How do the raw material sources for hydration and fermentation differ in terms of sustainability?

Hydration uses ethene derived from non-renewable crude oil, whereas fermentation uses sugar from renewable plant sources. This makes fermentation a more sustainable method as it relies on crops that can be regrown.

Compare the purity of ethanol produced directly from hydration versus fermentation.

Hydration produces high-purity ethanol in a continuous flow, while fermentation produces a dilute 15% mixture that contains yeast and water. Consequently, fermentation requires additional energy-intensive fractional distillation to reach high purity.

When is it more advantageous to use hydration over fermentation industrially?

Hydration is preferred when a fast, continuous supply of high-purity ethanol is needed and the infrastructure for oil cracking is available. It is more efficient for large-scale chemical manufacturing compared to the slower batch fermentation.

What happens to the fermentation process if the reaction vessel is not kept anaerobic?

If oxygen enters the vessel, yeast will respire aerobically, producing water and carbon dioxide instead of ethanol. Additionally, any ethanol already formed may be oxidized into ethanoic acid, ruining the batch.

Why is it a mistake to raise the temperature of fermentation to 100°C to speed it up?

High temperatures denature the enzymes in the yeast, which are biological catalysts made of protein. Once denatured, the enzymes lose their shape and can no longer catalyze the breakdown of glucose, stopping the process entirely.

What common error do students make when balancing the fermentation equation?

Students often forget that two molecules of carbon dioxide are produced for every one molecule of glucose. The correct balanced ratio is $C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2$.

What is the balanced chemical equation for the fermentation of glucose?

The equation is $C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2$. It shows glucose breaking down into ethanol and carbon dioxide gas.

Define the hydration of ethene and state its catalyst.

Hydration is the chemical addition of water (steam) to ethene to form ethanol. The reaction uses a concentrated phosphoric acid catalyst.

State the standard industrial conditions required for the hydration of ethene.

The process requires a temperature of 300°C, a pressure of 60–70 atmospheres (atm), and a concentrated phosphoric acid catalyst.

Why is fermentation referred to as a 'batch process'?

It is a batch process because the yeast is killed once the ethanol concentration reaches 15%. This requires the reaction to be stopped, the vessel cleared, and a new batch started from scratch.

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Manufacture of Ethanol

Summary

Industrial ethanol production is achieved through two distinct chemical pathways: the high-pressure catalytic hydration of ethene derived from petroleum and the biological fermentation of carbohydrates using yeast. Each method is governed by specific temperature and pressure conditions that optimize either chemical reaction rates or enzyme activity, with final purification typically requiring fractional distillation.

1. Hydration of Ethene

Chemical Synthesis: Ethanol can be synthesized by adding a water molecule across the double bond of ethene in a process known as hydration. This reaction is highly efficient for producing high-purity ethanol in large industrial quantities.
Industrial Conditions: The reaction requires a temperature of approximately 300°C and a high pressure of 60–70 atm. These extreme conditions are necessary to maintain a high rate of reaction while balancing equilibrium yield.
Catalytic Requirement: A concentrated phosphoric acid catalyst is used to lower the activation energy, allowing the ethene and steam to react more readily. Without this catalyst, the addition of water to the stable ethene molecule would be prohibitively slow.
Separation Technique: After the reaction, the mixture containing ethene, ethanol, and water is passed through a condenser. Because ethene has a significantly lower boiling point ( $-103^{\circ} C$ ) than ethanol ( $78^{\circ} C$ ) and water ( $100^{\circ} C$ ), it remains a gas and can be easily recycled back into the reactor.

Energy profile diagram showing the reduction in activation energy provided by the phosphoric acid catalyst in ethene hydration.

2. Biological Fermentation

3. Key Distinctions

4. Exam Strategy & Tips

5. Common Pitfalls & Misconceptions