Manufacture of Ethanol — Alcohols | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 6% of your exam marks.

stable

Low

Stable6%

Fermentation vs hydration routes and uses of ethanol are commonly tested.

Ethanol is made on an industrial scale by two completely different routes; one is fast and continuous, the other slow and batch-based. Both are important in different parts of the world.

Route A — Hydration of ethene

Ethene from cracking is reacted with water (as steam) in the presence of a catalyst:

C₂H₄(g) + H₂O(g) → C₂H₅OH(g)

An addition reaction across the C=C: H and OH from a water molecule attach to the two carbons of the double bond, leaving an ethanol molecule
Conditions:
- Temperature about 300 °C
- Pressure about 60–70 atm
- Catalyst: concentrated phosphoric acid, H₃PO₄
The reaction does not finish in a single pass: the gas mixture leaving the reactor still contains some ethene along with the ethanol and the water. The unreacted ethene condenses at a much lower temperature (boiling point −104 °C) than the other two and is separated off and pumped back into the reactor; the ethanol-and-water mixture is then split by fractional distillation
Hydration is a continuous industrial process — feedstock flows in, products flow out, around the clock
The route depends on having a cheap supply of ethene from cracking oil, so it dominates ethanol manufacture in oil-rich countries

Route B — Fermentation of sugar

Glucose or sucrose (in fruit juice, sugar-cane syrup or molasses) is dissolved in water with and left to react in the absence of air:

C₆H₁₂O₆(aq) → 2 C₂H₅OH(aq) + 2 CO₂(g)

The yeast contains enzymes that catalyse the breakdown of glucose into ethanol and carbon dioxide
Conditions:
- Temperature kept at about 25–35 °C (optimum near 30 °C)
  - Below 25 °C the enzymes work too slowly
  - Above 35 °C the enzymes start to (lose their shape and stop functioning)
- No oxygen — fermentation is an anaerobic process. If air is allowed in, Acetobacter would oxidise the ethanol back to vinegar (see section 2 part b)
The yeast keeps fermenting until the ethanol concentration reaches around 14–15%, at which point the alcohol kills the yeast and the reaction stops
The fermented mixture is then fractionally distilled to give purer ethanol (up to about 96%)
Fermentation is a batch process — each fermenter is filled, run, drained and refilled separately
The route depends only on plant material (a renewable source), so it dominates ethanol manufacture in countries with large agricultural surpluses such as Brazil

Exam tip

Conditions for fermentation

What comes up: "State two conditions needed for fermentation" (the absence of air is often given for you in the question stem, so you must name the other two).

Write (two marks): (1) a temperature of approximately 30 °C (the mark scheme accepts any value from 20 °C to 40 °C inclusive); (2) yeast (the mark scheme accepts "zymase" as an alternative name for the enzyme).

Watch out: simply writing "anaerobic" or "no oxygen" scores nothing when absence of air is already stated in the question — the examiner explicitly ignores those answers in that context. "Room temperature" is also ignored; you must give a numerical value or the 20–40 °C range.

Exam tip

Why fermentation must be anaerobic

What comes up: "Explain why fermentation needs to be carried out in the absence of air" (2 marks — needs a linked explanation, not just a fact).

Write (two marks): (M1) oxygen in the air would react with (oxidise) the ethanol; (M2) converting it into ethanoic acid (the mark scheme also accepts "carboxylic acid" or "vinegar"). Alternatively: (M1) the fermentation process is anaerobic; (M2) so without excluding air, only carbon dioxide and water would form instead of ethanol.

Watch out: stating only that "air would spoil the fermentation" without naming the specific product (ethanoic acid) drops the second mark.

Comparison

Feature	Hydration of ethene	Fermentation of sugar
Raw material	Ethene from cracking crude oil (non-renewable)	Glucose from plants (renewable)
Mode	Continuous, fast	Batch, slow (takes days)
Conditions	300 °C, 60–70 atm, H₃PO₄ catalyst	25–35 °C, yeast enzymes, no oxygen
Energy cost

Exam tip

Advantage and disadvantage of fermentation over hydration

What comes up: "Give one advantage and one disadvantage of using fermentation rather than hydration to produce ethanol" (4 marks — each point needs a reason to earn both marks).

Write (four marks): Advantage: (M1) the raw material, glucose from plants, is renewable; (M2) whereas ethene is obtained from crude oil, which is a finite (non-renewable) resource. Disadvantage: (M3) fermentation is slower than hydration (or: fermentation is a batch process whereas hydration is continuous); (M4) so hydration is more efficient (or: the ethanol produced by fermentation is impure and requires further purification).

Watch out: just writing "cheaper" or "less costly" without linking to the reason (lower temperature/pressure) is ignored by the mark scheme. Similarly, a reference to yield alone ("higher yield") is explicitly ignored — you must link the point to a named difference (speed, purity, or raw-material sustainability) and give the consequence for the second mark.