Alcohols

O Level / IGCSE Chemistry · Organic chemistry. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Ethanol, C2H5OH, is made in two ways. In fermentation, yeast is added to aqueous glucose at 25–35 °C with no oxygen present. Enzymes in the yeast turn glucose into ethanol and carbon dioxide. If it is too hot the enzymes are denatured. If oxygen gets in, the ethanol can be oxidised to ethanoic acid.

In the second method, steam is added to ethene at 300 °C and 6000 kPa (60 atm) with an acid catalyst.

Fermentation uses a renewable raw material and little energy, but it is slow, works in batches and gives impure, dilute ethanol. The ethene method is fast, continuous and gives pure ethanol, but ethene comes from petroleum, which is non-renewable, and the conditions need a lot of energy.

Rules to remember

Common mistake

Students mix up the two sets of conditions. Yeast and 25–35 °C belong to fermentation; 300 °C, 60 atm and an acid catalyst belong to the reaction of ethene with steam.

Worked example

What is the maximum mass of ethanol that can be made by fermenting 90 g of glucose? C6H12O6 → 2C2H5OH + 2CO2 (H 1, C 12, O 16)

  1. Mr of glucose = (6 × 12) + (12 × 1) + (6 × 16) = 180, so moles of glucose = 90 ÷ 180 = 0.50 mol.
  2. 1 mol of glucose gives 2 mol of ethanol, so moles of ethanol = 0.50 × 2 = 1.0 mol.
  3. Mr of C2H5OH = (2 × 12) + (6 × 1) + 16 = 46, so mass = 1.0 × 46 = 46 g.

Answer: 46 g of ethanol

Practice questions

Try each one, then open the answer.

1. Ethanol burns completely in excess oxygen: C2H5OH + 3O2 → 2CO2 + 3H2O. What mass of water forms when 4.6 g of ethanol burns completely? (H 1, C 12, O 16)

  1. A
    1.8 g
  2. B
    3.6 g
  3. C
    5.4 g
  4. D
    8.8 g
Show answer

Answer: C. Mr of ethanol = 46, so 4.6 g = 0.10 mol. The equation gives 3 mol of water for every 1 mol of ethanol, so 0.30 mol of water forms: 0.30 × 18 = 5.4 g. The mass 8.8 g is the carbon dioxide (0.20 mol × 44).

2. Which is an advantage of making ethanol by the catalytic addition of steam to ethene rather than by fermentation?

  1. A
    it uses a renewable raw material
  2. B
    it uses yeast, which is cheap and easily grown
  3. C
    it is fast and continuous and gives almost pure ethanol
  4. D
    it works at room temperature, so it uses little energy
Show answer

Answer: C. Fermentation is a slow batch process giving a dilute solution that must be distilled. The ethene route is fast and continuous but uses a non-renewable raw material.

3. Which is an advantage of making ethanol by fermentation rather than from ethene?

  1. A
    it needs a high temperature and pressure
  2. B
    it uses a renewable raw material
  3. C
    it is a much faster reaction
  4. D
    it is a continuous process that gives pure ethanol
Show answer

Answer: B. Fermentation uses sugars from crops, which can be regrown. Ethene comes from petroleum, which is non-renewable.

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