Worked example 1
For Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s), identify what is oxidised, what is reduced, and both agents.
Try it first: Assign oxidation numbers to zinc and copper on each side before looking at the sulfate.
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Chemical ReactionsRedox IdentificationContent level: Core 20 min
Identifying Redox Reactions and Agents
What you'll be able to do: Decide whether a reaction is redox, and name the species oxidised, reduced, and acting as each agent.
Introduction
Once oxidation numbers can be assigned, spotting a redox reaction becomes mechanical: compare each element before and after. If nothing changed, no electrons moved, and the reaction is something else.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Compare oxidation numbers before and after a reaction
- Identify which species is oxidised and which is reduced
- Name the oxidising agent and the reducing agent correctly
- Classify reactions as redox or non-redox, including disproportionation
Lesson
The definition in one comparison
Assign oxidation numbers to every atom on both sides. Oxidation is an increase in oxidation number, reduction is a decrease. If every element keeps its number, no electrons were transferred and the reaction is not redox, no matter how dramatic it looks.
OIL RIG: Oxidation Is Loss of electrons, Reduction Is Gain. LEO says GER works equally well.
Agents are named backwards from intuition
The oxidising agent is the species that causes oxidation in something else, which means it is itself reduced. The reducing agent is the species that is itself oxidised. Students lose more marks here than anywhere else in the unit, so state which species changed first, then attach the agent label second.
The oxidising agent is reduced. The reducing agent is oxidised. Write down the change before the label.
Electrons are conserved
Every electron lost by one species is gained by another. If one atom rises by 2 and another falls by 1, then two of the second atom must be involved. This is the arithmetic that makes balancing redox equations possible.
Zn → Zn²⁺ + 2 e⁻ paired with Cu²⁺ + 2 e⁻ → Cu
Which reaction types are redox
Single replacement and combustion are always redox. Synthesis and decomposition are redox when a free element is involved on either side. Double replacement, precipitation and acid-base neutralisation are never redox, because the ions keep the same charges throughout.
If a free element appears on one side and is combined on the other, the reaction is redox.
Disproportionation
Occasionally a single element is both oxidised and reduced in the same reaction. Chlorine in cold sodium hydroxide goes from 0 to -1 and to +1 at the same time. Check every element individually rather than stopping at the first change you find.
Key ideas
Definition
Oxidation
Loss of electrons, shown by an increase in oxidation number.
Definition
Reduction
Gain of electrons, shown by a decrease in oxidation number.
Definition
Oxidising agent
The species that accepts electrons and is therefore itself reduced.
Definition
Reducing agent
The species that donates electrons and is therefore itself oxidised.
Rule
Always paired
Oxidation and reduction occur together, and the electrons lost equal the electrons gained.
Worked examples
Worked example 2
Is AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq) a redox reaction?
Try it first: Look for any free element, then check each ion charge.
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Worked example 3
In 2 MnO₄⁻ + 5 H₂C₂O₄ + 6 H⁺ → 2 Mn²⁺ + 10 CO₂ + 8 H₂O, identify the oxidising agent.
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Common mistakes
Calling the oxidised species the oxidising agent.
Why it's wrong: An agent acts on something else, so the oxidising agent is the one being reduced.
Check instead: Identify the change first, then flip the label.
Assuming oxidation requires oxygen.
Why it's wrong: The modern definition is electron loss, and many redox reactions involve no oxygen at all.
Check instead: Compare oxidation numbers rather than looking for O atoms.
Treating any reaction with a colour change as redox.
Why it's wrong: Precipitation and acid-base reactions also change appearance without transferring electrons.
Check instead: Assign oxidation numbers on both sides and compare.
Stopping after finding one element that changed.
Why it's wrong: Disproportionation involves one element changing in two directions.
Check instead: Check every element before concluding.
Practice this skill
No practice questions are available for this topic yet. You can still practice the whole unit.
What you should now know
Oxidation is a rise in oxidation number caused by electron loss, and reduction is a fall caused by electron gain. The two always happen together and in matching electron amounts. The species that is oxidised is the reducing agent, and the species that is reduced is the oxidising agent, a pairing that is easy to reverse if you name it carelessly.
- Compare oxidation numbers on both sides to test for redox
- Oxidation is a rise, reduction is a fall
- The oxidising agent is reduced and the reducing agent is oxidised
- Electrons lost must equal electrons gained
- Double replacement and neutralisation are never redox
Sources and further reading
This lesson is original Chem Help content. No external sources were adapted.