Gas phase reaction PCl5(g)⇌PCl3(g)+Cl2(g) ∆H=∆E+∆ngRT ∆ng= Change in number of moles of product and reactant species. Since ∆ng=+ve, hence ∆H=+ve also one mole of PCl5 is dissociated into two moles of PCl3 and Cl2 in the same phase. Therefore, ∆S=Sproduct −Sreactant ∆S=+ ve.