Question

The effect of addition of helium gas to the following reaction in equilibrium state, is : \(PCl _5( g ) \rightleftharpoons PCl _3( g )+ Cl _2( g )\)

• A. addition of helium will not affect the equilibrium.
• B. the equilibrium will shift in the forward direction and more of\(Cl _2\) and \(PCl _3\) gases will be produced.
• C. the equilibrium will go backward due to suppression of dissociation of \(PCl _5\)
• D. helium will deactivate \(PCl _5\) and reaction will stop.

Hint:

Adding an inert gas at constant volume doesn’t affect the equilibrium. At constant pressure, the equilibrium shifts towards the side with more gas molecules.

Answer 1

The Correct Option is B

To solve this question, we need to consider the effect of adding an inert gas, such as helium, to a system in equilibrium. The reaction provided is:

\(PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g)\)

Let us analyze the situation step-by-step:

1. The addition of helium, an inert gas, increases the total pressure of the system, but it does not participate in the reaction. Since helium is not reactive, it does not affect the equilibrium concentrations directly.
2. According to Le Chatelier's Principle, if an inert gas is added to a gaseous equilibrium system at constant volume, it does not affect the equilibrium position because the partial pressures of the reactants and products remain unchanged.
3. However, if the addition of helium leads to an increase in volume (assuming constant pressure), the partial pressures of all gases would decrease. This situation might cause the equilibrium to shift towards the side with more moles of gas to counteract the change.
4. In the reaction \(PCl_5 \rightleftharpoons PCl_3 + Cl_2\), there is an increase from 1 mole (of \(PCl_5\)) to 2 moles (sum of \(PCl_3\) and \(Cl_2\)) of gases. Thus, if volume increases and pressure decreases, the equilibrium shifts to the right, producing more \(Cl_2\) and \(PCl_3\).
5. Therefore, the equilibrium will shift in the forward direction.

Thus, the correct answer is that the equilibrium will shift in the forward direction, and more \(Cl_2\) and \(PCl_3\) gases will be produced. This aligns with the provided correct answer.

Let's rule out the other options:

• Addition of helium will not affect the equilibrium: This is true in terms of equilibrium constant, but in terms of shift, the change in total volume affects the equilibrium shift.
• The equilibrium will go backward due to suppression of dissociation of \(PCl_5\): The equilibrium shifts forward rather than backward.
• Helium will deactivate \(PCl_5\) and reaction will stop: This is incorrect as helium is inert and does not deactivate reactants.