Q.What happens when $PCl_5$ is heated ?

Phosphorus pentachloride is thermally unstable and dissociates reversibly on heating into phosphorus trichloride and chlorine gas, an equilibrium that shifts further towards dissociation at higher temperature.

When $PCl_5$ (a solid that exists as an ionic $[PCl_4]^+[PCl_6]^-$ lattice, but as discrete covalent trigonal-bipyramidal molecules in the vapour phase) is heated, it undergoes reversible thermal decomposition: $PCl_5 \rightleftharpoons PCl_3 + Cl_2$ This is an equilibrium reaction: increasing the temperature favours the forward (dissociation) direction (since bond-breaking is endothermic), while the re …