Balance the yeast anaerobic chemical equation
Conserve atoms while forming ethanol and carbon dioxide.
Section B12.1 · Respiration
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall cell processes, enzymes and the distinction between gas exchange and breathing.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many hydrogen atoms are in one C₂H₅OH?
What you will learn
- Use the ethanol formula correctly
- Check all element totals
Balance the yeast anaerobic chemical equation
| Feature | Meaning |
|---|---|
| Glucose input | C6 · H12 · O6 |
| 2 ethanol | C4 · H12 · O2 |
| 2 carbon dioxide | C2 · H0 · O4 |
| Combined products | C6 · H12 · O6 |
Use the ethanol formula correctly
The Supplement yeast equation is C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Ethanol is written C₂H₅OH: each molecule contains two carbon, six hydrogen and one oxygen atoms. The final OH contributes one hydrogen, so counting only the five written next to H gives an incorrect total.
Two ethanol molecules contain four carbon, twelve hydrogen and two oxygen atoms. Two carbon-dioxide molecules add two carbon and four oxygen atoms. The combined products therefore contain six carbon, twelve hydrogen and six oxygen atoms, matching one glucose molecule. Oxygen is conserved in the products even though no O₂ reactant is used.
Check all element totals
Balance by coefficients rather than changing the substance formulae. A coefficient of two multiplies the entire ethanol formula, including its OH group. Equal atom totals do not require equal numbers of molecules: one glucose corresponds to four product molecules in this overall model.
This is the specified yeast pathway and not the aerobic or muscle equation. State energy release alongside the chemical equation without treating it as extra matter. The balanced equation gives relative amounts for the described overall reaction; it is not a complete account of all reactions that can occur in a yeast culture.
Sources for this lesson
Pause and explain
Which is the balanced yeast equation?
Worked example
How many hydrogen atoms are in 2C₂H₅OH?
Show the worked solution
- Each ethanol has five plus one hydrogen atoms.
- Two times six is twelve.
Answer Twelve hydrogen atoms.
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.
State the balanced anaerobic yeast equation.
Give me a hint
Both product coefficients are two.
Compare my reasoning
- Use the correct ethanol formula.
- Balance with two ethanol and two CO₂.
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.
Look for these in your work
- Preserve the formulae
- Give both coefficients
Count carbon, hydrogen and oxygen atoms in the products.
Give me a hint
Multiply each whole formula by its coefficient.
Compare my reasoning
- Two ethanol supply C4 H12 O2.
- Two CO₂ supply C2 O4.
Products total C6 H12 O6, matching one glucose molecule.
Look for these in your work
- Include OH hydrogen
- Combine both products
How can oxygen atoms appear in the products when the pathway does not use O₂?
Give me a hint
Look at the glucose formula.
Compare my reasoning
- Glucose already contains six oxygen atoms.
- Those atoms are conserved in ethanol and CO₂.
The oxygen atoms are already in glucose and are redistributed into the products; O₂ is not needed as a reactant in this model.
Look for these in your work
- Identify the atom source
- Separate oxygen atoms from O₂ use
Common mistakes
- Omitting the hydrogen in OH.
- Adding oxygen as a reactant to balance this anaerobic model.
What can you explain now?
Which is the balanced yeast equation?
Compare with the explanation
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
All three elements balance.
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Make it stick
Count all ethanol atoms and state the balanced equation.
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.