Oxidation and reduction half-reactions are a clear way to track how electrons move during a redox reaction. In every redox process, one substance loses electrons while another gains electrons. Separating the reaction into two half-reactions makes it easier to see what is oxidized, what is reduced, and how charge is conserved.
This skill matters in batteries, corrosion, electroplating, metabolism, and many industrial chemical processes.
The memory aid OIL RIG means Oxidation Is Loss and Reduction Is Gain of electrons. In a half-reaction, electrons are written as reactants when reduction occurs and as products when oxidation occurs. Balanced redox equations must conserve both atoms and total charge, so the number of electrons lost must equal the number of electrons gained.
For example, Zn + Cu2+ -> Zn2+ + Cu can be split into Zn -> Zn2+ + 2e- and Cu2+ + 2e- -> Cu.
Key Facts
- Oxidation is loss of electrons: X -> Xn+ + ne-.
- Reduction is gain of electrons: Yn+ + ne- -> Y.
- OIL RIG means Oxidation Is Loss, Reduction Is Gain.
- Electrons appear on the product side in oxidation half-reactions.
- Electrons appear on the reactant side in reduction half-reactions.
- In a balanced redox reaction, electrons lost = electrons gained.
Vocabulary
- Oxidation
- Oxidation is the loss of electrons by an atom, ion, or molecule.
- Reduction
- Reduction is the gain of electrons by an atom, ion, or molecule.
- Half-reaction
- A half-reaction shows only the oxidation part or only the reduction part of a redox reaction.
- Oxidizing agent
- An oxidizing agent causes another substance to be oxidized and is itself reduced.
- Reducing agent
- A reducing agent causes another substance to be reduced and is itself oxidized.
Common Mistakes to Avoid
- Putting electrons on the wrong side. Electrons are products in oxidation and reactants in reduction, so reversing them changes the meaning of the half-reaction.
- Forgetting to balance charge. A half-reaction can have the right atoms but still be wrong if the total charge is not the same on both sides.
- Calling the electron gainer the reducing agent. The species that gains electrons is reduced, but it acts as the oxidizing agent because it oxidizes the other species.
- Adding half-reactions before electron numbers match. The electrons must cancel completely, so multiply one or both half-reactions before combining them.
Practice Questions
- 1 Write the oxidation and reduction half-reactions for Zn + Cu2+ -> Zn2+ + Cu, and identify the oxidizing agent and reducing agent.
- 2 Balance the electron transfer between Al -> Al3+ + 3e- and Ag+ + e- -> Ag. How many Ag+ ions are reduced for every 1 Al atom oxidized?
- 3 A student says that Fe3+ -> Fe2+ is oxidation because the charge becomes less positive. Explain why this is incorrect using electron transfer.