Oxidising and Reducing Agents
This topic covers how to identify oxidising and reducing agents in redox reactions, which are fundamental to understanding electron transfer in chemistry. You must be able to recognise which reactant causes oxidation and which causes reduction.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- An oxidising agent takes electrons from another substance, causing it to be oxidised.
- A reducing agent gives electrons to another substance, causing it to be reduced.
- Crucially, the oxidising agent is itself reduced during the reaction.
- Similarly, the reducing agent is itself oxidised during the reaction.
- In terms of oxygen transfer, an oxidising agent provides oxygen, while a reducing agent removes it.
- Oxidising and reducing agents are always found on the reactant side of a chemical equation.
Definitions
- Oxidising Agent (Oxidant)
- A chemical species that causes another reactant to be oxidised by accepting its electrons. The oxidising agent undergoes reduction.
- Reducing Agent (Reductant)
- A chemical species that causes another reactant to be reduced by donating its electrons. The reducing agent undergoes oxidation.
Worked example
Consider the thermite reaction: Fe2O3 + 2Al → 2Fe + Al2O3. Identify the oxidising agent and the reducing agent.
- 1
First, identify which species are oxidised and which are reduced by examining their oxidation states.
Reactants are Fe2O3 and Al.
- 2
In the reactants, aluminium (Al) is an element, so its oxidation state is 0.
Iron in Fe2O3 has an oxidation state of +3 (since oxygen is -2).
- 3
In the products, iron (Fe) is now an element with an oxidation state of 0.
Aluminium in Al2O3 has an oxidation state of +3.
- 4
Aluminium's oxidation state increased from 0 to +3.
This is oxidation (loss of electrons).
- 5
Iron's oxidation state decreased from +3 to 0.
This is reduction (gain of electrons).
- 6
The reducing agent is the substance that gets oxidised.
Therefore, aluminium (Al) is the reducing agent.
- 7
The oxidising agent is the substance that gets reduced.
Therefore, iron(III) oxide (Fe2O3) is the oxidising agent.
Answer: Oxidising agent: Fe2O3; Reducing agent: Al
Common mistakes
- ×Confusing the agent with the process. Remember the name describes what it does to the *other* substance. The oxidising agent gets reduced; the reducing agent gets oxidised.
- ×Identifying a product as an agent. The oxidising and reducing agents are always reactants.
- ×Forgetting that the entire compound can be the agent, not just the single atom that changes oxidation state (e.g., Fe2O3 is the agent, not just Fe).
No-calculator tips
- ✓Use the mnemonic OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons). The reducing agent loses electrons (is oxidised), and the oxidising agent gains them (is reduced).
- ✓Quickly scan the reactants for elements in their elemental form (like Mg, Cl2, O2). They almost always undergo a change in oxidation state and are part of the redox process.
- ✓Look for common patterns: metals often act as reducing agents (they lose electrons to form positive ions), while non-metals like halogens or oxygen often act as oxidising agents.