Half equations are most often used in redox reactions and in electrolysis. They involve either atoms and molecule gaining or losing electrons to become ions, or ions gaining or losing electrons to become atoms and molecules.
Either way, the process for writing the equation is the same.
For example, you may be asked to write the half equation for the conversion of oxide ions into an oxygen molecule.
|1||Write down the reactant and the product.||O2- → O2|
|2||Balance the atoms.||2O2- → O2|
|3||Write the total charge underneath each species in the equation.|
|4||Balance the charge by adding electrons.||2O2- → O2 + 4e-|