Alcohols · Grade 10
In Grade 10 you learn: 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 oxidized to carboxylic acids. Ethanol, the alcohol in drinks, is made by fermenting sugar with yeast, warm and without air.
Key facts
- Fermentation: yeast turns sugar into ethanol and carbon dioxide, warm (about 30 °C) and without air: C6H12O6 → 2C2H5OH + 2CO2.
Worked examples
Example 1
What conditions are used to make ethanol by fermentation?
Answer: sugar solution with yeast, warm (about 30 °C), without air
Example 2
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 3
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 4
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 oxidize the ethanol to ethanoic acid (vinegar)
Example 5
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