Copyright©2000 by Houghton Mifflin Company. All rights reserved. 1 Chemistry FIFTH EDITION by Steven S. Zumdahl University of Illinois Chapter 15 Applications of Aqueous Equilibria Schedule Chapter 15 on Website

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 2 Section 15.1 Solutions of Acids or Bases containing a Common Ion

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 3 Common Ion Effect The shift in equilibrium that occurs because of the addition of an ion already involved in the equilibrium reaction. AgCl(s)  Ag + (aq) + Cl  (aq) It is an application of Le Chatelier’s Principle.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 4 Example: 1.0 M HF (Weak acid) HF (aq)  H + (aq) + F - (aq) Add NaF (s) Addition of Na + does not affect the solution; it has neutral properties. Addition of F - shifts the above weak acid equilibrium to the left Therefore, less dissociation of the weak acid, HF

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 5 Let’s Do these Problems!! (1)What is the pH of M HONH 2 ? (K b = 1.1 x ) (2)What is the pH of a mixture containing M HONH 2 & M HONH 3 Cl?

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 6 Section 15.2 A Buffered Solution... resists change in its pH when either H + or OH  are added. 1.0 L of 0.50 M H 3 CCOOH – M H 3 CCOONa pH = 4.74 Adding mol solid NaOH raises the pH of the solution to 4.76, a very minor change.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 7 One of the most practical applications of buffers is our blood. Most important one is the HCO 3 - and H 2 CO 3.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 8 BUFFERS Weak acid and its Salt HF & NaF Or Weak base and its Salt NH 3 & NH 4 Cl The systematic approach used in Chapter 14 for weak acids & bases applies to buffered solutions.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 9 Key Points on Buffered Solutions 1.They are weak acids or bases containing a common ion. Let’s do together #34 on page 782.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 10 Key Points on Buffered Solutions 1.They are weak acids or bases containing a common ion. 2.After addition of strong acid or base, deal with stoichiometry first, then equilibrium. Let’s do together # 36 on page 782.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 11 HOW DOES A BUFFER WORK? Read

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 12 Buffered Solution Characteristics 4 Buffers contain relatively large amounts of weak acid and corresponding base.  Added OH  reacts to completion with the weak acid. 4 OH - + HA  A - + H 2 O 4 The net result is the hydroxide is replaced by A -.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 13 Buffered Solution Characteristics 4 The pH is determined by the ratio of the concentrations of the weak acid and weak base. 4 [H + ] = K a [HA]/[A - ]

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 14 If the buffer contains large quantities of HA & A -, then when OH - is added and HA is converted to A - the change in the ratio [HA]/[A - ] is small. Therefore, the pH doesn’t change much.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 15 Buffered Solution Characteristics 4 Buffers contain relatively large amounts of weak acid and corresponding base. 4 Added H + reacts to completion with the weak base. 4 H + + A -  HA 4 Free H + converted to HA

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 16 Buffered Solution Characteristics 4 AGAIN!!! 4 The pH is determined by the ratio of the concentrations of the weak acid and weak base. 4 [H + ] = K a [HA]/[A - ]

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 17 If the buffer contains large quantities of HA & A -, then when H + is added and A - is converted to HA the change in the ratio [HA]/[A - ] is small. Therefore, the pH doesn’t change much.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 18 Since [H+] = K a [HA]/[A - ] Mathematical rearrangement Take – log of both sides

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 19 Henderson-Hasselbalch Equation  Useful for calculating pH when the [A  ]/[HA] ratios are known.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 20 For a particular buffering system (i.e., same acid-conjugate base pair) All sol’ns that have the same [A - ]/[HA] ratio will have the same pH.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 21 Henderson-Hasselbalch Equation This equation can be used to calculate the pH of a buffer solution.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 22 It is assumed that the equilibrium concentrations of HA & A- are equal to the initial concentrations of HA & A-. This is true if the amount of acid that dissociates is small (i.e., “x is small”). HA & A - must be relatively large.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 23 Read Sample Exercise 15.4.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 24 Let’s do together #40, 42a & 41a

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 25 What happens if you add strong acid or base to the buffer? Do # 44.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 26 Buffered Solution Characteristics 4 Buffers contain relatively large amounts of weak acid and corresponding base. 4 Added H + reacts to completion with the weak base.  Added OH  reacts to completion with the weak acid. 4 The pH is determined by the ratio of the concentrations of the weak acid and weak base.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 27 Section 15.3 Buffering Capacity... represents the amount of H + or OH  the buffer can absorb without a significant change in pH.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 28 pH of a buffered sol’n: Determined by the [A - ]/[HA] Capacity of a buffered sol’n: Determined by magnitude of [HA] & [A - ] A buffer with a large capacity contains large concentrations of the buffering components.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 29 Most Effective Buffering: [HA] = [A - ] pH = pK a + log [A - ]/[HA] = pK a + log 1 = pK a pK a of the weak acid to be used in a buffer should be as close as possible to the desired pH.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 30 Let’s Calculate the change in pH that occurs when mole of gaseous HCl is added to 1.0 liter of 1)5.00 M acetic acid & 5.00 M sodium acetate 2)0.050 M acetic acid & M sodium acetate K a = 1.8 x 10 -5

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 31 Titration and pH Curves Important Section! Please Read!!

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 32 Titration (pH) Curve A plot of pH of the solution being analyzed as a function of the amount of titrant added. Equivalence (stoichiometric) point: Enough titrant has been added to react exactly with the solution being analyzed.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 33 Strong Acid – Strong Base Titrations Net ionic equation is H + (aq) + OH - (aq)  H 2 O (l) Solution is neutral at the equivalence point.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 34 Figure 15.1 The pH Curve for the Titration of 50.0 mL of M HNO 3 with M NaOH Strong Acid With Strong Base

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 35 Figure 15.2 The pH Curve for the Titration of mL of 0.50 M NaOH with 1.0 M HCl Strong Base With Strong Acid

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 36 Weak Acid - Strong Base Titration Step 1 -A stoichiometry problem - reaction is assumed to run to completion - then determine remaining species. Step 2 -An equilibrium problem - determine position of weak acid equilibrium and calculate pH.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 37 Figure 15.3 The pH Curve for the Titration of 50.0 mL of M HC 2 H 3 O 2 with M NaOH Weak Acid With Strong Base

Copyright©2000 by Houghton Mifflin Company. All rights reserved Halfway to Equivalence Point 4 [HA] = [A - ] (Buffering phase) 4 [H + ] = K a [HA]/[A - ] 4 [H + ] = K a 4 Therefore, pH = pK a

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 39 Weak Acid - Strong Base Titration Equivalence Point always greater than 7. Acetic Acid + Sodium hydroxide form the salt Sodium Acetate. Na + CH 3 COO - is a salt which produces a basic pH (i.e. > 7) CH 3 COO - + H 2 O  CH 3 COOH + OH -

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 40 Figure 15.3 The pH Curve for the Titration of 50.0 mL of M HC 2 H 3 O 2 with M NaOH Weak Acid With Strong Base

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 41 Figure 15.4 The pH Curves for the Titrations of 50.0-mL Samples of 0.10 M Acids with Various K a Values with 0.10 M NaOH 1)Amt. of acid, not strength determines the equivalence pt. 2)pH of equivalence pt. is affected by acid strength; The weaker the acid, the stronger Its conjugate base  pH higher at equivalence pt.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 42 Figure 15.5 The pH Curve for the Titration of mL of M NH 3 with 0.10 M HCl Titration of Weak Bases With Strong Acids Equivilence Point at pH < 7.