Suppose the reaction, ${\mathbf{PCl}\ }_{5\ (s)}\rightarrow{\ \mathbf{PCl}}_{3\ (s)}+\ {\mathbf{Cl}}_{2\ (g)}$ in a closed vessel at equilibrium stage. What is the effect on the equilibrium concentration of Cl₂​ (g) by adding PCl₅​ at constant temperature?
A. Decreases
B. Increases
C. Unaffected
D. Cannot be described without the value of K_p

Hint: In chemical equilibrium the reaction tries to maintain and balance the concentration of the reactant and the product. The reaction will shift forward if the concentration of reactant increases and the reaction will shift backward if the concentration of products increases.

Complete Step by Step Answer:
Chemical Equilibrium is a state of a reaction when the rate of formation of product is equal to the rate of formation of reactant. In other words, the concentration of products and the concentration of reactants remain constant throughout the equilibrium state. If the concentration of any reactant or product increases in the reaction, the reaction tries to balance the equilibrium again. If the concentration of the reactants increases, the reaction will move forward and that will lead to the increase in the concentration of the products. And if the concentration of the products increases, the reaction will move backward and that will lead to the increase in the concentration of the reactants.

Now, in the given reaction:
${\mathbf{PCl}\ }_{5\ (s)}\rightarrow{\ \mathbf{PCl}}_{3\ (s)}+\ {\mathbf{Cl}}_{2\ (g)}$
If the PCl₅​ (s) is added or its concentration increases in the solution it will lead to an increase in the concentration of Cl₂​ (g) and vice versa.
Thus, Option (B) is correct

Note: Chemical equilibrium is a state of a reversible reaction, it can only occur in a reversible reaction. This state is also called dynamic equilibrium if there is no change in the concentration of the reactants or the products. The Haber-Bosch process that is the most important industrial process used for the formation/preparation of Ammonia goes through the equilibrium state.