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Chemistry Question 55 – JEE-MAIN 2025

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In the following system. PCl5(g)⇌PCl3(g)+Cl2(g) at equilibrium, upon addition of xenon gas at constant T & p, the concentration of

Understand how adding an inert gas affects equilibrium under different conditions (constant volume vs. constant pressure).

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Ninja Strategy•Le Chatelier's Principle Application

Recognize that adding an inert gas at constant pressure shifts the equilibrium towards the side with more gaseous moles, allowing for quick elimination of options suggesting no change or a shift in the opposite direction.

Step 1: Analyze the effect of inert gas addition at constant T and P✦ Active

When an inert gas (like xenon) is added to a system at equilibrium at constant temperature (T) and constant pressure (P), the total volume of the system must increase to accommodate the added gas while maintaining constant pressure. This increase in total volume leads to a decrease in the partial pressures (and thus concentrations) of all the reacting gases.

Step 2: Apply Le Chatelier's Principle○ Expand

According to Le Chatelier's Principle, the system will shift in a direction that counteracts the change. Since the effective partial pressures of the reacting gases have decreased (due to increased volume), the equilibrium will shift towards the side with a greater number of moles of gas to increase the total number of gaseous molecules and thus partially compensate for the decrease in partial pressures.

Step 3: Determine the shift and concentration changes○ Expand

Consider the given reaction: PCl5(g)⇌PCl3(g)+Cl2(g). There is 1 mole of gas on the reactant side and 1+1=2 moles of gas on the product side. Since the product side has more moles of gas, the equilibrium will shift to the right (forward direction) to produce more products. This means the concentration of PCl5 will decrease, and the concentrations of PCl3 and Cl2 will increase.

💡 Teacher's Secret Hint

Remember that adding an inert gas at constant volume has no effect on the equilibrium position.

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