4.3.6Chemical Changes
Half Equations in Electrolysis — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 18 of 18
Section 1 of 7Principles (HT only)
Cathode: reduction
- Cathode: reduction Positive ions gain electrons at the negative electrode and become neutral atoms.
- Electrons appear on the left of the half equation.
Anode: oxidation
- Anode: oxidation Negative ions lose electrons at the positive electrode.
- Electrons appear on the right of the half equation.
Balancing
- Balancing Both the number of each type of atom and the total charge must be equal on the two sides.
Section 2 of 7Molten compounds (HT only)
| Electrolyte | Cathode (reduction) | Anode (oxidation) |
|---|---|---|
| Lead(II) bromide | Pb2+ + 2e− → Pb | 2Br− → Br2 + 2e− |
| Aluminium oxide | Al3+ + 3e− → Al | 2O2− → O2 + 4e− |
| Sodium chloride | Na+ + e− → Na | 2Cl− → Cl2 + 2e− |
Section 3 of 7Writing any half equation (HT only)
1
Identify the ion that is discharged at the electrode.
2
Write the ion on one side and the neutral product on the other, using the correct formula (diatomic gases are X2).
3
Balance the atoms; for gases you often need 2 ions to make 1 molecule.
4
Add electrons to the correct side to balance the charge.
5
Label the process as reduction (cathode) or oxidation (anode).
Section 4 of 7Aqueous solutions (HT only)
Possible discharges
Cathode, active metal
- Cathode, active metal If the metal is more reactive than hydrogen: 2H+ + 2e− → H2.
Cathode, unreactive metal
- Cathode, unreactive metal If the metal is less reactive than hydrogen: Cu2+ + 2e− → Cu.
Anode, halide present
- Anode, halide present 2Cl− → Cl2 + 2e−.
Anode, no halide
- Anode, no halide 4OH− → O2 + 2H2O + 4e−.
Section 5 of 7Checking the electrons
1Same in each cell
- Same in each cell The electrons lost at the anode equal those gained at the cathode.
2Scale the equations
- Scale the equations Multiply a half equation so electron numbers match when comparing amounts of product.
3Example
- Example For Al2O3, 4Al3+ + 12e− → 4Al matches 6O2− → 3O2 + 12e−.
4Conclusion
- Conclusion Reduction and oxidation are always balanced overall.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Half equation | An equation showing what happens at one electrode, including electrons. |
| Oxidation | Loss of electrons; occurs at the anode. |
| Reduction | Gain of electrons; occurs at the cathode. |
| Discharge | Conversion of an ion into an atom or molecule at an electrode. |
| Diatomic | Made of molecules containing two atoms, such as Cl2 and O2. |
Section 7 of 7Exam technique
1
Writing chlorine as Cl instead of Cl2, which leaves the half equation unbalanced.
2
Putting the electrons on the wrong side of the arrow.
3
Forgetting to balance charge2Br− → Br2 needs 2e− on the right.
4
Mixing up which electrode shows oxidation; negative ions are oxidised at the anode.
Last updated: