Final answer:
The addition of an inert gas such as argon at constant volume does not affect the equilibrium of the given reactions (H₂ + I₂ ⇌ 2HI, PCl₅ ⇌ PCl₃ + Cl₂, and N₂ + 3H₂ ⇌ 2NH₃) as there is no change in the partial pressures of the reactants or products.
Step-by-step explanation:
The question is about the effect of adding inert gas, specifically argon, to different chemical reactions at equilibrium at a constant volume. To understand which reaction's equilibrium remains unaffected, we must consider Le Chatelier's Principle, which states that if a dynamic equilibrium is disturbed by changing the conditions (like concentration, temperature, pressure), the position of equilibrium moves to counteract the change.
In reactions where the number of moles of gas does not change, like A) H₂(g) + I₂(g) ⇌ 2HI(g), adding an inert gas at constant volume does not change the pressure of the reacting gases, and thus, the equilibrium position remains unaffected. Similarly, in B) PCl₅(g) ⇌ PCl₃(g) + Cl₂(g), and C) N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the equilibrium is not influenced by the introduction of argon because the volume and thus the total pressure is constant.
Therefore, given the mechanisms of how equilibrium is affected, the equilibrium in all three reactions will remain unaffected upon adding a small amount of argon at constant volume. This result corroborates Le Chatelier's Principle regarding the addition of inert gases.