Alcohols

6 marks · Normal

Ethanol is an important organic compound with many uses, including as a fuel and a chemical feedstock.

(a)

Describe one observable colour change when ethanol is oxidised by heating with potassium dichromate(VI) in dilute sulfuric acid.

[1 mark]

(b)

Write a word equation for the oxidation reaction described in part (a).

[1 mark]

(c)

State the role of sulfuric acid in this oxidation reaction.

[2 marks]

(d)

Describe how ethanol can be produced by fermentation.

[2 marks]

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(a) The orange colour of potassium dichromate(VI) turns green.

(b) ethanol + oxygen → ethanoic acid + water

(c) Sulfuric acid acts as a source of H+ ions making the medium acidic so the potassium dichromate(VI) can act as an oxidising agent.

(d) Ethanol is produced by adding yeast to a sugar solution and leaving it warm (around 30°C) in the absence of air. Enzymes in the yeast convert the sugar to ethanol and carbon dioxide.

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Marking points

B1 (a) · 1 mark · Correct statement of the observable colour change (orange to green).

B2 (b) · 1 mark · Correct word equation for oxidation of ethanol to ethanoic acid.

B3 (c) · 1 mark · Identifies sulfuric acid as source of H+ ions or requirement for oxidation.

B4 (c) · 1 mark · Links acidic medium or presence of H+ to potassium dichromate(VI) acting as oxidising agent.

B5 (d) · 1 mark · States presence of yeast and sugar solution, or outlines fermentation.

B6 (d) · 1 mark · Mentions warm temperature (about 30°C) and anaerobic/no air conditions.

Worked solution

(a) When ethanol is oxidised using potassium dichromate(VI) and dilute sulfuric acid, the solution changes colour from orange to green.

(b) The word equation for the reaction is: ethanol + oxygen → ethanoic acid + water.

(c) Sulfuric acid provides the H+ ions that make the medium acidic, which is required for potassium dichromate(VI) to act as an oxidising agent.

(d) Ethanol can be made by fermentation. Add yeast to a solution of sugar, keep it warm at about 30°C and in the absence of air. The yeast enzymes convert the sugar to ethanol and carbon dioxide.