I think it's 1. PCl5= PCl3 + Cl2. I can't tell the exact reason but it's something to do with Atomic Redox Reaction
Answer:
When a substance is cooled, its internal energy decreases: the movement of its particles decreases. bonds between particles form when a substance condenses or freezes, or sublimes to form a solid from a gas. What is called energy? Energy is the capacity of a physical system to do work.
Explanation: