8.4 Relative strengths of acids and bases  (Page 12/18)

1. Assume that the change in initial concentration of the acid as the equilibrium is established can be neglected, so this concentration can be assumed constant and equal to the initial value of the total acid concentration. 2. Assume we can neglect the contribution of water to the equilibrium concentration of H ₃ O ⁺ .

(b) The addition of HCl

(a) Adding HCl will add H ₃ O ⁺ ions, which will then react with the OH ⁻ ions, lowering their concentration. The equilibrium will shift to the right, increasing the concentration of HNO ₂ , and decreasing the concentration of ${\text{NO}}_2{}^{\text{−}}$ ions. (b) Adding HNO ₂ increases the concentration of HNO ₂ and shifts the equilibrium to the left, increasing the concentration of ${\text{NO}}_2{}^{\text{−}}$ ions and decreasing the concentration of OH ⁻ ions. (c) Adding NaOH adds OH ⁻ ions, which shifts the equilibrium to the left, increasing the concentration of ${\text{NO}}_2{}^{\text{−}}$ ions and decreasing the concentrations of HNO ₂ . (d) Adding NaCl has no effect on the concentrations of the ions. (e) Adding KNO ₂ adds ${\text{NO}}_2{}^{\text{−}}$ ions and shifts the equilibrium to the right, increasing the HNO ₂ and OH ⁻ ion concentrations.

This is a case in which the solution contains a mixture of acids of different ionization strengths. In solution, the HCO ₂ H exists primarily as HCO ₂ H molecules because the ionization of the weak acid is suppressed by the strong acid. Therefore, the HCO ₂ H contributes a negligible amount of hydronium ions to the solution. The stronger acid, HCl, is the dominant producer of hydronium ions because it is completely ionized. In such a solution, the stronger acid determines the concentration of hydronium ions, and the ionization of the weaker acid is fixed by the [H ₃ O ⁺ ] produced by the stronger acid.