chapter16

chapter16 - Applications of Aqueous Equilibria Buffers and...

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1 Dr. Ramón L ó pez de la Vega Florida International University Applications of Aqueous Equilibria Buffers and Titrations Common Ion: Two dissolved solutes that contain the same ion (either cation or anion). The presence of a common ion suppresses the ionization of a weak acid or a weak base. Common-Ion Effect: is the shift in equilibrium caused by the addition of a compound having an ion in common with the dissolved substance. The Common-Ion Effect
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2 Common ion effect – a manifestation of LeChatelier’s Principle A system is at equilibrium, HO 2 C 2 H 3 + H 2 O p H 3 O + + O 2 C 2 H 3 -1 Then the concentrations of the components are at equilibrium and Common ion effect – a manifestation of LeChatelier’s Principle now we add some NaO 2 C 2 H 3 HO 2 C 2 H 3 + H 2 O p H 3 O + + O 2 C 2 H 3 -1 NaO 2 C 2 H 3 Æ Na + + O 2 C 2 H 3 - The equilibrium will shift, so that Ka remains unchanged. According to LeChatelier’s Principle, this will result in a shift to the left. This will reduce the [ H + ]
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3 Buffer Solutions (an application of the common ion effect) A Buffer Solution: is a solution of (1) a weak acid or a weak base and (2) its salt; both components must be present. A buffer solution has the ability to resist changes in pH upon the addition of small amounts of either acid or base. Buffers are very important to biological systems. Buffers There are four ways that a buffer is made. 1. A solution of a weak acid and its conjugate base 2. A solution of a weak base and its conjugate acid 3. A solution of excess weak acid with a strong base. 4. A solution of excess weak base with a strong acid. The strong base must be the limiting reagent. The strong acid must be the limiting reagent
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4 Is it a buffer? A solution of 0.1 M NaCN and 0.1 M HCN? A solution of 0.1 M NH 3 and 0.1 M NH 4 Cl? A solution of 0.1 M HCl and 0.1M NaCl? A solution of 0.1 M NaOH and 0.1 M HCl? A solution of 0.1 M HCN and 0.05 M NaOH? A solution of 0.1 M HCN and 1.5 M NaOH? A solution of 0.1 M NH 3 and 0.05 M HCl? A solution of 0.1 M NH 3 and 1.5 M HCl? First question that must be answered, what really exists in solution? Buffers Buffer type no. 1 - weak acid and its conjugate base. A solution is made of 0.1 M KF and 0.1 M HF. What is the pH of this solution. 0.1 0 0 HF + H 2 O Æ Å H 3 O + +F - 0.1-x x x 0.1 0 0 KF + H 2 O Æ K + - 00 . 1 0 . 1 H 3 O + comes from only one source. F - is the common ion. It comes from two sources, the acid and the salt.
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5 Buffers Buffer type no. 1 - weak acid and its conjugate base. A solution is made of 0.1 M KF and 0.1 M HF. What is the pH of this solution. Ka = [x] [0.1+x] [0.1-x] 0.1 0 0 HF + H 2 O Æ Å H 3 O + +F - 0.1-x x x 0.1 0 0 KF + H 2 O Æ K + - 00 . 1 0 . 1 [H 3 O + ] = x [F - ] = x + 0.1 [HF] = 0.1-x Ka = [H 3 O + ] [F - ] [HF] We can neglect x since it is small and 0.1 + x = 0.1 and 0.1-x = 0.1 Buffers Buffer type no. 1 - weak acid and its conjugate base.
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This note was uploaded on 12/17/2011 for the course CHM 1046 taught by Professor Staff during the Fall '08 term at FIU.

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chapter16 - Applications of Aqueous Equilibria Buffers and...

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