Answer is: PCl3 + Cl2 → PCl5.
Balanced chemical reaction: PCl₃ + Cl₂ → PCl₅.
Oxidation half reaction: P⁺³ → P⁺⁵ + 2e⁻.
Reduction half reaction: Cl₂ + 2e⁻ → 2Cl⁻.
In this chemical reaction phosphorus lost two electrons and change oxidatation number from +3 to +5; chlorine gain those two electrons and change oxidation number form 0 to -1.
In other chemical reactions, elements do not change oxidation numbers.
