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PCl5 is prepared by the chlorination of PCl3. 

PCl3 + Cl2 -> PCl5 (?H = ?124 kJ/mol). This is an exothermic reaction.So phosphorus pentachloride decomposes to phosphorus trichloride at high temperatures (when heat is absorbed). 

PCl5(g) -> PCl3(g) + Cl2(g).