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- |
Posted on 07:55 / 0
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