Ethanol from fermentation: glucose, yeast and conditions
Use all the required fermentation conditions and explain where ethanol and carbon dioxide come from.
Core and Extended · Chemistry 0620, 2026–2028. Manufacture comparisons, ethanol oxidation and esterification are developed in Extended lessons.
Before you begin
- Identify ethanol's structural formula and count atoms in glucose.
- Read a balanced equation without confusing a condition with a reactant.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is aqueous glucose?
What you will learn
- Describe fermentation of aqueous glucose using yeast at 25–35°C without oxygen.
- Recognise ethanol and carbon dioxide as fermentation products and account for their atoms.
State the complete conditions
Ethanol can be made by fermenting aqueous glucose at 25–35°C, in the presence of yeast and in the absence of oxygen. Aqueous means the glucose is dissolved in water. Yeast supplies the biological machinery, including enzymes, for the fermentation; the named starting material is glucose, not ethene. Give all these conditions when describing the required route.
The temperature range supports the required yeast process. Simply using the much higher temperature of catalytic ethene hydration would not reproduce fermentation and can damage the enzymes. Absence of oxygen belongs to this fermentation description; it is different from the oxygen supply involved in burning ethanol or bacterial oxidation to ethanoic acid.
Count products rather than deleting glucose atoms
The model equation is C₆H₁₂O₆→2C₂H₅OH+2CO₂. Six C atoms become four in the two ethanol molecules and two in carbon dioxide. The twelve H atoms are retained in the ethanol, and the six O atoms become two in ethanol and four in carbon dioxide. Yeast and the temperature are conditions stated alongside the equation, not additional elements in the product formula.
Fermentation produces an aqueous mixture rather than magically separating pure ethanol. Separating ethanol from that mixture is a later physical step. The balanced equation is an idealised chemical model, not a promise of complete conversion or a practical fermentation procedure; comparisons of feedstocks, processing and purity are considered in the Extended manufacture lesson.
Pause and explain
Which condition set belongs to fermentation?
Worked example
Describe the required fermentation route and check the atom totals in its equation.
Show the worked solution
- Start with aqueous glucose, yeast, 25–35°C and absence of oxygen.
- Write C₆H₁₂O₆→2C₂H₅OH+2CO₂.
- Products contain C₆H₁₂O₆ overall: C 4+2, H 12, O 2+4.
Answer Aqueous glucose with yeast at 25–35°C without oxygen forms ethanol and carbon dioxide: C₆H₁₂O₆→2C₂H₅OH+2CO₂.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Give the fermentation starting material, biological agent, temperature and oxygen condition.
Give me a hint
Organise the answer as four separate required features.
Compare my reasoning
- The starting material is aqueous glucose.
- Yeast is present and the temperature is 25–35°C.
- Oxygen is absent in the required fermentation description.
Aqueous glucose; yeast; 25–35°C; absence of oxygen.
Look for these in your work
- All four features are present.
- The answer is specific to fermentation.
Check the C, H and O totals in C₆H₁₂O₆→2C₂H₅OH+2CO₂ and explain why CO₂ is needed.
Give me a hint
Include the OH hydrogen in each ethanol formula.
Compare my reasoning
- Two ethanol molecules contain C₄H₁₂O₂.
- Two CO₂ molecules add C₂O₄, giving total C₆H₁₂O₆.
- Omitting CO₂ loses two C and four O atoms, so ethanol alone is not the balanced product set.
Products total C₆H₁₂O₆: C₄H₁₂O₂ in ethanol plus C₂O₄ in carbon dioxide.
Look for these in your work
- All three element totals are correct.
- The carbon dioxide contribution is explicit.
A plan uses glucose and yeast at 300°C with abundant oxygen and claims pure ethanol is the only product. Identify the mismatches with the specified route.
Give me a hint
Compare conditions, products and separation independently.
Compare my reasoning
- The required yeast route is 25–35°C without oxygen, not 300°C with oxygen.
- The model produces ethanol and carbon dioxide, not only ethanol.
- The aqueous fermentation mixture requires a separate physical separation step to obtain more concentrated ethanol.
Use 25–35°C without oxygen; include carbon dioxide; distinguish the aqueous product mixture from separated ethanol.
Look for these in your work
- Both condition errors are corrected.
- Product formation and separation are distinguished.
Common mistakes
- Omitting yeast, aqueous glucose, the temperature range or oxygen absence.
- Using hydration's 300°C condition for the yeast route.
- Forgetting the carbon dioxide product.
- Calling the fermentation mixture pure separated ethanol.
What can you explain now?
State the four required features of the fermentation route.
Compare with the explanation
Aqueous glucose, yeast, 25–35°C and absence of oxygen.
Each belongs to this biological manufacture route; ethene and high pressure belong to hydration.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Write fermentation's four required features and its equation from memory. Count the carbon atoms entering each product.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.