Balance redox equations using oxidation-number changes
Use oxidation-state changes and electron conservation to set balanced redox coefficients, then check atoms and charge.
Section 6 · Electrochemistry
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Check atom balance, total charge and oxidation-state changes separately. Written practice is self-checked
Before you begin
- Recognise common ions and write formulas for simple compounds.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does an increase in oxidation number represent?
What you will learn
- Use oxidation-number changes to help balance redox equations
- Match total electron loss with total electron gain
- Check final atom and charge balance
Oxidation-number changes count electron transfer
An increase in oxidation number represents oxidation and corresponds to electron loss. A decrease represents reduction and corresponds to electron gain. The total increase in oxidation number must balance the total decrease after accounting for atom counts.
Use the atom counts to find coefficients that equalise electron loss and gain. Then balance remaining atoms and check the total charge on both sides. In aqueous ionic equations, include only species relevant to the stated reaction and conditions.
Keep the final equation independently checked
The oxidation-state method helps determine coefficients but does not replace atom or charge checks. Multiplying a change by the number of atoms matters; two atoms each changing by one correspond to two electrons transferred.
Do not change a chemical formula's subscripts to balance an equation. Change coefficients and verify every element and total ionic charge.
Pause and explain
What must balance between the oxidation and reduction changes?
Worked example
Balance Zn + Cu²⁺ → Zn²⁺ + Cu using oxidation-number changes.
Show the worked solution
- Zn changes 0 → +2, losing two electrons.
- Each Cu²⁺ changes +2 → 0, gaining two electrons.
- The electron changes match in a 1:1 ratio; check atoms and charge.
Answer Zn + Cu²⁺ → Zn²⁺ + Cu is balanced: one Zn and one Cu on each side, total charge +2 on each side.
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.
Balance Fe²⁺ + Cl₂ → Fe³⁺ + Cl⁻ using oxidation-number changes.
Give me a hint
Each iron loses one electron; one chlorine molecule gains two electrons total.
Compare my reasoning
- Two Fe²⁺ ions each change +2→+3, losing one electron each.
- Cl₂ changes from 0 to two Cl⁻ ions, gaining two electrons.
- Use coefficients 2 for Fe²⁺, Fe³⁺ and Cl⁻, then check charge.
2Fe²⁺ + Cl₂ → 2Fe³⁺ + 2Cl⁻. Each side has charge +4.
Look for these in your work
- Electron totals matched
- Atom counts balanced
- Charge checked
Balance Mg + Ag⁺ → Mg²⁺ + Ag using the oxidation-number changes.
Give me a hint
Magnesium loses two electrons; each silver ion gains one.
Compare my reasoning
- Mg: 0→+2, loss of two electrons.
- Ag⁺: +1→0, gain of one electron per Ag⁺.
- Use two Ag⁺ ions for each Mg atom.
Mg + 2Ag⁺ → Mg²⁺ + 2Ag.
Look for these in your work
- Electron changes compared
- Silver coefficient set
- Atoms and charge balanced
A proposed balance for Al + Cu²⁺ → Al³⁺ + Cu is Al + Cu²⁺ → Al³⁺ + Cu. Diagnose and correct it.
Give me a hint
Compare total electron changes before checking charge.
Compare my reasoning
- Al loses three electrons per atom.
- Cu²⁺ gains two electrons per ion.
- Use the least common electron total, six, then check atoms and charge.
2Al + 3Cu²⁺ → 2Al³⁺ + 3Cu. Six electrons are transferred; both sides have total charge +6.
Look for these in your work
- Electron loss/gain compared
- Coefficients scaled to six electrons
- Charge and atoms verified
Common mistakes
- Matching oxidation-number changes without multiplying by atom count
- Ignoring total ionic charge
- Changing subscripts instead of coefficients
- Assuming oxygen or hydrogen is balanced without checking
What can you explain now?
What does an increase in oxidation number represent?
Compare with the explanation
Oxidation and electron loss
The species has lost electron density in the bookkeeping model.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
How do oxidation-state changes set coefficients, and which two independent balances must be checked at the end?
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.