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A sample of soap was prepared in the laboratory by refluxing a mixture of approximately 5 g of animal fat, 2 g of sodium hydroxide pellets (in excess) and 25 cm³ of ethanol in an apparatus like that drawn on the right - Leaving Cert Chemistry - Question 2 - 2006
Question 2
A sample of soap was prepared in the laboratory by refluxing a mixture of approximately 5 g of animal fat, 2 g of sodium hydroxide pellets (in excess) and 25 cm³ of ... show full transcript
Worked Solution & Example Answer:A sample of soap was prepared in the laboratory by refluxing a mixture of approximately 5 g of animal fat, 2 g of sodium hydroxide pellets (in excess) and 25 cm³ of ethanol in an apparatus like that drawn on the right - Leaving Cert Chemistry - Question 2 - 2006
Step 1
Why was the reaction mixture refluxed? Name the type of reaction which occurs during the reflux stage of the preparation.
Answer
The reaction mixture was refluxed primarily to speed up the reaction. This allows the reactants to remain in the boiling mixture, ensuring complete reaction by continually providing heat without losing any of the reactants to evaporation. The type of reaction that occurs during this reflux stage is a base hydrolysis or saponification reaction.
Step 2
Step 3
What is the purpose of the ethanol? Why is it desirable to remove the ethanol after reflux? Describe with the aid of a labelled diagram how you would remove the ethanol after the reflux stage of the experiment.
Answer
The purpose of ethanol in the reaction is to act as a solvent, aiding in the dissolution of the reactants. It is desirable to remove the ethanol after reflux because it can interfere with the final soap product and needs to be isolated in order to purify the soap.
To remove the ethanol, a labelled diagram would show a distillation apparatus, where heat is applied to the mixture in a boiling flask to evaporate the ethanol. The ethanol vapour goes through a condenser, where it cools and is collected in a separate receiving flask, leaving the soap in the boiling flask.
Step 4
Describe how a sample of soap was obtained from the reaction mixture.
Answer
To obtain a sample of soap from the reaction mixture, the mixture would first be allowed to cool and then poured into a Buchner flask. Water would be added to wash any impurities, and the soap would precipitate out. The precipitated soap can then be collected by suction filtration.
Step 5
At the end of the experiment, what is the location of (i) the second product of the reaction? (ii) the excess sodium hydroxide?
Answer
(i) The second product of the reaction, which is soap, would be found in the Buchner flask after filtration. (ii) The excess sodium hydroxide would remain dissolved in the reaction mixture in the boiling flask unless it has been neutralized or removed during the washing process.
Step 6
What would you observe, upon shaking, if a little of the soap prepared in this experiment is added to (i) a test tube containing deionised water, (ii) a test tube containing mineral water from a limestone region?
Answer
(i) Upon shaking, the soap in deionised water would dissolve and create a lather due to the absence of calcium or magnesium ions. This would result in a clear solution with bubbles.
(ii) In contrast, when added to mineral water from a limestone region, the soap would not dissolve well and may form a precipitate due to the formation of insoluble salts (soap scum) with the calcium ions present in mineral water, resulting in less effective lathering.