Making Ethanol

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 4.32 — The two industrial routes to ethanol, with their conditions (bold C statement — Paper 2 only)
• 4.33 — The reasons for fermentation in the absence of air at an optimum temperature (bold C statement — Paper 2 only)

Specification section 4(e) Alcohols and (f) carboxylic acids — also taught in Alkenes, Alkene Reactions And The Bromine Test, Alcohols, Carboxylic Acids.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Ethanol is made industrially in two completely different ways. One is thousands of years old and uses a living organism; the other was designed in the twentieth century and uses a catalyst and a pressure vessel. They produce the same molecule, and the specification wants both sets of conditions and an argument about which is preferable.

Route one — FERMENTATION

1. The raw material is SUGAR, obtained from plants such as sugar cane or from starch crops.
2. The sugar solution is mixed with YEAST, a single-celled fungus, which supplies the enzymes that do the work.
3. The mixture is kept at an OPTIMUM TEMPERATURE of about 30 °C.
4. It is kept in the ABSENCE OF AIR — the vessel is sealed, apart from a valve that lets carbon dioxide escape.
5. The sugar is converted to ethanol and carbon dioxide: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.
6. The process stops when the ethanol concentration rises high enough to kill the yeast, at around fifteen per cent.
7. The dilute ethanol produced is then separated and concentrated by fractional distillation.

Statement 4.33 — why no air, and why that temperature

1. WHY NO AIR, reason one: if oxygen is present, bacteria in the air oxidise the ethanol you have just made into ethanoic acid, and the product is spoiled — this is how wine becomes vinegar.
2. WHY NO AIR, reason two: with oxygen available the yeast respires aerobically instead, giving carbon dioxide and water rather than ethanol, so the yield falls.
3. WHY AN OPTIMUM TEMPERATURE, the cold side: enzymes in the yeast work slowly at low temperatures, so the process becomes uneconomically slow.
4. WHY AN OPTIMUM TEMPERATURE, the hot side: above about 40 °C the enzymes are denatured and the yeast dies, so the process stops altogether.
5. The best temperature is therefore a compromise between the two — fast enough to be worth doing, cool enough that the yeast survives.

Think of it like the difference between growing wheat and milling flour

Growing wheat is slow, seasonal, dependent on weather, and you take what the field gives you. Milling is fast, continuous and controllable — you turn the machine on and flour comes out at a fixed rate, but you had to buy the grain from somewhere. Fermentation is the field: living, patient, working from something a farmer can grow again next year. Hydration of ethene is the mill: quick, continuous and pure, but only as long as somebody keeps supplying crude oil, which nobody can grow.

Route two — HYDRATION OF ETHENE

1. The raw material is ETHENE, obtained by cracking fractions from crude oil.
2. Ethene is mixed with STEAM and passed over a catalyst of PHOSPHORIC ACID.
3. The temperature is about 300 °C and the pressure about 60–70 atmospheres.
4. Water adds across the carbon-to-carbon double bond: C₂H₄ + H₂O → C₂H₅OH.
5. This is an ADDITION reaction — two molecules become one, with nothing given off.
6. The process runs CONTINUOUSLY rather than in batches, and the ethanol produced is essentially pure.

The comparison is genuinely balanced, which is why it makes a good examination question. Fermentation uses a renewable raw material, needs only gentle conditions and simple equipment, and can be carried out anywhere sugar grows. Against that it is slow, works in batches, gives a dilute product that must be distilled, and the crops occupy land that could grow food.

Hydration of ethene is fast, continuous and gives a pure product with a very high yield, and needs far less labour. Against that its raw material is a finite one — ethene comes from crude oil — and the high temperature and pressure make the plant expensive to build and costly in energy to run.

Notice that the two routes fail in opposite directions, which is the point worth carrying into an exam answer. Fermentation is cheap to set up and expensive in time. Hydration is expensive to set up and cheap in time. Which is preferable depends entirely on what is scarce where the plant is being built.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

The two routes to ethanol

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Conditions: Yeast, about 30 °C, in the absence of air. Glucose gives ethanol and carbon dioxide.

C₂H₄ + H₂O → C₂H₅OH

Conditions: Steam, phosphoric acid catalyst, about 300 °C and 60–70 atmospheres. Water adds across the double bond.

Fermentation, in the order an answer needs

1. Dissolve sugar from a plant source in water.
2. Add yeast, which supplies the enzymes for the reaction.
3. Seal the vessel to exclude air, leaving a valve for carbon dioxide to escape.
4. Keep the mixture at about 30 °C.
5. Leave until the ethanol concentration stops rising, then separate and concentrate it by fractional distillation.

Statement 4.33 — the four reasons, learn all four

• NO AIR: oxygen lets bacteria oxidise the ethanol to ethanoic acid, spoiling the product
• NO AIR: with oxygen the yeast respires aerobically, giving carbon dioxide and water instead of ethanol
• NOT TOO COLD: the yeast's enzymes work too slowly, so the process is uneconomic
• NOT TOO HOT: above about 40 °C the enzymes are denatured and the yeast dies

The vocabulary these two routes need

Fermentation

The conversion of sugar into ethanol and carbon dioxide by yeast, in the absence of air.

Hydration

Addition of water across a carbon-to-carbon double bond.

Batch process

One that is set up, run to completion and emptied, then started again.

Continuous process

One in which reactants are fed in and product drawn off without stopping.

Model answer [4 marks]

Explain why fermentation must be carried out in the absence of air and at about 30 °C. [4]

Air must be excluded for two reasons. If oxygen is present, bacteria in the air oxidise the ethanol that has been produced into ethanoic acid, spoiling the product; and the yeast respires aerobically instead of fermenting, giving carbon dioxide and water rather than ethanol. The temperature is a compromise. Below about 30 °C the enzymes in the yeast work too slowly for the process to be economic. Above about 40 °C those enzymes are denatured and the yeast dies, so no ethanol is produced at all. About 30 °C is fast enough to be worthwhile while keeping the yeast alive.

Model answer [4 marks]

Give one advantage and one disadvantage of making ethanol by hydration of ethene rather than by fermentation. [4]

An advantage is that hydration of ethene runs as a continuous process and gives essentially pure ethanol at a high rate, whereas fermentation is a batch process producing a dilute solution that must then be concentrated by fractional distillation. A disadvantage is that its raw material, ethene, is obtained by cracking fractions from crude oil, which is a finite resource that will eventually run out, whereas fermentation uses sugar from plants, which can be grown again each year and is therefore renewable.

Not this: Fermentation is kept warm, so the hotter it is the more ethanol you get.

This: There is an OPTIMUM. Raising the temperature speeds the process up only until about 40 °C, at which point the yeast's enzymes are denatured and the yeast dies. Past that point the yield does not rise more slowly — it stops.

Mark-losing trap. Fermentation is about 30 °C; hydration of ethene is about 300 °C. Losing the zero loses the mark.

Mark-losing trap. Give BOTH reasons for excluding air — the ethanol is oxidised, and the yeast respires aerobically instead.

Mark-losing trap. The catalyst for hydration is phosphoric acid, not sulfuric acid and not a metal.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — Which organism is used in the fermentation of sugar to make ethanol?
2. Grade 7 · State [2 marks] — Give the temperature and the catalyst used in the industrial hydration of ethene to make ethanol.
3. Grade 8 · Explain [3 marks] — Explain why a fermentation vessel is sealed except for a valve that lets gas out.
4. Grade 9 · Explain [4 marks] — Select every statement that belongs in a full explanation of why fermentation is carried out at about 30 °C rather than at 10 °C or at 60 °C.
5. 9+ · Evaluate [6 marks] — A country with plentiful sugar cane and no oil reserves must choose a route to ethanol. Evaluate which route it should use and give the strongest argument against your choice.

The people behind this science

Two ways into the same idea — the one who proved fermentation is something a living organism does, and the one who went to look at the gas coming off the vat. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Louis Pasteur — the one who proved fermentation is something a living organism does

Statement 4.33 is Pasteur's discovery almost word for word. In the 1850s and 1860s he established that fermentation is not a spontaneous chemical decomposition, as the leading chemists of the day insisted, but the work of living yeast cells; that yeast ferments sugar to alcohol only when air is excluded, and switches to ordinary respiration when it is not — an effect still named after him; and that the souring of wine is a different organism getting in. Every condition this page specifies is a condition he identified experimentally, and he is the right figure to ask why they matter.

• “Why does the yeast make alcohol only when the air is kept out?”
• “How did you show that fermentation needs something alive?”
• “What did the chemists of your day think was happening instead?”
• “What gets into the wine when it goes sour?”
• “How would you keep an experiment free of the wrong organisms?”

Joseph Priestley — the one who went to look at the gas coming off the vat

Fermentation gives two products, and this page tends to treat the carbon dioxide as an afterthought. Priestley did not. Living next to a brewery in Leeds in the 1760s, he became curious about the layer of gas sitting above the fermenting vats, found that it put out a candle and would not support a mouse, and worked out how to dissolve it in water — which is how carbonated water was invented, and how he was drawn into the study of gases that led to his isolating oxygen. He is the right person to ask what the second product of fermentation actually is and how you would investigate it.

• “What was the gas sitting above the fermenting vats?”
• “How did you test a gas you could not see?”
• “What made you curious about a brewery in the first place?”
• “How did you get the gas to dissolve in water?”
• “What can you learn from a gas that puts a candle out?”

Carry on through the course

• Previous lesson: Alcohols
• Next lesson: Carboxylic Acids
• What Organic Chemistry Is — Two facts about carbon generate several million compounds — and three words keep them in order.
• Naming, Drawing And Classifying — A name that tells you the structure, a method that finds every isomer, and three words that cover every organic reaction here.
• Crude Oil And Its Fractions — One mixture, separated by boiling point into six useful materials — and one causal chain that explains every trend down the column.
• Burning Fuels — Two ways a hydrocarbon can burn, and why one of them fills a room with a gas you cannot smell.
• Pollutants And Acid Rain — One pollutant comes out of the fuel and one comes out of the air — and telling them apart decides which fix works.
• All of Chemistry · Chemistry key terms · required practicals · equations · Incandio Science · The 4CH1 specification