Oxidation, reduction and electron transfer
Key idea: Oxidation is electron loss and reduction is electron gain; they happen together. The oxidising agent accepts electrons and is reduced, while the reducing agent supplies electrons and is oxidised.
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Guided lessons · coming soon
Guided lesson
Guided steps
- Match oxygen, hydrogen and electron changes to each process.
- Decide how magnesium becomes a positive ion.
- Follow electrons and check charge in both half-equations.
- Compare what happens to each substance with what it does.
- Follow oxygen transfer and name both agents.
- Find electron counts, oxidation and the oxidising agent.
- Write half-equations and check atoms, states, electron sides and charge.
- Correct the agent names while keeping the valid oxidation statement.
- Track hydrogen and explain both processes and the reducing agent.
- Explain oxygen transfer and what each agent does.
- Match electron totals and find the smallest coefficients.
- Write scaled half-equations and the ionic equation, then check conservation.
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ContinueOxidation and reduction always happen together: electrons lost by one species are gained by another. There are three ways to recognise the two processes, and the substances that cause them have names of their own.
Oxidation and reduction
A redox reaction is one in which oxidation and reduction happen together. You can recognise the two processes in three ways:
| Basis for comparison | Oxidation | Reduction |
|---|---|---|
| oxygen | gain of oxygen | loss of oxygen |
| hydrogen | loss of hydrogen | gain of hydrogen |
| electrons | loss of electrons | gain of electrons |
For these redox reactions, use the description that matches what the equation shows: oxygen or hydrogen moving between substances, or ions forming from atoms and atoms from ions. A movement of hydrogen ions in an acid–base reaction is not, by itself, evidence of redox. Oxidation states give a further way to check.
Check your understanding 1 An atom becomes a positive ion
A neutral magnesium atom becomes Mg²⁺. Before reading the electron model, decide whether it gains or loses electrons and how many move.
Show answer
The atom loses two electrons. Its charge changes from 0 to +2 because two negative charges have been removed.
Electron transfer
In electron terms, the electrons lost by one species are gained by another. The numbers must balance: oxidation and reduction happen together.
Oxygen transfer between compounds is useful when the equation shows it, but it is not the explanation for every redox reaction. For example, when magnesium burns in oxygen, magnesium is oxidised and oxygen is reduced to oxide ions. Oxygen has not been removed from another compound.
For zinc in copper(II) sulfate solution:
split the change into the part each species undergoes:
Each half-equation conserves atoms and total charge. Zinc has charge 0 on the left and + 2 + 2(-1) = 0 on the right. Copper has + 2 + 2(-1) = 0 on the left and 0 on the right. Put electrons on the side that balances charge, then check that the numbers lost and gained agree.
The two electrons lost by each zinc atom are the two gained by each copper(II) ion. No oxygen is involved, and the reaction is still redox.
OIL RIG — Oxidation Is Loss, Reduction Is Gain, of electrons — helps you recall the electron definitions. It does not tell you which species loses electrons; the equation does.
Oxidising and reducing agents
“Oxidised” and “reduced” describe what happens to a species. “Oxidising agent” and “reducing agent” describe what a species does to the other one.
- The oxidising agent causes the other substance to be oxidised. In the reactions here it accepts electrons, or supplies oxygen to the other substance, so the oxidising agent is itself reduced.
- The reducing agent causes the other substance to be reduced. In the reactions here it supplies electrons, or removes oxygen from the other substance, so the reducing agent is itself oxidised.
In the zinc reaction, zinc supplies the electrons that reduce copper(II) ions, so zinc is the reducing agent and is oxidised. Copper(II) ions accept those electrons, so they are the oxidising agent and are reduced.
Worked example 1
Follow the oxygen, then name the agents
Problem
Show solution step by step
Choose the definition the equation supports
Method
Track oxygen.Reason
Oxygen visibly moves from one substance to another; no ions are shown.Working
Answer
Oxygen starts in CuO and ends in H₂O.Name each process
Method
Apply gain and loss of oxygen.Reason
Gaining oxygen is oxidation; losing oxygen is reduction.Working
Answer
Hydrogen gains oxygen: oxidised. Copper(II) oxide loses oxygen: reduced.Name each agent
Working
Answer
Copper(II) oxide oxidises hydrogen, so it is the oxidising agent. Hydrogen reduces copper(II) oxide, so it is the reducing agent.
Quick check
Guided practice 2
Use electrons when ions are shown
Problem
Decide each role
Hints
Hint 1
charges
Compare each species’ charge before and after: an atom becoming a positive ion has lost electrons.Hint 2
agent
The oxidising agent is the species that takes those electrons.Show solution step by step
Write the two electron changes
Method
Split the equation into what each species does.Reason
Ions are shown, so the electron definition is the one the equation supports.Working
Answer
Mg → Mg²⁺ + 2e⁻; Cu²⁺ + 2e⁻ → Cu.Name process, then agent
Working
Answer
Magnesium loses electrons and is oxidised; it is the reducing agent. Copper(II) ions gain electrons and are reduced; they are the oxidising agent.
Quick check
Check your understanding 2 Construct the two half-equations
For Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s), write both half-equations with states. Check charge on each side. Work on paper or use notes.
Show answer
Mg(s) → Mg²⁺(aq) + 2e⁻ and Cu²⁺(aq) + 2e⁻ → Cu(s).
Compare three features: unchanged atoms and correct states; electrons on the product side for magnesium and reactant side for copper(II); equal total charge on both sides of each equation. Both sides of each half-equation have total charge 0. Two electrons are lost and two gained, so they cancel when you add the equations.
Spot the mistake 3
“The oxidising agent is oxidised”
Learner response
Correct the answer
Show solution step by step
Keep what is right
Method
Check the process first.Reason
Zinc does lose electrons, so “zinc is oxidised” is correct.Working
Answer
Zn → Zn²⁺ + 2e⁻: oxidised.Fix the agent
Working
Answer
Zinc causes iron(II) ions to be reduced, so zinc is the reducing agent. Iron(II) ions are the oxidising agent: they oxidise zinc and are themselves reduced.
Common mistake
Try it yourself 4
A reaction with hydrogen, not oxygen
Problem
Track the hydrogen
Hints
Hint 1
Where hydrogen goes
Hydrogen starts in H₂S. Where does it end up?Hint 2
Hydrogen definition
Losing hydrogen is oxidation; gaining hydrogen is reduction.Show solution step by step
Track hydrogen
Method
Follow the hydrogen atoms.Reason
No oxygen appears, but hydrogen moves from one substance to another.Working
Answer
H₂S loses hydrogen to form sulfur; chlorine gains hydrogen to form HCl.Name processes and the reducing agent
Working
Answer
Hydrogen sulfide is oxidised and chlorine is reduced. Hydrogen sulfide reduces chlorine, so it is the reducing agent.
Quick check
Check your understanding 3 Iron(III) oxide and carbon monoxide
Iron(III) oxide is heated with carbon monoxide: Fe₂O₃(s) + 3CO(g) → 2Fe(s) + 3CO₂(g). Use oxygen transfer to identify the substances oxidised and reduced, then explain the role of each agent.
Show answer
Use gain and loss of oxygen: oxygen moves from iron(III) oxide to carbon monoxide, and no ions are shown. Carbon monoxide gains oxygen to form carbon dioxide, so it is oxidised. Iron(III) oxide loses oxygen to form iron, so it is reduced. Iron(III) oxide is the oxidising agent and carbon monoxide is the reducing agent. Compare the oxygen movement, both processes and what each agent does to the other substance.
Check your understanding 4 Make the electron counts agree
Aluminium atoms react with copper(II) ions to form aluminium ions and copper: Al(s) + Cu²⁺(aq) → Al³⁺(aq) + Cu(s). This outline is not balanced. Find the electron counts and the smallest whole-number coefficients of aluminium and copper(II) ions.
Show answer
Each aluminium atom loses 3 electrons; each copper(II) ion gains 2. Match 6 electrons by using 2 aluminium atoms and 3 copper(II) ions. The product coefficients must also conserve the numbers of aluminium and copper atoms.
Check your understanding 5 Write and check the complete reaction
For aluminium reacting with copper(II) ions, write the scaled half-equations and add them to obtain the balanced ionic equation. Name both agents and check total charge. Work on paper or use notes.
Show answer
2Al(s) → 2Al³⁺(aq) + 6e⁻
3Cu²⁺(aq) + 6e⁻ → 3Cu(s)
Adding and cancelling six electrons gives 2Al(s) + 3Cu²⁺(aq) → 2Al³⁺(aq) + 3Cu(s).
Compare the electron sides and cancellation, both atom counts, states and total charge: +6 on each side of the ionic equation. Aluminium supplies electrons, so it is oxidised and is the reducing agent. Copper(II) ions accept them, so they are reduced and are the oxidising agent.
Common mistakes
- Saying the oxidising agent is oxidised. It causes oxidation and is itself reduced.
- Looking for oxygen transfer when no oxygen moves. Use electron transfer for atom–ion changes.
- Treating any reaction with hydrogen ions as redox. An acid–base reaction is not.
- Saying only one substance changes. Oxidation and reduction always happen together.
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Syllabus and review details
- SEC G3 Combined Science Chemistry component 2027 · 2027
Chemistry content structure and subject content, PDF pages 26–36
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