Acetic acid, CH₃COOH, has the peculiar property of being a retarding agent for concrete at low concentrations and an accelerator at high concentrations. A solution of acetic acid in equilibrium with hydrogen ions and acetate ions, CH₃COO^–, can be disturbed in several ways. Predict the change in the reaction quotient, Q, when,

(a) Sodium acetate is added to the system, 

(b) Additional acetic acid is added to the system, and

(c) Sodium hydroxide is added to the system. Then use those predictions to explain how the equilibrium shifts in response to each stress.

Strategy In each case, determine what change the added substance induces in the equilibrium concentrations. By noting whether the concentration change affects the numerator or denominator in the equilibrium expression, we can then judge whether the resulting reaction quotient is larger than or smaller than k. Comparing the expected Q with K, we can predict the direction that equilibrium will move to offset the stress.