Making ethanol by fermentation
2 marks · HardState two reasons why anaerobic conditions are important during the fermentation of glucose to make ethanol.
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Prevents yeast converting glucose into products other than ethanol; stops the formation of water instead of ethanol; prevents oxidation of ethanol to ethanoic acid; yeast produces ethanol only in absence of air.
Also accept: Prevents production of water instead of ethanol; Prevents oxidation of ethanol to ethanoic acid; Ensures yeast produces ethanol; Excludes air which would allow aerobic respiration
Common mistakes: Omitting reason related to ethanol not being made; Stating only that air is not needed without explaining why
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Marking points
B1 · 1 mark · States that anaerobic conditions are needed to prevent production of water instead of ethanol
B2 · 1 mark · States that anaerobic conditions are needed to prevent oxidation of ethanol to ethanoic acid, or to ensure yeast produces ethanol
Worked solution
Anaerobic conditions are needed because otherwise (i) the yeast would produce carbon dioxide and water instead of ethanol, and (ii) any oxygen present could cause ethanol to be oxidised to ethanoic acid.