Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

3.4.3Chemical Changes

Redox

5 min read 5 parts Lesson 17 of 24 Unit quiz

0%read

Section 1 of 5Oxidation and Reduction

3.4.3 Redox

Oxidation and reduction are defined by electron transfer, and they always occur together in a redox reaction.

Oxidation
  • Loss of electrons by a substance; in terms of oxygen, oxidation means gaining oxygen.
Reduction
  • Gain of electrons by a substance; in terms of oxygen, reduction means losing oxygen.
Redox
  • A reaction in which oxidation and reduction take place at the same time, so electrons move between species.
Mnemonic
  • OIL RIGoxidation is loss of electrons and reduction is gain of electrons.

Section 2 of 5Electrons at the Electrodes

Each electrode reaction is a half of the overall redox change.

1
At the anode, negative ions lose electrons, so they are oxidised, as when bromide ions form bromine.
2
At the cathode, positive ions gain electrons, so they are reduced, as when lead ions form lead metal.
3
The negative terminal of the power supply pushes electrons to the cathode, and electrons are drawn away from the anode towards the positive terminal.
4
Half-equations must balance both the atoms and the charges, so the number of electrons gained or lost must be correct.

Section 3 of 5Identifying Redox Changes

Identifying Redox Changes
Iron and copper
  • Fe + Cu²⁺ → Fe²⁺ + Cuiron is oxidised and copper ions are reduced.
Zinc chloride
  • Zn²⁺ gains two electrons to become zinc atoms, so it is reduced at the cathode.
Chloride ions
  • 2Cl⁻ → Cl₂ + 2e⁻: chloride ions lose electrons, so they are oxidised at the anode.
Checking
  • Identify which species gains electrons and which loses them, then confirm the two changes balance.

Section 4 of 5Worked Example

Question

  • Complete the half-equations for the electrolysis of molten zinc chloride: cathode, Zn²⁺ + ... → Zn;
    • anode, ...
  • Cl⁻ → Cl₂ + ...

  1. CathodeZn²⁺ needs two electrons to become a zinc atom, so Zn²⁺ + 2e⁻ → Zn (reduction).
  2. Anodetwo Cl⁻ ions are needed to form one Cl₂ molecule, so 2Cl⁻ → Cl₂ + ....
  3. Each chloride ion loses one electron, so two electrons are lost in total: 2Cl⁻ → Cl₂ + 2e⁻ (oxidation).

Answer

  • Cathode: Zn²⁺ + 2e⁻ → Zn;
    • anode: 2Cl⁻ → Cl₂ + 2e⁻.

Section 5 of 5Exam Checklist & Tips

3.4.3 – Key Points for the Exam

  • Oxidation
    • Loss of electrons;
      • or, in terms of oxygen, gain of oxygen.
  • Reduction
    • Gain of electrons;
      • or, in terms of oxygen, loss of oxygen.
  • Redox reaction A reaction in which oxidation and reduction take place at the same time.
  • Half-equation An equation showing only the electrons gained or lost by one species.

Common Mistakes

  • Defining oxidation as gain of electrons; oxidation is loss of electrons.
  • Stating that oxidation occurs at the cathode; reduction occurs at the cathode and oxidation at the anode.
  • Balancing only the atoms in a half-equation and forgetting to balance the charges.
  • Believing that electrons flow through the electrolyte; the current in the electrolyte is carried by ions.
\n