Edexcel IGCSE Chemistry · Spec 4.31C-4.33C
Ethanol: Oxidation, Hydration and Fermentation (Paper 2)
Covers describe oxidation of ethanol, Compare hydration and fermentation conditions and Explain the absence of air and optimum fermentation temperature.
Chemistry revision video
Ethanol: Oxidation, Hydration and Fermentation (Paper 2)
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Explained
Making ethanol two ways, and oxidising it
Ethanol is C2H5OH, and this topic covers three reactions: the two industrial routes to making it, and what happens when it is oxidised.
Oxidation
Warming ethanol with acidified potassium dichromate(VI) oxidises it to ethanoic acid.
The colour change is the observation to give: the solution turns from orange to green as the dichromate is reduced.
The same conversion happens slowly and without a reagent when ethanol is left exposed to air, because bacteria oxidise it. That is why an opened bottle of wine turns to vinegar, and vinegar is a dilute solution of ethanoic acid.
Fermentation
Yeast converts glucose into ethanol and carbon dioxide.
The conditions are worth learning precisely. It runs at about 30 to 40 degrees Celsius, and it needs no oxygen, because it is anaerobic respiration in the yeast.
Below about 30 degrees the enzymes work too slowly. Above about 40 degrees the yeast cells and their enzymes are denatured, and the process stops altogether.
Fermentation is a batch process, so the vessel has to be emptied and restarted, and the product is a dilute impure solution that must be distilled. Against that, its raw material is a plant crop, which is renewable.
Hydration of ethene
Ethene reacts with steam to give ethanol, using a catalyst at high temperature and pressure.
This is a continuous process, so it runs without stopping, and the product is pure ethanol needing no separation. The drawback is the feedstock: ethene comes from cracking crude oil, which is finite.
A comparison question wants that trade off. Renewable but slow, batch and impure, against fast, continuous and pure but dependent on a non renewable source.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme asks for the condition needed for fermentation and credits anaerobic, no oxygen, or the absence of air. It then instructs the examiner to ignore respiration.
Respiration is what the yeast is doing, not the condition being asked for. The question wanted a condition, and naming the process does not supply one.
On the industrial conditions for hydration, the same series gives a pressure of 60 to 70 atmospheres and phosphoric acid as the catalyst, allowing sulfuric acid. An examiner report on an earlier paper says directly that these conditions are often tested and should be learned, which is unusually plain advice from a report.
A separate mark scheme, on a safety question about heating a mixture containing ethanol, wants it described as flammable or able to ignite. The examiner report adds that some candidates wrote about the water bath without ever saying why one was needed, and lost the mark.
The water bath is the answer to a question nobody asked. Flammable is the answer to the one that was.
Why the temperature matters twice
Fermentation has an optimum rather than a maximum, which is unusual among the reactions in this course and is worth understanding.
An ordinary chemical reaction goes faster the hotter it gets. Fermentation is catalysed by enzymes, and enzymes are proteins with a specific shape, so heating them past a point destroys the active site and the rate collapses.
So when a graph of fermentation rate against temperature rises to a peak and then falls sharply, the rise is about kinetic energy and collisions and the fall is about denaturing. Two halves, two different explanations.
Spec 4.31C-4.33C
What you need to know
- Describe oxidation of ethanol
- Compare hydration and fermentation conditions
- Explain the absence of air and optimum fermentation temperature
Active recall
Quick check
Answer each question before opening the answer.
What colour change shows dichromate reduction?
Orange to green
State the hydration conditions
Phosphoric acid, about 300 degrees C and 60-70 atm
Why is air excluded from fermentation?
To favour anaerobic respiration and ethanol production
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