Philip Dutton University of Windsor, Canada N9B 3P4 Prentice-Hall © 2002 General Chemistry Principles and Modern Applications Petrucci Harwood Herring 8 th Edition Chapter 17: Additional Aspects of Acid-Base Equilibria

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 2 of 42 Contents 17-1The Common-Ion Effect in Acid-Base Equilibria 17-2Buffer Solutions 17-3Acid-Base Indicators 17-4Neutralization Reactions and Titration Curves 17-5Solutions of Salts of Polyprotic Acids 17-6Acid-Base Equilibrium Calculations: A Summary Focus On Buffers in Blood

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 3 of The Common-Ion Effect in Acid- Base Equilibria The Common-Ion Effect describes the effect on an equilibrium by a second substance that furnishes ions that can participate in that equilibrium. The added ions are said to be common to the equilibrium.

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 4 of 42 Solutions of Weak Acids and Strong Acids Consider a solution that contains both M CH 3 CO 2 H and M HCl. CH 3 CO 2 H + H 2 O ↔ CH 3 CO H 3 O + HCl + H 2 O ↔ Cl - + H 3 O + (0.100-x) M x M x M M [H 3 O + ] = ( x) M essentially all due to HCl

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 5 of 42 Acetic Acid and Hydrochloric Acid 0.1 M HCl0.1 M CH 3 CO 2 H 0.1 M HCl M CH 3 CO 2 H

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 6 of 42 Example 17-1 Demonstrating the Common-Ion Effect: A Solution of a weak Acid and a Strong Acid. (a) Determine [H 3 O + ] and [CH 3 CO 2 - ] in M CH 3 CO 2 H. (b) Then determine these same quantities in a solution that is M in both CH 3 CO 2 H and HCl. CH 3 CO 2 H + H 2 O → H 3 O + + CH 3 CO 2 - Recall Example 17-6 (p 680): [H 3 O + ] = [CH 3 CO 2 - ] = 1.333·10 -3 M

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 7 of 42 Example 17-1 CH 3 CO 2 H + H 2 O → H 3 O + + CH 3 CO 2 - Initial concs. weak acid0.100 M0 M0 M strong acid0 M0.100 M0 M Changes-x M+x M+x M Eqlbrm conc.( x) M ( x) M x M Assume x << M, – x ≈ x ≈ M

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 8 of 42 Example 17-1 Eqlbrm conc.( x) M ( x) M x M Assume x << M, – x ≈ x ≈ M CH 3 CO 2 H + H 2 O → H 3 O + + CH 3 CO 2 - [H 3 O + ] [CH 3 CO 2 - ] [CH 3 CO 2 H] Ka=Ka= x · ( x) ( x) = x · (0.100) (0.100) = = 1.8·10 -5

Example 17-1: Conclusion General Chemistry: Chapter 18Slide 9 of 42 In the presence of the strong acid, the concentration of the weak acid anion [A - ] is very much less than it is when only the weak acid (HA) is present. [CH 3 CO 2 - ] = 1.8·10 -5 M compared to 1.3·10 -3 M. In the presence of the strong acid, the concentration of the [OH - ] ions originating from the water self ionization is much less than it is in the pure water. Le Chatellier’s Principle

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 10 of 42 Suppression of Ionization of a Weak Acid

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 11 of 42 Suppression of Ionization of a Weak Base

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 12 of 42 Solutions of Weak Acids and Their Salts

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 13 of 42 Solutions of Weak Bases and Their Salts

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 14 of Buffer Solutions Two component systems that change pH only slightly on addition of acid or base. –The two components must not neutralize each other but must neutralize strong acids and bases. A weak acid and it’s conjugate base. A weak base and it’s conjugate acid

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 15 of 42 Buffer Solutions Consider [CH 3 CO 2 H] = [CH 3 CO 2 - ] in a solution. [H 3 O + ] [CH 3 CO 2 - ] [CH 3 CO 2 H] Ka=Ka= = 1.8·10 -5 = [CH 3 CO 2 H] [CH 3 CO 2 - ] KaKa [H 3 O + ] = pH = -log[H 3 O + ] = -logK a = -log(1.8·10 -5 ) = 4.74

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 16 of 42 How A Buffer Works

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 17 of 42 The Henderson-Hasselbalch Equation A variation of the ionization constant expression. Consider a hypothetical weak acid, HA, and its salt NaA: HA + H 2 O ↔ A - + H 3 O + [H 3 O + ] [A - ] [HA] Ka=Ka=[H 3 O + ] = KaKa [HA] [A - ]

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 18 of 42 Henderson-Hasselbalch Equation pK a + log [HA] pH = [A - ] pK a + log [acid] pH = [conjugate base] -log[H 3 O + ] = -logK a - [HA] [A - ] log

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 19 of 42 Henderson-Hasselbalch Equation Only useful when you can use initial concentrations of acid and salt. –This limits the validity of the equation. Limits can be met by: 0.1 < [HA] < 10 [A - ] [A - ] > 10·K a and [HA] > 10·K a pK a + log [acid] pH= [conjugate base]

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 20 of 42 Example 17-5 Preparing a Buffer Solution of a Desired pH. What mass of NaC 2 H 3 O 2 must be dissolved in L of 0.25 M HC 2 H 3 O 2 to produce a solution with pH = 5.09? (Assume that the solution volume is constant at L) HC 2 H 3 O 2 + H 2 O ↔ C 2 H 3 O H 3 O + Equilibrium expression: [H 3 O + ] [HC 2 H 3 O 2 ] Ka=Ka= [C 2 H 3 O 2 - ] = 1.8·10 -5

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 21 of 42 Example 17-5 [H 3 O + ] [HC 2 H 3 O 2 ] Ka=Ka= [C 2 H 3 O 2 - ] = 1.8·10 -5 [H 3 O + ] = = 8.13·10 -6 M [HC 2 H 3 O 2 ] = 0.25 M Solve for [C 2 H 3 O 2 - ] [H 3 O + ] [HC 2 H 3 O 2 ] = K a [C 2 H 3 O 2 - ] = M 8.13· = 1.80·10 -5

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 22 of 42 Example mol NaC 2 H 3 O g NaC 2 H 3 O 2 mass C 2 H 3 O 2 - = L [C 2 H 3 O 2 - ] = M 1 L mol 1 mol C 2 H 3 O mol NaC 2 H 3 O 2 ·· · = g NaC 2 H 3 O 2

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 23 of 42 Six Methods of Preparing Buffer Solutions

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 24 of 42 Calculating Changes in Buffer Solutions

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 25 of 42 Buffer Capacity and Range Buffer capacity is the amount of strong acid or base that a buffer can neutralize before its pH changes appreciably. –Maximum buffer capacity exists when [HA] and [A - ] are large and approximately equal to each other. Buffer range is the pH range over which a buffer effectively neutralizes added acids and bases. –Practically, range is 2 pH units around pK a

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 26 of Acid-Base Indicators Color of some substances (e.g. weak acids) depends on the pH. HIn + H 2 O ↔ In - + H 3 O + >90% acid form the color appears to be the acid color >90% base form the color appears to be the base color Intermediate color is seen in between these two states. Complete color change occurs over 2 pH units.

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 27 of 42 Indicator Colors and Ranges

Several indicators in a large pH trange Csonka GáborÁltalános kémia -sav-bázis 2Dia 28 /34 pH = 0 Benzil- orange Bromcresol- purple Thymolftaleine Brom- phenol blue Congo-red Thymolblue pH = 14

Two indicators, pH = pK ± 1, in narrow range Csonka GáborÁltalános kémia -sav-bázis 2Dia 29 /34 Thymol blue Cresol red

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 30 of Neutralization Reactions and Titration Curves Equivalence point: –The point in the reaction at which both acid and base have been consumed. –Neither acid nor base is present in excess. End point: –The point at which the indicator changes color. Titrant: –The known solution added to the solution of unknown concentration. Titration Curve: –The plot of pH vs. titrant volume.

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 31 of 42 The millimole Typically: –Volume of titrant added is less than 50 mL. –Concentration of titrant is less than 1 mol/L. –Titration uses less than 1/1000 mole of acid and base. L/1000 mol/1000 = M = L mol mL mmol =

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 32 of 42 Titration of a Strong Acid with a Strong Base

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 33 of 42 Titration of a Strong Acid with a Strong Base The pH has a low value at the beginning. The pH changes slowly –until just before the equivalence point. The pH rises sharply –perhaps 6 units per 0.1 mL addition of titrant. The pH rises slowly again. Any Acid-Base Indicator will do. –As long as color change occurs between pH 4 and 10.

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 34 of 42 Titration of a Strong Base with a Strong Acid

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 35 of 42 Titration of a Weak Acid with a Strong Base

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 36 of 42 Titration of a Weak Acid with a Strong Base

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 37 of 42 Titration of a Weak Polyprotic Acid H 3 PO 4 ↔ H 2 PO 4 - ↔ HPO 4 2- ↔ PO 4 3- NaOH

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 38 of Solutions of Salts of Polyprotic Acids The third equivalence point of phosphoric acid can only be reached in a strongly basic solution. The pH of this third equivalence point is not difficult to caluclate. –It corresponds to that of Na 3 PO 4 (aq) and PO 4 3- can ionize only as a base. PO H 2 O → OH - + HPO 4 2- K b = K w /K a3 = 2.4·10 -2

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 39 of 42 Example 17-9 K b = 2.4·10 -2 PO H 2 O → OH - + HPO 4 2- Initial concs.1.0 M0 M0 M Changes-x M+x M+x M Eqlbrm conc.( x) M x M x M Determining the pH of a Solution Containing the Anion (A n- ) of a Polyprotic Acid. Sodium phosphate, Na 3 PO 4, is an ingredient of some preparations used to clean painted walls before they are repainted. What is the pH of 1.0 M Na 3 PO 4 ?

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 40 of 42 Example 17-9 x x – = 0 x = M pOH = pH = [OH - ] [HPO 4 2- ] [PO 4 3- ] Kb=Kb= x · x ( x) = = 2.4·10 -2 It is more difficult to calculate the pH values of NaH 2 PO 4 and Na 2 HPO 4 because two equilibria must be considered simultaneously.

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 41 of 42 Concentrated Solutions of Polyprotic Acids For solutions that are reasonably concentrated (> 0.1 M) the pH values prove to be independent of solution concentrations. for H 2 PO 4 - for HPO 4 2- pH = 0.5 (pK a1 + pK a2 ) = 0.5 ( ) = 4.68 pH = 0.5 (pK a2 + pK a3 ) = 0.5 ( ) = 9.79

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 42 of Acid-Base Equilibrium Calculations: A Summary Determine which species are potentially present in solution, and how large their concentrations are likely to be. Identify possible reactions between components and determine their stoichiometry. Identify which equilibrium equations apply to the particular situation and which are most significant.

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 43 of 42 Focus On Buffers in Blood CO 2 (g) + H 2 O ↔ H 2 CO 3 (aq) H 2 CO 3 (aq) + H 2 O(l) ↔ HCO 3 - (aq) K a1 = 4.4·10 -7 pK a1 = 6.4 pH = 7.4 = pH = pK a1 + log [H 2 CO 3 ] [HCO 3 - ]

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 44 of 42 Buffers in Blood 10/1 buffer ratio is somewhat outside maximum buffer capacity range but… The need to neutralize excess acid (lactic) is generally greater than the need to neutralize excess base. If additional H 2 CO 3 is needed CO 2 from the lungs can be utilized. Other components of the blood (proteins and phosphates) contribute to maintaining blood pH.

Prentice-Hall © 2002General Chemistry: Chapter 18Slide 45 of 42 Chapter 18 Questions Develop problem solving skills and base your strategy not on solutions to specific problems but on understanding. Choose a variety of problems from the text as examples. Practice good techniques and get coaching from people who have been here before.