Chap19Notes - Chem 1036 Chapter 19 1 of 50 Chem 1036 Chapter 19 3 of 50 How does a buffer work Example HC2H3O2 C2H3O2 There is an equilibrium between

Chap19Notes - Chem 1036 Chapter 19 1 of 50 Chem 1036...

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Chem 1036 Chapter 19 1 of 50 Chapter 19 Ionic Equilibria in Aqueous Systems Buffer Solutions Demonstration: What happens when Strong acid is added to: Strong base is added to: Water Water Water with Alka-Seltzer Water with Alka-Seltzer The Alka-Seltzer provides a buffer solution. Chem 1036 Chapter 19 2 of 50 Buffer: A solution containing approximately equal amounts of a weak acid and its conjugate base (or a weak base and its conjugate acid). A buffer resists change in pH when small amounts of acid or base are added to the solution. Examples of buffers: HC 2 H 3 O 2 /NaC 2 H 3 O 2 NH 4 Cl/NH 3 H 2 CO 3 /NaHCO 3 (buffer in blood) K a = 1.8x10 –5 K a = 5.6x10 –10 K a = 4.5x10 –7 p K a = 4.75 p K a = 9.25 p K a = 6.35 Note the spectator ions (Na + and Cl ). Chem 1036 Chapter 19 3 of 50 How does a buffer work? Example: HC 2 H 3 O 2 / C 2 H 3 O 2 There is an equilibrium between the conjugate acid/base pair: HC 2 H 3 O 2 + H 2 O H 3 O + + C 2 H 3 O 2 An increase in the concentration of HC 2 H 3 O 2 shifts the equilibrium to the right and an increase in concentration of C 2 H 3 O 2 shifts the equilibrium to the left. Addition of base to the buffer: The acid component of the buffer reacts with added base: HC 2 H 3 O 2 ( aq ) + OH ( aq ) in buffer added The pH becomes only slightly more basic because the acid component in the buffer (HC 2 H 3 O 2 ) consumes the small amount of added OH . A small amount of the buffer weak base component (C 2 H 3 O 2 ) is made, the equilibrium between HC 2 H 3 O and and C 2 H 3 O 2 shifts to the left and the pH increases slightly: HC 2 H 3 O 2 + H 2 O H 3 O + + C 2 H 3 O 2 Chem 1036 Chapter 19 4 of 50 Addition of acid to the buffer: The base component of the buffer reacts with added acid: C 2 H 3 O 2 ( aq ) + H 3 O + ( aq ) in buffer added The pH becomes only slightly more acidic because the base component in the buffer (C 2 H 3 O 2 ) consumes the small amount of added H 3 O + . A small amount of the buffer weak acid component (HC 2 H 3 O 2 ) is made, the equilibrium between HC 2 H 3 O and and C 2 H 3 O 2 shifts to the right and the pH decreases slightly: HC 2 H 3 O 2 + H 2 O H 3 O + + C 2 H 3 O 2
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Chem 1036 Chapter 19 5 of 50 Chem 1036 Chapter 19 6 of 50 Buffer Calculations Example Problem : What is the pH of a buffer that is composed of 0.25 M HC 2 H 3 O 2 and 0.35 M NaC 2 H 3 O 2 ? K a = 1.8 x 10 –5 . HC 2 H 3 O 2 ( aq ) + H 2 O( l ) H 3 O + ( aq ) + C 2 H 3 O 2 ( aq ) Initial: 0.25 M 0 0.35 M Change:______________________________________________ Equilibrium: K a = 1.8 x 10 –5 = > @ + 3 2 3 2 2 3 2 H O C H O HC H O ± ª º ª º ¬ ¼ ¬ ¼ = Chem 1036 Chapter 19 7 of 50 Henderson-Hasselbalch Equation for buffers : HA( aq ) + H 2 O( l ) H 3 O + ( aq ) + A ( aq ) K a = > @ 3 H O A HA ² ± ª º ª º ¬ ¼ ¬ ¼ (p K a = –log[ K a ]) a [base] pH p log [acid] K ² Chem 1036 Chapter 19 8 of 50 Example Problem : What is the pH of a buffer that is composed of 0.25 M HC 2 H 3 O 2 and 0.35 M NaC 2 H 3 O 2 ? K a = 1.8 x 10 –5 .
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