Final answer:
Adding HCl to a solution with equal acetic acid and sodium acetate concentrations disrupts equilibrium and triggers the common ion effect, forming more acetic acid by reacting with acetate ions, shifting equilibrium to the left and keeping the ionization constant unchanged.
Step-by-step explanation:
When hydrochloric acid (HCl) is added to a solution containing equal concentrations of acetic acid and sodium acetate, the reaction that occurs involves the common ion effect. The additional H+ ions from the HCl will react with the acetate ions (CH3COO−) from the sodium acetate to form acetic acid (CH3COOH). As a result of this addition, the equilibrium of the acetic acid ionization reaction is disturbed, causing the equilibrium to shift to the left, which decreases the concentration of the acetate ions and increases the concentration of acetic acid.
The chemical equation representing this reaction is:
H+(aq) + CH3COO− (aq) → CH3COOH (aq)
This is a classic illustration of Le Chatelier's principle, where the system counteracts the change imposed on it. Even though the concentration of H+ ions increases, the ionization constant (Ka) for acetic acid remains constant during the process because the addition of HCl changes only the equilibrium composition, not the inherent strength of the acetic acid.