Redox Reactions

Full Notes

Oxidation and Reduction

Oxidation and reduction can be defined in two ways:

• Electron transfer:
  • Oxidation = loss of electrons
  • Reduction = gain of electrons
  • Mnemonic: OIL RIG
• Change in oxidation number:
  • Oxidation = oxidation number becomes more positive
  • Reduction = oxidation number becomes more negative

These principles apply across s-, p- and d-block elements.

Example Zinc and copper(II) ions

Zn + Cu²⁺ → Zn²⁺ + Cu
Zn is oxidised: 0 → +2
Cu is reduced: +2 → 0

Redox Reactions of Metals with Acids

Metals often react with dilute acids to form a salt and hydrogen gas:

For Example:

• Mg + 2HCl → MgCl₂ + H₂
• Zn + H₂SO₄ → ZnSO₄ + H₂
• Fe + 2HCl → FeCl₂ + H₂

These are redox reactions where:

• Metal is oxidised (loses electrons)
• H⁺ is reduced (gains electrons)

Only reactions that release H₂ (i.e. H⁺ reduced to H₂ gas) are included in this section. Reactions involving HNO₃ or H₂SO₄ as oxidising agents are covered under electrode potentials later in the course (here).

Interpreting Redox Equations

Redox equations can be interpreted by tracking oxidation numbers:

Example Aluminium and copper(II) ions

2Al + 3Cu²⁺ → 2Al³⁺ + 3Cu
Al goes from 0 → +3 (oxidised)
Cu²⁺ goes from +2 → 0 (reduced)

You can use this information to quickly identify which species are oxidised/reduced and predict products.

Summary

• Oxidation = loss of electrons or increase in oxidation number.
• Reduction = gain of electrons or decrease in oxidation number.
• Metals reacting with acids form salts and hydrogen via redox.
• Track oxidation numbers to interpret and predict redox reactions.