Oxidising & Reducing Agents
Step 1: Assign Oxidation Numbers. Use standard rules (e.g., Oxygen is usually , Hydrogen is , pure elements are ) to determine the oxidation state of every atom in the reactants and products.
Step 2: Identify Changes. Compare the oxidation states of the same element across the reaction arrow. Look for which element increased in value and which decreased.
Step 3: Apply Definitions. The reactant containing the element that increased in oxidation state is the reducing agent. The reactant containing the element that decreased in oxidation state is the oxidising agent.
Step 4: Verify Electron Balance. Ensure that the total number of electrons lost by the reducing agent equals the total number of electrons gained by the oxidising agent.
| Feature | Oxidising Agent | Reducing Agent |
|---|---|---|
| Electron Action | Gains electrons | Loses electrons |
| Oxidation State | Decreases (becomes more negative) | Increases (becomes more positive) |
| Internal Process | Undergoes Reduction | Undergoes Oxidation |
| Typical Elements | Non-metals (e.g., , ) | Metals (e.g., , ) |
Identify the Reactants: Always select the agent from the left side of the equation. A common mistake is naming a product as the agent.
Look for Spectator Ions: In ionic equations, ignore ions that do not change oxidation state (like or in many aqueous reactions) to focus on the species actually transferring electrons.
Check for 'Disproportionation': Be aware of rare cases where the same substance acts as both the oxidising and reducing agent, with one part of it being oxidised and another part reduced.
Sanity Check: If you identify two oxidising agents and no reducing agent, re-calculate your oxidation numbers; every redox reaction must have exactly one of each (or one substance doing both).
The 'Agent' Confusion: Students often think an 'oxidising agent' is the one being oxidised. Remember: an agent causes a change in others. A travel agent doesn't go on vacation; they make the vacation happen for you.
Oxygen Dependency: Do not assume oxygen must be present for a substance to be an oxidising agent. While oxygen is a common oxidant, many reactions (like those involving halogens) are redox reactions without any oxygen involved.
Hydrogen as a Marker: While loss of hydrogen is often oxidation in organic chemistry, always verify with oxidation numbers to ensure accuracy in inorganic contexts.