Cambridge O Level Chemistry · Syllabus 5070 · Organic Chemistry
Fermentation
What is Fermentation?
The process in which yeast converts aqueous glucose into ethanol and carbon dioxide, carried out at 25 to 35 degrees Celsius in the absence of oxygen.
This definition is part of the Organic Chemistry chapter in Cambridge O Level Chemistry.
Common mistakes with Fermentation
- “Air is bubbled through the fermentation mixture to keep the yeast alive.” why it fails With oxygen present, the process is no longer fermentation, and any ethanol that does form can be oxidised further. correct Fermentation requires the absence of oxygen, with yeast, aqueous glucose and a temperature of 25–35 °C. Taught in 11.6.
- 11. “Air is bubbled through the fermentation mixture to keep the yeast alive.” defect Oxygen introduced into a process defined by its absence. why Fermentation is what yeast does without oxygen. With oxygen present, any ethanol formed can itself be oxidised to ethanoic acid. repair Add four words: “in the absence of oxygen”. transfer Explain, using section 11.7B, exactly what would go wrong if air were admitted.
- 12. “Ethene is hydrated at 300 °C and 60 kPa.” defect A unit error of a factor of 100. why 60 kPa is below atmospheric pressure. The answer describes a partial vacuum rather than a high-pressure plant. repair Write 6000 kPa, or write 60 atm. The number 60 belongs only with “atm”. transfer State both conditions for hydration and both for fermentation, with correct units throughout.
Questions students ask about Fermentation
What are the two ways of making ethanol, and how do they differ?
Fermentation uses yeast acting on aqueous glucose at 25-35°C in the absence of oxygen, as a batch process from a renewable source. Catalytic hydration reacts ethene with steam over an acid catalyst at about 300°C and 6000 kPa, as a continuous process from crude oil. Both give the same product, ethanol, by different routes with different trade-offs.
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