American Chemical Society (ACS) Chemistry Test 2025 – 400 Free Practice Questions to Pass the Exam

In the reaction represented by the equilibrium PCl5 ⇌ PCl3 + Cl2, shifting the equilibrium to the right means favoring the production of products, PCl3 and Cl2, from the reactant PCl5. According to Le Chatelier's principle, changes in concentration can affect the position of equilibrium.

When Cl2 is removed from the equilibrium mixture, the reaction will respond by shifting to the right to replace the Cl2 that has been removed. This shift occurs because the equilibrium seeks to counteract the change that has been imposed upon it—in this case, the loss of Cl2.

The introduction of a catalyst, on the other hand, would not cause any shift in the equilibrium position; it only speeds up the attainment of equilibrium without favoring one side over the other. Adding PCl3 would increase the concentration of one of the products, which might push the equilibrium to the left, while adding Cl2 would increase the concentration of one of the products, potentially leading to a leftward shift as it increases reactants.