Making ethanol by fermentation
2 marks · EasyEthanol may be made by fermentation or by reacting ethene with steam.
Explain why it is important to keep air out of the fermentation vessel.
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To prevent aerobic respiration of the sugar; in the presence of air, yeast produces carbon dioxide and water, not ethanol.
Also accept: to ensure ethanol is produced and not just carbon dioxide and water; air would lead to respiration instead of fermentation
Common mistakes: Saying air 'kills' the yeast; oxygen needed for fermentation
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Marking points
M1 · 1 mark · States air must be excluded to ensure anaerobic conditions (prevents aerobic respiration).
A1 · 1 mark · Links presence of air to production of carbon dioxide and water instead of ethanol.
Worked solution
If air is allowed in, the process is aerobic. Yeast would respire sugar aerobically, producing carbon dioxide and water, not ethanol. Keeping air out ensures anaerobic conditions, so ethanol is formed.