Practical: making and testing the smell of some esters in the lab
3 marks · NormalDescribe how esters can be made and separated in the laboratory. In your answer, explain the roles of each of the following:
(a) concentrated sulfuric acid,
(b) the hot water bath,
(c) adding sodium carbonate solution after the reaction.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
Also accept: conc. sulfuric acid acts as catalyst; hot water bath offers gentle heating for safety; sodium carbonate neutralises excess acid, reduces the strong acid smell, and allows ester to be detected as a layer.
Common mistakes: Saying sodium carbonate 'separates' the ester without mentioning neutralisation; Missing role of catalyst for sulfuric acid; Not mentioning safety with water bath
Show mark scheme and worked solution
Marking points
B1 · 1 mark · Explains concentrated sulfuric acid acts as a catalyst in esterification (or as a dehydrating agent to speed up the reaction).
B2 · 1 mark · Explains that a hot water bath is used for gentle heating because reactants/ester are flammable and cannot be heated over a direct flame.
B3 · 1 mark · Explains sodium carbonate neutralises excess acid (producing effervescence) and helps reveal the smell and separate the ester layer.
Worked solution
- Concentrated sulfuric acid is added because it acts as a catalyst, speeding up the esterification reaction and increasing yield. It also acts as a dehydrating agent.
- The mixture is heated in a hot water bath rather than directly over a flame because the alcohols and esters produced are flammable. The hot water bath allows for safe heating.
- After cooling, sodium carbonate solution is added to neutralise the remaining carboxylic acid, which produces carbon dioxide and causes effervescence. This neutralises strong acid smells and causes the ester to form a layer on top, making it easier to separate and smell.