The Reactivity Series

In terms of oxygen

• Oxidation is the gain of oxygen by a substance
• Reduction is the loss of oxygen from a substance
• The two always happen together in the same reaction — a redox reaction
• In Zn + CuO → ZnO + Cu:
  • Zinc gains oxygen → zinc is oxidised → zinc is the reducing agent (it caused the reduction of CuO)
  • Copper(II) oxide loses oxygen → it is reduced → CuO is the oxidising agent

In terms of electrons

• The more general definitions use electron transfer:
  • Oxidation is loss of electrons
  • Reduction is gain of electrons
• Mnemonic: OIL RIG (Oxidation Is Loss, Reduction Is Gain)
• For the displacement reaction Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s):
  • Mg → Mg²⁺ + 2 e⁻ (oxidation — Mg loses electrons)
  • Cu²⁺ + 2 e⁻ → Cu (reduction — Cu²⁺ gains electrons)
• Mg is the reducing agent (gives away electrons); Cu²⁺ is the oxidising agent (accepts them)

Spectator ions and ionic equations

• In the full equation Mg(s) + CuSO₄(aq) → MgSO₄(aq) + Cu(s), the sulfate ion SO₄²⁻ appears on both sides unchanged
• An ion that is unchanged in a reaction is a spectator ion — it does not take part in the chemistry
• Leaving out the spectator ions gives the ionic equation, which highlights the actual redox change: Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s)

A practice prompt

• In the reaction Cu(s) + 2 AgNO₃(aq) → Cu(NO₃)₂(aq) + 2 Ag(s), Cu loses two electrons to form Cu²⁺ (oxidation) while each Ag⁺ gains one electron to form Ag (reduction)
• Copper is therefore the reducing agent here, and silver(I) is the oxidising agent