Identify the shift in equilibruim and the effect on the pH of the system when a small quantity of hydrochloric acid is added at equilibrium
When a small quantity of hydrochloric acid (HCl) is added at equilibrium, the equilibrium will shift to the left in the direction of the reactants. This is because HCl is a strong acid, and it will react with one of the products in the equilibrium reaction.
This reaction can be represented by the equation:
HCl + H2O ⇌ H3O+ + Cl-
As HCl is added, it will react with water (H2O) to form hydronium ions (H3O+) and chloride ions (Cl-). This reaction consumes water and increases the concentration of H3O+ ions in the solution.
Since an increase in the concentration of H3O+ ions corresponds to a decrease in pH, the addition of HCl at equilibrium will lower the pH of the system. Thus, the pH of the solution will decrease.