Oxidation & Reduction Using Electrons
- As well as understanding oxidation and reduction in terms of oxygen, you need to understand these reactions in terms of electrons
- When a substance loses electrons it is oxidised
- When a substance gains electrons it is reduced
- If this occurs in the same reaction, the reaction is a redox reaction
- For example, when iron reacts with a compound of copper such as copper sulfate a displacement reaction occurs
iron + copper sulfate → iron(II) sulfate + copper
Fe + CuSO4 → FeSO4 + Cu
- We can write this as an ionic equation
Fe + Cu2+ + SO42– → Fe2+ + SO42– + Cu
- We can then remove the spectator ions to see the overall change
Fe + Cu2+→ Fe2+ + Cu
- The iron has lost electrons to become a positive ion, so has been oxidised
- The positive copper ion has gained electrons to become an atom, so have been reduced
The redox reaction between Fe and Cu2+
The Fe atom is oxidised (loses electrons) and the Cu2+ ion is reduced (gains electrons)
Electrolysis
- Oxidation and reduction take place during the process of electrolysis at the anode (positive electrode) and the cathode (negative electrode)
- Positive ions are attracted towards the cathode
- Reduction (gain of electrons) takes place here
- E.g. Pb2+ + 2e– → Pb
- Negative ions are attracted towards the anode
- Oxidation (loss of electrons) takes place here
- E.g. 2Br– → Br2 + 2e–
Examiner Tip
Remember: OIL RIG - Oxidation Is Loss, Reduction Is Gain of electrons
You can also remember which electrodes oxidation takes place by remembering: RED CAT (reduction at the cathode) and AN OX (oxidation at the anode)
Worked example
Which change in the following equation is oxidation?
V3+ + Fe3+ → V4+ + Fe2+
Answer:
Step 1
Identify the changes for each species
- V3+ to V4+
- V3+ has lost 1 electron
- Fe3+ to Fe2+
- Fe3+ has gained 1 electron
Step 2
Identify each change as either oxidation and reduction
- V3+ to V4+ is oxidation
- Fe3+ to Fe2+ is reduction
Therefore V3+ has been oxidised
