chm1046 Chapter 15 Slides

chm1046 Chapter 15 Slides - 8/22/2010 Le Chateliers...

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8/22/2010 1 Le Chatelier’s Principle Common Ion Effect: The shift in equilibrium that occurs because of the addition of an ion already involved in the equilibrium reaction equilibrium reaction. •AgCl( s ) ֐ Ag + ( aq ) + Cl - ( aq ) Common Ion Effect HCN( aq ) + H 2 O( l ) H 3 O + ( aq ) + CN - ( aq ) Addition of NaCN will shift the equilibrium to the left because of the addition of CN - which is 2 , which is already involved in the equilibrium reaction. A solution of HCN and NaCN is less acidic than a solution of HCN alone. Common Ion Effect Calculate the pH of each of the following solutions: a. 0.200M HC 2 H 3 O 2 (K a = 1.8 x 10 -5 ) b. 0.200M HC 2 H 3 O 2 and 0.500M NaC 2 H 3 O 2 Buffered Solutions A buffered solution contains: 1. a weak acid and its salt or 2. a weak base and its salt A buffered solution resists change in its pH when either H + or OH - are added due to the common ion effect. The common ion depresses the equilibrium
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8/22/2010 2 Buffering: How Does It Work? 5 After addition of strong acid or base, deal with stoichiometry first, then equilibrium. 6 • Calculate the pH of the HC 2 H 3 O 2 /NaC 2 H 3 O 2 solution above after adding 0.010 mol solid NaOH to 1.0L of the solution. (0.200M HC 2 H 3 O 2 and 0.500M NaC 2 H 3 O 2 , K a = 1.8 x 10 -5 ) (left) Pure water at pH 7.000. (right) When 0.01 mol NaOH is added to 1.0 L of pure water, the pH jumps to 12.000 .
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8/22/2010 3 Buffered Solution Characteristics ¾ Buffers contain relatively large amounts of weak acid and corresponding base. ¾ Added OH - reacts to completion with the weak acid, but since there are large amounts of weak acid and its corresponding base, their equilibrium concentrations do not change significantly ¾ The pH is determined by the ratio of the concentrations of the weak acid and weak base.
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chm1046 Chapter 15 Slides - 8/22/2010 Le Chateliers...

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