Alcohols
6 marks · NormalEthanol can be prepared by fermentation or it can be oxidised in the laboratory.
Describe how ethanol can be oxidised using potassium dichromate(VI) in dilute sulfuric acid. Include what you would observe during the process.
Describe the method used, including reagents and condition.
State the observable colour change that shows oxidation has occurred.
Name the organic product formed.
Explain how ethanol can be produced by fermentation. Include two conditions that are essential for this process.
State the process used and the starting material.
Give two conditions that ensure ethanol is produced, not other products.
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
(a.i) Add ethanol to potassium dichromate(VI) in dilute sulfuric acid and heat under reflux.
(a.ii) The colour changes from orange to green.
(a.iii) Ethanoic acid.
(b.i) Fermentation of glucose/sugar using yeast.
(b.ii) Keep warm (about 30°C), absence of air (anaerobic), and/or presence of yeast (any two).
Show mark scheme and worked solution
Marking points
B1 (a(i)) · 1 mark · Describes adding ethanol to potassium dichromate(VI) in dilute sulfuric acid and heating under reflux.
B2 (a(ii)) · 1 mark · States that the colour change is from orange to green.
B3 (a(iii)) · 1 mark · Names ethanoic acid as the organic product formed.
B4 (b(i)) · 1 mark · States that ethanol is produced by fermentation of glucose/sugar with yeast.
B5 (b(ii)) · 2 marks · Gives two correct essential conditions for fermentation (any two of: 30°C, no air, yeast).
Worked solution
a.i) Ethanol is oxidised by heating it with potassium dichromate(VI) in dilute sulfuric acid. The mixture is often heated under reflux to prevent loss of volatile substances.
a.ii) As the reaction proceeds, the solution changes colour from orange (Cr2O7^2-) to green (Cr^3+).
a.iii) The organic product is ethanoic acid.
b.i) Ethanol is produced by fermentation of glucose or sugar using yeast.
b.ii) The fermentation must be carried out at about 30°C and in the absence of air (anaerobic conditions). Yeast must be present as it provides the enzymes.