A few drops of oxidising reagent were added to a 0.5 cm³ sample of ethanal (CH₃CHO) in a test-tube - Leaving Cert Chemistry - Question (b) - 2022
Question (b)
A few drops of oxidising reagent were added to a 0.5 cm³ sample of ethanal (CH₃CHO) in a test-tube.
What was observed when the oxidising reagent used was
(i) warm ... show full transcript
Worked Solution & Example Answer:A few drops of oxidising reagent were added to a 0.5 cm³ sample of ethanal (CH₃CHO) in a test-tube - Leaving Cert Chemistry - Question (b) - 2022
Step 1
What was observed when the oxidising reagent used was warm dilute acidified KMnO₄?
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Answer
When warm dilute acidified KMnO₄ is added to ethanal, a color change is observed. The solution changes from purple (or violet) to colorless, indicating that the KMnO₄ has been reduced.
Step 2
What was observed when the oxidising reagent used was Fehling's reagent, pre-heated to about 40 °C?
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Answer
When Fehling's reagent is added to ethanal, it changes from a blue solution to a brick red precipitate of copper(I) oxide (Cu₂O), showing the reduction of Cu²⁺ ions.
Step 3
Identify the organic product of the oxidation of ethanal using these reagents.
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The organic product formed from the oxidation of ethanal is ethanoic acid (acetic acid, CH₃COOH).
Step 4
Is ethanal very easy or very difficult to oxidise?
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Ethanal is very easily oxidised due to the presence of the aldehyde functional group, which is readily converted to a carboxylic acid.
Step 5
Write a balanced half-equation to show the reduction of the Cu²⁺ ions of the Fehling's reagent in the test.
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The balanced half-equation for the reduction of Cu²⁺ ions in Fehling's reagent is:
Cu2++2e−→Cu0
Step 6
Describe the polarity across the carbonyl group in ethanol.
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In ethanol, the carbonyl (C=O) group is polar. The carbon atom of the carbonyl is slightly positively charged (δ⁺), while the oxygen atom is slightly negatively charged (δ⁻).
Step 7
Refer to the polarity across the carbonyl group to explain why ethanol is very soluble in water.
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Ethanol is very soluble in water due to hydrogen bonding. The polarity of the C=O bond allows ethanol to form hydrogen bonds with the water molecules, which enhances its solubility.
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