Q.What happens when PCl5 is heated?

Step 1. Write the reaction.

PCl5(g) <=> PCl3(g) + Cl2(g).

Step 2. Describe the nature of the change.

This is a reversible thermal dissociation (decomposition) reaction, not a simple one-way breakdown: on heating, PCl5 dissociates into PCl3 and chlorine gas, but on cooling (or under increased pressure), the reverse combination reaction (PCl3 + Cl2 -> PCl5) can occur, so the system reaches a dynamic equilibrium.

Step 3. Note the effect of conditions. …

Write the thermal dissociation equilibrium for PCl5 and note that it is reversible, with the position of equilibrium shifting wit …

Describing the reaction as a simple, irreversible one-way decomposition rather than recognising it as a true dissociation equilibrium (PCl5 …