A half-equation shows you what happens at one of the electrodes during electrolysis. Electrons are shown as e-. A half-equation is balanced by adding, or taking away, a number of electrons equal to the total number of charges on the ions in the equation.
Positive ions gain electrons at the negative electrode, so are reduced.
Negative ions or neutral atoms lose electrons at the positive electrode and are oxidised. For example, chlorine is produced during the electrolysis of sodium chloride solution:
2Cl- - 2e- → Cl2
This half-equation can be rewritten as 2Cl- → Cl2 + 2e-
The simulation below shows what happens during the purification of copper by electrolysis.
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