumping. — Anhydrous CaCl2 absorbs dissolved water by forming a hydrate. The drying is complete when the solid no longer clumps — this indicates no free water remains.
5. Decant or filter the dried organic layer into a round-bottomed flask. Distil and collect the fraction boiling at 77 °C. — Distillation separates the ester from any remaining high-boiling impurities by exploiting the difference in boiling points. Collecting only the fraction at 77 °C ensures purity.

Answer: Wash with sodium carbonate solution (removes acid and ethanol) → separate layers → optional brine wash → dry with anhydrous CaCl2 → filter → distil and collect fraction at 77 °C.

5. Identifying the Ester Product

Once the pure ester has been collected, its identity can be confirmed by several methods. The most immediate is smell — esters have characteristic fruity or floral odours. Ethyl ethanoate smells of pear drops, while other esters have distinctive scents that can be used as a preliminary check.

A more rigorous confirmation uses the measured boiling point. If the fraction collected during distillation boils sharply at the expected temperature (77 °C for ethyl ethanoate), this is strong evidence of purity. A broad boiling range indicates the presence of impurities.

In an IR spectrum of the pure ester, you should see the characteristic ester carbonyl (C=O) absorption at approximately 1735 cm⁻¹ and the C–O–C stretch in the fingerprint region. Critically, the broad O–H absorptions that would indicate residual ethanoic acid (2500–3300 cm⁻¹) or ethanol (3200–3550 cm⁻¹) should be absent.

6. Calculating Percentage Yield

Because esterification is reversible and some product is inevitably lost during purification, the actual yield of ester is always less than the theoretical maximum. Calculating percentage yield allows you to evaluate the efficiency of the preparation.

Worked example: Calculating Percentage Yield of Ethyl Ethanoate

A student uses 15.0 cm³ of ethanol (density 0.789 g cm⁻³) and excess ethanoic acid. After purification, they collect 8.20 g of ethyl ethanoate. Calculate the percentage yield. (Molar masses: ethanol = 46.0 g mol⁻¹; ethyl ethanoate = 88.0 g mol⁻¹)

1. mass = volume × density = 15.0 × 0.789 = 11.835 g — Density converts volume to mass. Ethanol is the limiting reagent here because ethanoic acid is in excess.
2. n(ethanol) = mass ÷ molar mass = 11.835 ÷ 46.0 = 0.2573 mol — Moles = mass / molar mass. Keep sufficient significant figures at this intermediate stage.
3. From the 1:1 stoichiometry: n(ester) = 0.2573 mol — The balanced equation shows 1 mol ethanol produces 1 mol ester, so moles of ester theoretically possible equals moles of limiting reagent.
4. theoretical mass = 0.2573 × 88.0 = 22.64 g — Theoretical mass = moles × molar mass of product.
5. percentage yield = (actual mass ÷ theoretical mass) × 100 = (8.20 ÷ 22.64) × 100 = 36.2% — A yield of around 30–40% is typical for this preparation due to the reversible equilibrium and losses during purification. This is not a sign of poor technique — it reflects the chemistry.

Answer: Percentage yield = 36.2% (to 3 significant figures)

7. Evaluation, Error Analysis, and Exam Technique

Evaluation questions are among the most frequently examined aspects of Required Practical 10. You need to be able to identify sources of error, assess their significance, and suggest realistic improvements. The key is to link each source of error to its specific effect on the result.

When writing evaluation answers, avoid vague statements such as 'human error' or 'the equipment was inaccurate'. Examiners award marks for specific, chemically justified points. Name the impurity, state how it affects the measurement, and give a concrete improvement.

Required Practical 10 tests your ability to connect practical technique with chemical understanding. Every procedural decision — why reflux rather than open heating, why sodium carbonate rather than sodium hydroxide, why anhydrous rather than hydrated calcium chloride — has a specific chemical justification that examiners expect you to articulate precisely.