The Common Ion Effect

1. The Common Ion Effect By: Shannon Kirby Tyler Brown Jeffrey Hao(the emperor)

2. Big Ideas • SWBAT Learn about The Common Ion Effect. • SWBAT Work out Common Ion Effect icebox problems.

3. What is The Common Ion Effect? • The Common Ion effect views the effect of adding a strong electrolyte to a weak electrolyte solution, where both have an ion in common, from the perspective of Le Chatelier’s principle. • The common-ion effect: The extent of ionization of a weak electrolyte is decreased by adding to the solution a strong electrolyte that has an ion in common with the weak electrolyte.

4. The Common Ion Effect: explained • When a soluble salt, ex: A+C- is added to a solution of another salt (ex: A+B-) containing a common ion (A+), the dissociation of AB is suppressed. • AB A-+ B+ • AC A-+ C+ • By the addition of the salt, AC, the concentration of A+ increases. Therefore, according to Le Chatelier's principle, the equillibrium will shift to the left, thereby decreasing the concentration of A+ ions. • Of that, the degree of dissociation of AB will be reduced.

5. Potassium nitrite (KNO2) is a soluble ionic compound; a strong electrolyte. It dissociates completely in an aqueous solution: • KNO2(aq) K+ (aq) + NO2-(aq) • In contrast, HNO2 is a weak electrolyte that ionizes like so: • HNO2(aq) H+ (aq) + NO2-(aq) • The addition of NO2 from KNO2 causes this equilibrium to shift to the left, thereby decreasing the equilibrium concentration of H+(aq) • HNO2(aq) H+ (aq) + NO2-(aq) • This decrease in the dissociation of the weak acid HNO2 is the common-ion effect.

6. When a strong electrolyte supplies the common ion NO2-, the equilibrium shifts to form more HNO2(aq). • HNO2(aq)  H+ (aq) + NO2-(aq) Added NO2- Equilibrium shifts to form less NO2(aq)

7. The ionization of a weak base is also decreased by the addition of a common ion. For example, the addition of NH4+ (from the strong electrolyte NH4Cl) causes the base-dissociation equilibrium of NH3 to shift to the left, decreasing the equilibrium concentration of OH- and lowering the pH: • NH3(aq) + H2O(l) NH4+(aq)+OH-(aq) • addition of NH4+ shifts equilibrium, reducing [OH-]

8. Important Info • When, in a problem, the strong electrolyte is dissolved in a solution that is already a certain molarity solution of weak electrolyte, the source of this molarity is unimportant. Let’s just disregard math and assume that the molarity of the solution came a random outside source. 

9. Example • Calculate the pH of a solution that is 0.10M NH4Cl and .25M NH3(aq)

10. Answer • pH = 9.65

11. Practice example • Calculate the fluoride ion concentration and pH of a solution that is 0.20M in HF and 0.10M in HCl

12. Answer • Fluoride ion concentration: 1.4x10-3 M • pH = 1.00

13. Practice exercises • Calculate the pH of a solution containing 0.085M nitrous acid(HNO2; Ka = 4.5 x 10-4) and 0.10M potassium nitrite(KNO2) • Calculate the formate ion concentration and pH of a solution that is 0.050M in formic acid(HCHO2; Ka=1.8x10-4)and 0.10M in HNO3.

14. Answers • #1: pH = 3.42 • #2: [CHO2-] = 9.0x10-5; pH = 1.00

15. The End! 