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:

 

StepResult
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.

 

UnbalancedBalanced
Al3+ + e → AlAl3+ + 3e → Al
Cl2+ + e → CuCu2+ + 2e → Cu
H+ + e → H22H+ + 2e → H2
Br- → Br2 + e-2Br- → Br2 + 2e-
O2- → O2 + e-2O2 → O2 + 4e-

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