Balance the aerobic respiration chemical equation
Preserve molecule formulae and count all three elements.
Section B12.1 · Respiration
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
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 oxygen atoms are in 3O₂?
What you will learn
- Balance coefficients not subscripts
- Check carbon hydrogen and oxygen
Balance the aerobic respiration chemical equation
| Feature | Meaning |
|---|---|
| Glucose | C₆H₁₂O₆ |
| Oxygen | 6O₂ |
| Carbon dioxide | 6CO₂ |
| Water | 6H₂O |
| Oxygen atom check | 18 on each side |
Balance coefficients not subscripts
The balanced aerobic equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. Each formula names a substance. Coefficients multiply whole formula units; changing a subscript changes the substance. Glucose contains six carbon, twelve hydrogen and six oxygen atoms in each molecule, while an oxygen molecule contains two oxygen atoms.
Start from one glucose. Six carbon atoms require six carbon-dioxide molecules. Twelve hydrogen atoms require six water molecules. The products then contain twelve oxygen atoms in carbon dioxide and six in water, making eighteen. The glucose supplies six; the other twelve come from six oxygen molecules.
Check carbon hydrogen and oxygen
Check each element on both sides: carbon six and six, hydrogen twelve and twelve, oxygen eighteen and eighteen. A balanced equation conserves atom counts, not the count of molecules. It does not assert that all the enzyme-catalysed reactions occur in one collision or that a single event can be seen in an anatomical diagram.
This symbolic equation is a Supplement requirement for Extended learners. Write the correct formulae before balancing and retain the whole-number coefficients. Energy release belongs to the process explanation and does not add chemical atoms. Coefficients show relative amounts under the specified overall reaction, not a respiratory rate unless a time is supplied.
Sources for this lesson
Pause and explain
Which coefficient multiplies water in the balanced one-glucose equation?
Worked example
Why is the oxygen coefficient six?
Show the worked solution
- Products contain 18 oxygen atoms; glucose supplies 6.
- The remaining 12 require 6 molecules of O₂.
Answer Six O₂ molecules supply the remaining twelve oxygen 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 aerobic equation for one glucose molecule.
Give me a hint
Balance carbon and hydrogen before the remaining oxygen.
Compare my reasoning
- Use 6CO₂ and 6H₂O for carbon and hydrogen.
- Add 6O₂ to supply the remaining oxygen.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.
Look for these in your work
- Retain formulae
- Give all three coefficients
Count the oxygen atoms on each side of that balanced equation.
Give me a hint
Include oxygen already in glucose.
Compare my reasoning
- Left: 6 + 6 × 2 = 18.
- Right: 6 × 2 + 6 × 1 = 18.
Eighteen oxygen atoms on each side.
Look for these in your work
- Include glucose oxygen
- Check both products
A student changes water to H₁₂O to fix hydrogen. Explain the error and repair it.
Give me a hint
A coefficient preserves the water formula.
Compare my reasoning
- H₂O is the formula of water.
- Six H₂O supplies twelve hydrogen atoms.
Keep H₂O and use coefficient 6; changing its subscripts changes the substance rather than balancing water.
Look for these in your work
- Preserve the formula
- Use a coefficient
Common mistakes
- Changing O₂ to O₆ to balance.
- Balancing only carbon while ignoring oxygen.
What can you explain now?
Which coefficient multiplies water in the balanced one-glucose equation?
Compare with the explanation
Six
Six H₂O gives twelve hydrogen atoms.
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Make it stick
Balance the equation by coefficients and check all three elements.
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.