For both IB Chemistry SL and HL, identify which species gains and loses electrons. The species that gains electrons is the oxidizing agent, while the species that loses electrons is the reducing agent.
The Reasoning
A redox reaction involves simultaneous oxidation and reduction. Oxidation is the loss of electrons and an increase in oxidation state; reduction is the gain of electrons and a decrease in oxidation state.
| Species | What happens to it? | Its role |
|---|---|---|
| Gains electrons | Is reduced; oxidation state decreases | Oxidizing agent |
| Loses electrons | Is oxidized; oxidation state increases | Reducing agent |
The names describe what each agent causes in the other species. An oxidizing agent oxidizes another species by accepting its electrons, while a reducing agent reduces another species by supplying electrons.
Use this three-step method:
- Assign oxidation states to relevant atoms on both sides of the equation.
- Identify which oxidation state increases and which decreases.
- Name the entire reactant species containing the changing atom as the agent.
For example:
Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)
The half-equations are:
Zn(s) → Zn²⁺(aq) + 2e⁻
Cu²⁺(aq) + 2e⁻ → Cu(s)
Zinc changes from oxidation state 0 to +2, so it loses electrons and is oxidized. Therefore, Zn(s) is the reducing agent. Copper changes from +2 to 0, so Cu²⁺(aq) gains electrons and is reduced. Therefore, Cu²⁺(aq) is the oxidizing agent.
A common misconception is that the oxidizing agent is oxidized. This is incorrect: the oxidizing agent causes oxidation but is itself reduced.
IB Exam Technique
For a deduce question, show the oxidation-state changes or half-equations before naming the agents. Always identify the complete reactant species, include charges and state symbols where given, and remember: oxidation occurs at the anode, while reduction occurs at the cathode.