10_LectureWeb

# 10_LectureWeb - Learning Objectives You should 1 understand...

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Learning Objectives 2/7/08 You should: 1. understand what autoprotolysis is and when it occurs. 2. be able to predict the pH of a solution containing an intermediate salt of a polyprotic acid. 3. understand how the common ion effect impacts acid and base ionization equilibria. 4. know that the pH of a solution containing equal amounts of a weak acid and its conjugate base is equal to p K a . 5. know what a buffer solution is and how it maintains pH over a narrow range. 6. be able to apply the Henderson-Hasselbalch equation. 7. know how to prepare a buffer solution with a desired pH. 8. be able to breakdown an acid-base neutralization reaction into a sum of acid ionization, base ionization, and water autoionization reactions in order to find the equilibrium con - stant for the neutralization reaction. Salts of Polyprotic Acids Salts of polyprotic acids can be either acidic (ex. HSO 4 - ) or basic (ex. HCO 3 - ). Consider H 2 PO 4 - At equilibrium the species present in solution will be H 3 PO 4 , H 2 PO 4 - , HPO 4 2 - , PO 4 3 - , H 2 O, H 3 O + , OH - with the relative quantities depending on the relevant acid and base ionization constants. What are the relative strengths of the predominant acidic and basic species? Species such as H 2 PO 4 - and HPO 4 2 - that can behave either as acids or bases are said to be amphiprotic / amphoteric . 2 H 2 PO 4 - will react by with itself in a process called autoprotolysis because H 2 PO 4 - is the strongest base present in a significant quantity. H 2 PO 4 - H aq L + H 2 PO 4 - H aq L F H 3 PO 4 H aq L + HPO 4 2 - H aq L The equilibrium constant for the autoprotolysis reaction can be determined by adding the following reactions with known equilibrium constants to get the autoprotolysis reaction and then combining the known equilibrium constants using the rules previously developed. H 2 PO 4 - H aq L + H 2 O H l L F H 3 O + H aq L + HPO 4 2 - H aq L K a 2 H acid ionization L H 2 PO 4 - H aq L + H 2 O H l L F H 3 PO 4 H aq L + OH - H aq L K b = K w ê K a 1 H hydrolysis L H 3 O + H aq L + OH - H aq L F 2H 2 O H l L 1 ê K w Addition of these three equations yields the autoprotolysis reaction, consequently K autoprotolysis = K a 2 i k j j K w ÅÅÅÅÅÅÅÅÅ K a 1 y { z z i k j j 1 ÅÅÅÅÅÅÅ K w O = K a 2 ÅÅÅÅÅÅÅÅÅ K a 1 = @ H 3 PO 4 DA HPO 4 2 - E ÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅ ÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅ @ H 2 PO 4 - D 2 But the concentrations of H 3 PO 4 and HPO 4 2 - formed by the autoprotolysis reaction are equal. K

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10_LectureWeb - Learning Objectives You should 1 understand...

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