Chapter 17

# Chapter 17 - Chapter 17 Additional Aspects of Equilibria...

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Chapter 17 Additional Aspects of Equilibria

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Salt Solutions in Water Salt solutions completely ionize in H 2 O. If their ions can react with H 2 O to form H + of OH - , the pH of the solution will change.
Anions that are the conjugate bases of weak acids will react with H 2 O to form OH - ions. Anions that are conjugate bases of strong acids are very weak bases and aren’t strong enough to react with H 2 O. No pH change.

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Anions that have ionizable protons (HSO 4 - ) are amphoteric. Can act as an acid or a base and must be determined based on the K a and K b for the ion.

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Will K 2 HC 6 H 5 O 7 form an acidic or basic solution in H 2 O? - - 3 7 5 6 2 - 2 7 5 6 - 3 7 5 6 3 2 - 2 7 5 6 OH O H C O H O H HC O H C O H O H O H HC + + + + + or Must look at K a and K b . From table 16.3 in book get K a3 = 4.0x10 -7 Must calculate K b from appropriate K ax . - 3 7 5 6 - 7 5 6 2 a2 - 7 5 6 2 - 2 7 5 6 O H HC H O H C H K O H C H H O H HC + + + + +2 +1 3-
K w = K a x K b b 10 - 5 - -14 a2 w K 10 x 9 . 5 10 x 1.7 10 x 1 K K = = Now compare for our situation K a = 4.0x10 -7 K b = 5.9x10 -10 K a is 700 times larger than K b so the acid reaction will predominate and solution will be acidic.

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All cations (except alkali and heavy alkaline earth metal ions) act as weak acids. Rules Salts derived from strong acid and strong base from neutral solutions in H 2 O. ex. NaCl Na + from NaOH Cl - from HCl Salt derived from strong base/weak acid. Anion will be relatively strong conjugate base. pH > 7. ex. Ba(C 2 H 3 O 2 ) 2
Salt derived from weak base/strong acid. Strong conjugate acid. pH < 7. ex. Al(NO 3 ) 3 Al 3+ + 3H 2 O Al(OH 3 ) + 3H + Salt derived from weak base/weak acid. ex. NH 3 C 2 H 3 O 2 . Both conjugate acid and base will be fairly strong in solution. pH depends on which ion hydrolyzes H 2 O better. Must compare K a and K b .

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Additional aspects of Aqueous Equilibria 1. Common Ion Effect Remember LeChatlier - 2(aq) 3 2 (aq) equil 2(aq) 3 2 O H C H O H HC + + What happens if I add more C 2 H 3 O 2 - to the solution?
The dissociation of a weak electrolyte is decreased by adding a strong electrolyte that has an ion in common with the weak electrolyte.

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