Experiment: Esterification of ethanol and ethanoic acid

Esterification is a Form 5 practical in the Carbon Compounds chapter that shows how an alcohol and a carboxylic acid react to form an ester. It brings together the two homologous series studied earlier and produces a compound whose sweet smell is easy to recognise. This guide sets out the aim, the apparatus, a numbered procedure, the observations to expect, and the science process skills the practical papers reward.

Aim

To prepare an ester, ethyl ethanoate, by warming ethanol with ethanoic acid in the presence of concentrated sulfuric acid, and to identify the ester by its characteristic smell.

Apparatus and materials

• Boiling tube
• Dropper or teat pipette
• Measuring cylinder
• Beaker of hot water for a water bath, plus a beaker of cold water
• Bunsen burner, tripod and gauze (to heat the water bath)
• Ethanol
• Glacial (concentrated) ethanoic acid
• Concentrated sulfuric acid (the catalyst)

Procedure

1. Measure a small, equal volume of ethanol and ethanoic acid into a clean boiling tube.
2. Carefully add a few drops of concentrated sulfuric acid to the mixture, adding the acid slowly and mixing gently.
3. Place the boiling tube in a beaker of hot water (a water bath) and warm the mixture gently for a few minutes; do not heat it directly with a flame.
4. Observe the mixture as it warms and note any smell that develops, wafting the vapour gently towards you.
5. After warming, pour the mixture carefully into a beaker of cold water.
6. Waft the vapour above the cold water towards you and note the smell of the product floating on the surface.
7. Record all observations, including the smell before and after pouring into water.

Expected observations

As the mixture is warmed in the water bath, a sweet, fruity, pleasant smell develops, quite different from the sharp smell of the ethanoic acid used. When the warm mixture is poured into cold water, a thin, oily, colourless layer of the ester can be seen floating on top of the water, and the sweet, fruity smell becomes clearer above the cold water because it is separated from the acids. This pleasant smell is the sign that an ester has formed.

Inference and conclusion

The alcohol and the carboxylic acid react together in the presence of concentrated sulfuric acid to form an ester and water; this reaction is called esterification. The ester formed, ethyl ethanoate, has the sweet, fruity smell observed, and its low solubility in water explains the oily layer that floats when the mixture is poured into cold water. The concentrated sulfuric acid acts as a catalyst and dehydrating agent, speeding the reaction and helping to remove water. The conclusion is that an alcohol and a carboxylic acid react to form a sweet-smelling ester.

Science process skills (Paper 3 style)

Making an inference. From the observation of a sweet, fruity smell, infer that an ester has been formed by the reaction of the alcohol and the carboxylic acid.

Identifying variables. If the investigation tested whether the concentrated acid is needed, the manipulated variable is whether concentrated sulfuric acid is added, the responding variable is whether a sweet-smelling ester forms, and the controlled variables are the volumes of ethanol and ethanoic acid and the warming.

Operational definition. Define successful esterification operationally: the reaction has produced an ester if a sweet, fruity smell is detected after warming and an oily layer floats when the mixture is added to cold water.

Communicating. Describe the smell precisely (sweet and fruity, unlike the sharp acid) and the appearance of the ester layer, so a reader can tell that the product is an ester and not unreacted acid.

Safety precautions

• Wear safety goggles, because concentrated sulfuric acid and glacial ethanoic acid are very corrosive.
• Add the concentrated sulfuric acid slowly and carefully, so that the mixture does not splash or heat up suddenly.
• Warm the mixture in a hot water bath, not directly over a flame, because ethanol and the ester are flammable.
• Waft the smell towards you rather than inhaling directly, so that the concentrated acid vapours are not breathed in.
• Handle glacial ethanoic acid in a fume cupboard or well-ventilated area, so its sharp vapour is not inhaled.

Common errors

• Heating directly with a flame. A naked flame can ignite the flammable ethanol and ester; always use a hot water bath.
• Adding too much or too fast concentrated acid. Adding concentrated sulfuric acid quickly makes the mixture spit and heat suddenly; add only a few drops, slowly.
• Confusing the acid smell with the ester smell. The starting ethanoic acid is sharp; the product is sweet and fruity. Pour into cold water to separate the ester and make its smell clearer.
• Not pouring into cold water. Skipping this step leaves the ester mixed with the acids and its smell masked; pouring into cold water reveals the floating ester.
• Reporting a colour change. This reaction is identified by smell and the oily layer, not by a colour change; do not invent one.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we make sure students can state the reactants, the role of the concentrated sulfuric acid, and the evidence that an ester has formed, the sweet smell and the floating layer. Esterification is a favourite topic across SPM Chemistry because it links alcohols, carboxylic acids and esters and connects to everyday flavourings and scents, and the same skills of careful observation and safe handling are rewarded in the Paper 3 practical assessment (Paper 3 is a practical test assessing science process skills).