In equilibrium + – + H The equilibrium constant may change when — Chemical Equilibrium Chemistry Question

💡 Solution & Explanation

# Solution For the equilibrium: $CH_3COOH + H_2O \rightleftharpoons CH_3COO^- + H_3O^+$ The equilibrium constant $K_a$ is defined as: $$K_a = \frac{[CH_3COO^-][H_3O^+]}{[CH_3COOH]}$$ **When does $K_a$ change?** $K_a$ depends only on **temperature**. It is a thermodynamic constant related to the standard free energy change: $$\Delta G° = -RT \ln K_a$$ **Analysis of common disturbances:** - **Concentration changes** (adding more acid/base) → Shifts equilibrium position but $K_a$ remains constant - **Pressure/Volume changes** → No significant effect on $K_a$ for weak acid equilibria in solution - **Catalyst addition** → Speeds up equilibration but doesn't change $K_a$ - **Temperature change** → Changes $\Delta H°$ relationship, directly altering $K_a$ value Since option D must be the answer, it refers to **temperature change**. When temperature increases, the equilibrium constant increases for endothermic reactions (like weak acid dissociation, which is endothermic). When temperature decreases, $K_a$ decreases. **Answer: $K_a$ changes only when temperature is changed.**