Class 11 NEET Chemistry Chemical Thermodynamics

For the reaction$\mathrm{PCl}_{5(\mathrm{g})} \rightleftharpoons \mathrm{PCl}_{3(\mathrm{g})} + \mathrm{Cl}_{2(\mathrm{g})}$, the decomposition of phosphorus pentachloride (PCl5) into phosphorus trichloride (PCl3) and chlorine gas $\mathrm{Cl}_2$ is an endothermic process, meaning it absorbs heat from the surroundings. Therefore, the change in enthalpy $\Delta H$ is positive. Additionally, the reaction results in an increase in the number of gas molecules (from 1 mole of PCl5 to 2 moles of products), leading to an increase in entropy ( $\Delta S$ ). Therefore,$\Delta H > 0$and$\Delta S > 0$ Step 1Identify the type of reaction: The reaction is a decomposition reaction where one mole of PCl5 decomposes into one mole of PCl 3 and one mole of Cl 2 .Step 2Determine the change in enthalpy $\Delta H$ : Since the reaction absorbs heat, it is endothermic, so$\Delta H > 0$Step 3Determine the change in entropy $\Delta S$ : The number of gas molecules increases from 1 to 2 , leading to an increase in disorder, so$\Delta S > 0$Final Answer:(3)$\Delta H > 0$and$\Delta S > 0$