Chapter 19

Chapter 19 - CHAPTER 19 IONIC EQUILIBRIA IN AQUEOUS SYSTEMS...

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19-1 CHAPTER 19 IONIC EQUILIBRIA IN AQUEOUS SYSTEMS 19.1 The purpose of an acid-base buffer is to maintain a relatively constant pH in a solution. 19.2 The weak acid component neutralizes added base and the weak base component neutralizes added acid so that the pH of the buffer solution remains relatively constant. The components of a buffer do not neutralize one another when they are a conjugate acid/base pair. 19.3 The presence of an ion in common between two solutes will cause any equilibrium involving either of them to shift in accordance with Le Châtelier’s principle. For example, addition of NaF to a solution of HF will cause the equilibrium H F ( aq ) + H 2 O( l ) ' H 3 O + ( aq ) + F - ( aq ) to shift to the left, away from the excess of F - . 19.4 A buffer is a mixture of a weak acid and its conjugate base (or weak base and its conjugate acid). The pH of a buffer changes only slightly with added H 3 O + because the added H 3 O + reacts with the base of the buffer. The net result is that the concentration of H 3 O + does not change much from the original concentration, keeping the pH nearly constant. 19.5 A high buffer capacity results when the weak acid and weak base are both present at high concentration. Addition of 0.01 mol of HCl to a high-capacity buffer will cause a smaller change in pH than with a low-capacity buffer, since the ratio [HA] / [A - ] will change less. 19.6 Only (c) has an affect on the buffer capacity. In theory, any conjugate pair (of any p K a ) can be used to make a high capacity buffer. With proper choice of components, it can be at any pH. The buffer range changes along with the buffer capacity, but does not determine it. A high-capacity buffer will result when comparable quantities (i.e., buffer-component ratio < 10:1) of weak acid and weak base are dissolved so that their concentrations are relatively high. 19.7 The buffer component ratio refers to the ratio of concentrations of the acid and base that make up the buffer. When this ratio is equal to 1, the buffer resists changes in pH with added acid to the same extent that it resists changes in pH with added base. The buffer range extends equally in both the acidic and basic direction. When the ratio shifts with higher [base] than [acid], the buffer is more effective at neutralizing added acid than base so the range extends further in the acidic than basic direction. The opposite is true for a buffer where [acid] > [base]. Buffers with a ratio equal to 1 have the greatest buffer range. The more the buffer component ratio deviates from 1, the smaller the buffer range. 19.8 p K a (formic) = 3.74; p K a (acetic) = 4.74. Formic acid would be the buffer choice, since its p K a is closer to the desired pH. If acetic acid were used, the buffer component ratio would be far from 1:1 and the buffer’s effectiveness would be lower (see 18.7). The NaOH serves to partially neutralize the acid and produce its conjugate base.
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This note was uploaded on 01/18/2009 for the course CHEM 1C taught by Professor Roland during the Spring '08 term at UCSC.

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Chapter 19 - CHAPTER 19 IONIC EQUILIBRIA IN AQUEOUS SYSTEMS...

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