Half-equations - Higher tier

A half-equation shows you what happens at one of the electrodes during electrolysis. You should be able to complete and balance half-equations for the reactions that happen during electrolysis.

Writing a half-equation

Electrons are shown as e^- in half-equations. 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.

For example, here is how you would write the balanced half-equation for chloride ions forming chlorine gas:

 

Step	Result
1. Write the formulae of the reactant and product.	Cl- → Cl2
2. Adjust the number of ions, if needed.	2Cl- → Cl2
3. Count the number of charges. Add enough electrons so that both sides have the same total number of charges.	2Cl- → Cl2 2e-

You may also see this half-equation written as: 2Cl^- – 2e^– → Cl₂

The table shows some examples of half-equations before and after balancing.

 

Unbalanced	Balanced
Al3+ + e– → Al	Al3+ + 3e– → Al
Cl2+ + e– → Cu	Cu2+ + 2e– → Cu
H+ + e– → H2	2H+ + 2e– → H2
Br- → Br2 + e-	2Br- → Br2 + 2e-
O2- → O2 + e-	2O2 → O2 + 4e-