The equilibrium will remain unaffected in all the three cases.
Solution
Noble gas or inert gas does not react itself due to the extremely low reactivity of noble gas with other substances.
As \(\mathrm{Ar}\) is a noble gas, it is also unreactive but it decreases the activation energy in both forward as well as backward directions by the same amount of reversible reaction.
When Ar is added at constant volume, the equilibrium will not change as there is no change in the no. of moles of the reaction (forward and backward) of either reactant or products.
\(\Rightarrow \mathrm{H}_2(\mathrm{g})+\mathrm{I}_2(\mathrm{g}) ightleftharpoons 2 \mathrm{HI}(\mathrm{g})\)
$\cdot$ No. of moles for forward reaction \(=2\) for reactant and 2 for product
$\cdot$ No. of moles for backward reaction \(=2\) for reactant and 2 for product
\(\Rightarrow \mathrm{PCl}_5(\mathrm{g}) ightleftharpoons \mathrm{PCl}_3(\mathrm{g})+\mathrm{Cl}_2(\mathrm{g})\)
$\cdot$ No. of moles for forward reaction \(=2\) for reactant and 1 for product
$\cdot$ No. of moles for backward reaction \(=1\) for reactant and 2 for product
\(\Rightarrow \mathrm{N}_2(\mathrm{g})+3 \mathrm{H}_2(\mathrm{g}) ightleftharpoons 2 \mathrm{NH}_3(\mathrm{g})\)
$\cdot$ No. of moles for forward reaction \(=4\) for reactant and 2 for product
$\cdot$ No. of moles for backward reaction \(=2\) for reactant and 4 for product
Hence, in all cases equilibrium remains unaffected.
Conclusion:
Therefore, the equilibrium will remain unaffected in all three cases.
Hence, the correct answer is option 4.