Half-reactions and balancing redox: write oxidation and reduction half-reactions showing electron transfer, and balance them so that electrons lost equal electrons gained.

What this topic is asking

The Core Curriculum asks you to write half-reactions, the separate oxidation and reduction steps of a redox reaction, with the electrons shown explicitly, and to balance them so that electrons lost equal electrons gained. The Regents tests writing and balancing half-reactions in Part B-2, building on the oxidation-number page.

What a half-reaction shows

Splitting a redox reaction into half-reactions makes the electron transfer explicit and easy to check. For zinc reacting with copper ions, the oxidation half-reaction is $\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^-$ and the reduction half-reaction is $\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}$. Whether electrons are on the left or right tells you immediately which half is oxidation and which is reduction.

Balancing a half-reaction

The number of electrons needed equals the change in oxidation number times the number of atoms changing. Magnesium going from $0$ to $+2$ loses two electrons per atom; chlorine going from $0$ to $-1$ gains one electron per atom, so $\text{Cl}_2$ gains two.

Equalizing the electrons

This is the key step in balancing a full redox reaction by the half-reaction method. For example, if one half-reaction releases two electrons and the other accepts three, multiply the first by three and the second by two so both involve six electrons.

Try this

Q1. Write the oxidation half-reaction for aluminum forming $\text{Al}^{3+}$. [1 point]

• Cue. $\text{Al} \rightarrow \text{Al}^{3+} + 3e^-$.

Q2. State where the electrons appear in a reduction half-reaction. [1 point]

• Cue. On the reactant (left) side, because they are gained.