1. Identify the Oxidation and Reduction Half-Reactions:

* Oxidation: P₄ → POCl₃ (Phosphorus is oxidized from 0 to +5)

* Reduction: Cl₂ → HCl (Chlorine is reduced from 0 to -1)

2. Balance the Atoms (Except H and O):

* Oxidation: P₄ → 4POCl₃

* Reduction: Cl₂ → 2HCl

3. Balance Oxygen by Adding H₂O:

* Oxidation: P₄ + 12H₂O → 4POCl₃

* Reduction: Cl₂ → 2HCl

4. Balance Hydrogen by Adding H⁺:

* Oxidation: P₄ + 12H₂O → 4POCl₃ + 24H⁺

* Reduction: Cl₂ + 2H⁺ → 2HCl

5. Balance Charge by Adding Electrons (e⁻):

* Oxidation: P₄ + 12H₂O → 4POCl₃ + 24H⁺ + 20e⁻

* Reduction: Cl₂ + 2H⁺ + 2e⁻ → 2HCl

6. Make the Number of Electrons in Both Half-Reactions Equal:

* Multiply the reduction half-reaction by 10:

10Cl₂ + 20H⁺ + 20e⁻ → 20HCl

7. Combine the Balanced Half-Reactions:

P₄ + 12H₂O + 10Cl₂ → 4POCl₃ + 20HCl

8. Final Check:

* Make sure the number of atoms of each element is the same on both sides of the equation.

* Make sure the total charge is the same on both sides (in this case, it's 0).

Balanced Redox Reaction:

P₄ + 12H₂O + 10Cl₂ → 4POCl₃ + 20HCl