Half-reaction
Definition and meaning of Half-reaction in chemistry.
Half-reaction is one of the two parts of a redox (oxidation–reduction) reaction, written separately to show either the loss of electrons (oxidation) or the gain of electrons (reduction).
In more detail
Splitting a redox reaction into half-reactions makes it easier to balance atoms, charge, and electron transfer separately before combining them. Each half-reaction is balanced for mass and charge on its own, adding H+, OH-, and H2O as needed in acidic or basic solution, then multiplied by whole numbers so the electrons lost equal the electrons gained. Adding the two half-reactions and canceling the electrons gives the overall balanced redox equation. In electrochemical cells, the two half-reactions occur physically apart: oxidation at the anode and reduction at the cathode.
Key facts
| Field | General Chemistry |
|---|---|
| Represents | Oxidation (electron loss) or reduction (electron gain) |
| Balancing rule | Mass and charge balanced independently, then electrons canceled |
| Occurs at | Anode (oxidation) and cathode (reduction) in electrochemical cells |
For zinc metal reacting with copper(II) ions, the oxidation half-reaction is Zn(s) → Zn2+(aq) + 2e-, and the reduction half-reaction is Cu2+(aq) + 2e- → Cu(s); adding them and canceling electrons gives Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s).
Frequently asked questions
Why bother splitting a redox reaction into half-reactions?
It isolates electron loss from electron gain, which makes balancing complex redox equations, especially in acidic or basic solution, far more systematic than balancing the whole reaction at once.
How are two half-reactions combined into one overall equation?
Each half-reaction is multiplied by a whole number so the number of electrons released equals the number absorbed; the half-reactions are then added, and the electrons (and any other common species) cancel out.