Redox and ionic equations
Oxidation and reduction with oxygen
- is the gain of oxygen. Magnesium is oxidised when it burns: 2Mg + O₂ → 2MgO.
- is the loss of oxygen. Iron oxide is reduced when carbon removes its oxygen: 2Fe₂O₃ + 3C → 4Fe + 3CO₂.
In the second reaction, iron oxide is reduced and carbon is oxidised at the same time. A reaction where oxidation and reduction both happen is a redox reaction.
Key terms
OIL RIG (Higher tier only)
Oxidation and reduction can also be described in terms of :
Remember
- Oxidation Is Loss of electrons
- Reduction Is Gain of electrons
When a metal reacts, its atoms lose electrons to form positive ions, so the metal is . This explains why the most reactive metals are the ones that lose electrons most easily.
Key terms
Ionic equations (Higher tier only)
An shows only the particles that actually change. Ions that are there before and after without changing are , and they are left out.
Mg + CuSO₄ → MgSO₄ + Cu
The sulfate ions (SO₄²⁻) don't change, so they are spectators:
Mg + Cu²⁺ → Mg²⁺ + Cu
Zn + 2HCl → ZnCl₂ + H₂
The chloride ions are spectators:
Zn + 2H⁺ → Zn²⁺ + H₂
Check that both atoms and charges balance on each side.
Key terms
Half equations (Higher tier only)
A shows what happens to one substance, with electrons written as e⁻. An ionic equation splits into two halves:
Mg + Cu²⁺ → Mg²⁺ + Cu
- Mg → Mg²⁺ + 2e⁻ — magnesium loses electrons, so it is
- Cu²⁺ + 2e⁻ → Cu — copper ions gain electrons, so they are
Zn + 2H⁺ → Zn²⁺ + H₂
- Zn → Zn²⁺ + 2e⁻ (oxidation)
- 2H⁺ + 2e⁻ → H₂ (reduction)
The number of electrons lost must equal the number gained.