Question 19.9: Predicting the Effect on Solubility of Adding Strong Acid Pr......
Predicting the Effect on Solubility of Adding Strong Acid
Problem Write balanced equations to explain whether addition of H_3O^+ from a strong acid affects the solubility of each ionic compound:
(a) Lead(II) bromide (b) Copper(II) hydroxide (c) Iron(II) sulfide
Plan We write the balanced dissolution equation and note the anion:
• Weak-acid anions react with H_3O^+ and increase solubility when a strong acid is added.
• Strong-acid anions do not react with H_3O^+, so addition of a strong acid has no effect.
Solution (a) PbBr_2(s) \xrightleftharpoons[]{} Pb^{2+}(aq) + 2Br^−(aq)
No effect. Br^− is the anion of HBr, a strong acid, so it does not react with H_3O^+.
(b) Cu(OH)_2(s) \xrightleftharpoons[]{} Cu^2(aq) + 2OH^−(aq)
Increases solubility. OH^− is the anion of H_2O, a very weak acid, so it reacts with the added H_3O^+:
As OH^− is removed from solution by this reaction, the solubility equilibrium shifts to the right and additional Cu(OH)_2 dissolves.
(c) FeS(s) \xrightleftharpoons[]{} Fe^{2+}(aq) + S^{2−}(aq)
Increases solubility. S^{2−} is the anion of the weak acid HS^−, so it reacts with the added H_3O^+, removing it from solution; the solubility equilibrium shifts to the right and additional FeS dissolves:
S^{2−}(aq) + H_3O^+(aq) ⟶ HS^−(aq) + H_2O(l)