Redox & electron transfer
Extended tier only
- Redox reactions can also be defined in terms of electron transfer
- Oxidation is a reaction in which an element, ion or compound loses electrons
- The oxidation number of the element is increased
- This can be shown in a half-equation, e.g. when silver reacts with chlorine, silver is oxidised to silver ions:
Ag → Ag+ + e-
- Reduction is a reaction in which an element, ion or compound gains electrons
- The oxidation number of the element is decreased
- This can be shown in a half-equation, e.g. when oxygen reacts with magnesium, oxygen is reduced to oxide ions:
O2 + 4e- → 2O2-
- 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 atom 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)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
- V3+ to V4+
- 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
Examiner Tip
Use the mnemonic OIL-RIG to remember oxidation and reduction in terms of the movement of electrons:
- Oxidation Is Loss
- Reduction Is Gain.
