The effect of addition of helium gas to the following reaction in equilibrium state, is :
`PCl_5(g) iff PCl_3(g) + Cl_2(g)`

Options

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

Answer

To solve the question regarding the effect of adding helium gas to the equilibrium reaction:

\[ \text{PCl}_5(g) \iff \text{PCl}_3(g) + \text{Cl}_2(g) \]

we need to analyze the situation based on Le Chatelier's principle, which states that if an external change is applied to a system at equilibrium, the system will adjust to counteract that change and restore a new equilibrium.

### Step-by-Step Solution:

1. **Identify the Reaction and Its Equilibrium**:
The reaction involves the decomposition of phosphorus pentachloride (PCl₅) into phosphorus trichloride (PCl₃) and chlorine gas (Cl₂).

2. **Determine the Number of Moles of Gases**:
- On the left side (reactants), we have 1 mole of PCl₅.
- On the right side (products), we have 1 mole of PCl₃ and 1 mole of Cl₂, totaling 2 moles of gases.

3. **Understand the Effect of Inert Gas Addition**:
Helium is an inert gas and does not participate in the reaction. Its addition will affect the total pressure and volume of the system, but not the concentrations of the reactants or products directly.

4. **Consider the Two Scenarios**:
- **Constant Volume**: If helium is added at constant volume, the total pressure increases, but the partial pressures of PCl₅, PCl₃, and Cl₂ remain unchanged. Therefore, there is no shift in the equilibrium position.
- **Constant Pressure**: If helium is added at constant pressure, the volume of the system increases. This results in a decrease in the partial pressures of all gases involved. According to Le Chatelier’s principle, the system will shift towards the side with more moles of gas to counteract this change. Since the products side has more moles (2 moles) compared to the reactants side (1 mole), the equilibrium will shift to the right, favoring the formation of PCl₃ and Cl₂.

5. **Conclusion**:
- At constant volume, the equilibrium does not shift.
- At constant pressure, the equilibrium shifts to the right, producing more PCl₃ and Cl₂.

### Final Answer:
- If helium is added at constant volume, the equilibrium does not shift.
- If helium is added at constant pressure, the equilibrium shifts to the right.