Alcohols

6 marks · Normal

The two main industrial sources of ethanoic acid are laboratory oxidation of ethanol and microbial fermentation to produce ethanol. The steps below investigate both processes.

(a)

A laboratory technician oxidises 4.6 g of ethanol (CH2OH, molar mass = 46 g/mol) using excess potassium dichromate(VI) in dilute sulfuric acid as the oxidising agent. Calculate the maximum mass of ethanoic acid (COOH, molar mass = 60 g/mol) that could be obtained.

[3 marks]

(b)

Explain the role of dilute sulfuric acid in the oxidation of ethanol with potassium dichromate(VI).

[1 mark]

(c)

Another technician produces ethanol by fermentation. State the conditions required for the fermentation of glucose to produce ethanol.

[2 marks]

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(a) 6.0 g

(b) Dilute sulfuric acid provides H+ ions allowing potassium dichromate(VI) to act as an oxidising agent.

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Marking points

M1 (a) · 1 mark · Converts mass of ethanol into moles correctly.

M2 (a) · 1 mark · Correctly uses chemical equation to relate moles of ethanol to moles of ethanoic acid (1:1 ratio). (follow-through)

A1 (a) · 1 mark · Calculates correct final mass of ethanoic acid, with correct units. (follow-through)

B1 (b) · 1 mark · States the role of dilute sulfuric acid as a provider of H+ ions needed for oxidation.

B2 (c) · 1 mark · Gives at least two correct conditions for fermentation: yeast, temperature (around 30°C), absence of air (anaerobic).

B3 (c) · 1 mark · Includes absence of air (anaerobic) or explains necessity for enzyme activity (not denatured).

Worked solution

a) Step 1: Calculate moles of ethanol: moles = mass / molar mass = 4.6 g / 46 g/mol = 0.1 mol.

The oxidation equation is: CH3CH2OH + 2[O] → CH3COOH + H2O so 1 mole of ethanol gives 1 mole of ethanoic acid.

Step 2: Moles of ethanoic acid = 0.1 mol.

Step 3: Mass of ethanoic acid = moles × molar mass = 0.1 mol × 60 g/mol = 6.0 g.

b) Dilute sulfuric acid provides H+ ions, which are needed to allow potassium dichromate(VI) to work as an oxidising agent and start the redox reaction.

c) Conditions for fermentation: Yeast is added to the sugar solution and the mixture is kept warm (about 30°C) in the absence of air (anaerobic conditions) for several days.