White phosphorus reacts with thionyl chloride to give :

Solution

Chemistry ma’am · 4 steps

Phosphorus is oxidised, so something must be reduced: here it is sulphur, which ends up in S₂Cl₂.

1. The reaction. Thionyl chloride chlorinates white phosphorus to phosphorus trichloride. The sulphur is split between SO₂ and S₂Cl₂.

   P₄ + 8SOCl₂ → 4PCl₃ + 4SO₂ + 2S₂Cl₂

2. Check the electrons. Each P goes from 0 to +3, so the 4 P atoms lose 4 × 3 = 12 electrons. In S₂Cl₂ sulphur is +1, so the 4 S atoms that go from +4 to +1 gain 4 × 3 = 12 electrons. The S in SO₂ stays at +4.
3. Why no chlorine gas. Chlorine is -1 in SOCl₂, PCl₃ and S₂Cl₂. Forming Cl₂ would oxidise chloride as well as phosphorus, and with the sulphur sitting at +4 in SO₂, nothing would be reduced. So Cl₂ cannot be a product here.
4. So. The products are PCl₃, SO₂ and S₂Cl₂, which is option (b).

The idea to keep. In any product list, find what is oxidised and what is reduced. Thionyl chloride, SOCl₂, gives PCl₃. Sulphuryl chloride, SO₂Cl₂, gives PCl₅. Keep the two apart.

Answer from the NTA final answer key. Solution by PrepWiser.