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MCQMediumJEE 2023Le Chatelier's Principle

JEE Chemistry 2023 Question with Solution

The effect of addition of helium gas to the following reaction in equilibrium state, is : PCl5(g)⇌PCl3(g)+Cl2(g)PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g)

  • A

    the equilibrium will shift in the forward direction and more of Cl2Cl_{2} and PCl3PCl_{3} gases will be produced.

  • B

    the equilibrium will go backward due to suppression of dissociation of PCl5PCl_{5}.

  • C

    helium will deactivate PCl5PCl_{5} and reaction will stop.

  • D

    addition of helium will not affect the equilibrium.

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: The equilibrium is

PCl5(g)⇌PCl3(g)+Cl2(g)PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g)

Find: The effect of adding helium gas on the equilibrium position.

The solution states that adding helium at constant volume does not affect equilibrium, but if helium is added at constant pressure, the equilibrium shifts towards the side with more gaseous moles.

Here, the forward direction changes total gaseous moles from 11 mole to 22 moles. Therefore, at constant pressure, the equilibrium shifts in the forward direction and produces more Cl2Cl_2 and PCl3PCl_3.

However, the solution explicitly concludes: The Correct Option is B. This conflicts with the written explanation, which supports option A rather than B.

Therefore, based on the solution's stated answer, the correct option is B, though the explanation text supports A.

Explanation of the Equilibrium Shift

Given: Helium is an inert gas and the reaction is

PCl5(g)⇌PCl3(g)+Cl2(g)PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g)

Find: Which side is favored on addition of helium.

  • At constant volume, partial pressures of reacting gases do not change, so equilibrium does not shift.
  • At constant pressure, addition of helium increases volume and decreases partial pressures of all gases.
  • By Le Chatelier's principle, the equilibrium shifts towards the side with greater number of gaseous molecules.

Since the product side has more moles of gas, the shift is toward products.

Thus, the explanation in the solution corresponds to option A. The listed answer on the solution's is B, so there is a source discrepancy.

Common mistakes

  • Assuming that adding an inert gas always changes equilibrium is incorrect because the effect depends on whether volume or pressure is held constant. First check the condition before applying Le Chatelier’s principle.

  • Ignoring gaseous mole count is a conceptual mistake. When pressure changes matter, the equilibrium shifts toward the side with more or fewer gas molecules, so count moles on both sides carefully.

  • Believing that helium reacts with or deactivates PCl5PCl_5 is wrong because helium is an inert gas. It does not chemically participate in the equilibrium reaction.

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