Alcohols · Year 12

In Year 12 you get fluent with: alcohols: the –OH group and their formulae; reactions of alcohols: burning, sodium and oxidation; fermentation.

Alcohols are a homologous series with the –OH group: methanol, ethanol, propanol and butanol. They burn to give carbon dioxide and water, react with sodium to give hydrogen, and are oxidised to carboxylic acids. Ethanol, the alcohol in drinks, is made by fermenting sugar with yeast, warm and without air.

Key facts

  • Alcohols contain –OH: methanol CH3OH, ethanol C2H5OH, propanol C3H7OH, butanol C4H9OH.
  • Alcohols burn to give carbon dioxide and water, react with sodium to give hydrogen, and are oxidised to carboxylic acids.
  • Fermentation: yeast turns sugar into ethanol and carbon dioxide, warm (about 30 °C) and without air: C6H12O6 → 2C2H5OH + 2CO2.

Worked examples

Example 1

What is the functional group of the alcohols?

Answer: –OH

Example 2

A small piece of sodium is added to ethanol. Which gas bubbles off?

Answer: hydrogen

Example 3

Ethanol is oxidised (for example by acidified potassium dichromate, or by microbes in the air). What is made?

Answer: ethanoic acid

Example 4

In the complete combustion of one molecule of ethanol, chem C2H5OH, how many oxygen molecules are needed?

Answer: 3

Example 5

What conditions are used to make ethanol by fermentation?

Answer: sugar solution with yeast, warm (about 30 °C), without air

Example 6

Write the word and symbol equations for making ethanol by fermentation, and give the conditions.

Answer: glucose → ethanol + carbon dioxide; C6H12O6 → 2C2H5OH + 2CO2; yeast, about 35 °C, no air

Example 7

Why is fermentation carried out at about 35 °C, and not at 20 °C or 60 °C?

Answer: at 20 °C the enzymes in the yeast work slowly; at 60 °C they are denatured and stop working; about 35 °C gives a good rate

Example 8

Why must air be kept out of a fermentation vessel?

Answer: with oxygen the yeast respires aerobically, making carbon dioxide and water instead of ethanol; also, microbes in the air would oxidise the ethanol to ethanoic acid (vinegar)

Example 9

180 g of glucose is fermented: C6H12O6 → 2C2H5OH + 2CO2. What is the maximum mass of ethanol that can form? (Aᵣ: C = 12, H = 1, O = 16)

Answer: 92 g

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