1. Copyright © 2001 by Harcourt, Inc. All rights reserved. Chapter 13 Acids and Bases Copyright © 2001 by Harcourt, Inc. All rights reserved. Requests for permission to make copies of any part of the work should be mailed to the following address: Permissions Department, Harcourt, Inc. 6277 Sea Harbor Drive, Orlando, Florida 32887-6777 Bronsted-Lowry Acid-Base Model The Ion Product of Water pH and pOH Weak Acids and Their Equilibrium Constants Weak Bases and Their Equilibrium Constants Acid-Base Properties of Salt Solutions

2. Copyright © 2001 by Harcourt, Inc. All rights reserved. The Arrhenius Theory:A Brief Review HCl(g) → H + (aq) + Cl - (aq) NaOH(s) → Na + (aq) + OH - (aq) H2OH2O H2OH2O Na + (aq) + OH - (aq) + H + (aq) + Cl - (aq) → H 2 O(l) + Na + (aq) + Cl - (aq) H + (aq) + OH - (aq) → H 2 O(l) Arrhenius theory did not handle non OH - bases such as ammonia very well.

3. Copyright © 2001 by Harcourt, Inc. All rights reserved. Brønsted-Lowry Acid-Base Model Acid : a proton donor, base : a proton acceptor HB(aq)+A - (aq) HA(aq)+B - (aq) amphiprotic: H 2 O, HCO 3 - …. acidbase conjugate acid conjugate base

4. Copyright © 2001 by Harcourt, Inc. All rights reserved. Brønsted-Lowry Model of Acids and Bases Acid is proton donor, base is proton acceptor HB(aq)+A - (aq) HA(aq)+B - (aq) HB, HA are acids; A -, B - are bases. HB – B - and HA – A - are conjugate acid-base pairs. Amphiprotic

5. Copyright © 2001 by Harcourt, Inc. All rights reserved. Acidic and Basic Water Solutions Aqueous solution : acidic and basic properties of aqueous solutions are dependent on an equilibrium that involves the solvent, water pH : power of the hydrogen ion

6. Copyright © 2001 by Harcourt, Inc. All rights reserved. Aqueous solution In any aqueous solution, there is an equilibrium between H 3 0 + (H + ) ions and OH – ions: ion product constant of water H 2 O H + (aq) + OH – (aq) K W = [H + ]  [OH – ] = 1.0  10 -14 at 25  C 1. Pure water: [H + ] = [OH – ] = 1.0  10 –7 M; neutral solution 2. Acidic solution: [H + ] > 1.0  10 –7 M > [OH – ] 3. Basic solution: [OH – ] > 1.0  10 –7 M > [H + ] In seawater, [H + ] = 5  10 -9 M; [OH – ] = ? [OH – ] = (1.0  10 -14 ) / (5  10 -9 ) = 2  10 -6 M

7. Copyright © 2001 by Harcourt, Inc. All rights reserved. pH pH = –1og 10 [H + ] Neutral solution:pH = 7.0 Acidic solution:pH < 7.0 Basic solution:pH > 7.0 Suppose [H + ] = 2.4  10 -6 M; calculate pH pH = –1og 10 (2.4  10 –6 ) = 5.62 Suppose pH = 8.68; calculate [H + ] [H + ] = 10 –8.68 = 2.1  10 –9 M pOH = –1og 10 [OH – ]; pH + pOH = 14.00 at 25  C

8. Copyright © 2001 by Harcourt, Inc. All rights reserved. Ionization of aqueous HCl

9. Copyright © 2001 by Harcourt, Inc. All rights reserved. Relationship between pH and [H + ]

10. Copyright © 2001 by Harcourt, Inc. All rights reserved. Relationship between pH and [H + ]

11. Copyright © 2001 by Harcourt, Inc. All rights reserved. Strong Acids HCl, HBr, HI, HClO 4, HNO 3, H 2 SO 4 Strong Bases LiOH, NaOH, KOH, Ca(OH) 2, Sr(OH) 2, Ba(OH) 2 HCl solution Strong Acids & Bases

12. Copyright © 2001 by Harcourt, Inc. All rights reserved. Weak Acid and Their Eqilibrium Consts. Acetic AcidHC 2 H 3 O 2 or CH 3 CO 2 H

13. Copyright © 2001 by Harcourt, Inc. All rights reserved. Weak Acids Molecules : molecules containing an ionizable hydrogen atom Cations

14. Copyright © 2001 by Harcourt, Inc. All rights reserved. Molecules HF(aq) + H 2 O H 3 O + (aq) + F – (aq) There are literally thousands of molecular weak acids, most of them organic in nature.

15. Copyright © 2001 by Harcourt, Inc. All rights reserved. Cations NH 4 + (aq) + H 2 O H 3 O + (aq) + NH 3 (aq) Zn(H 2 O) 4 2+ (aq) + H 2 O H 3 O + (aq) + Zn(H 2 O) 3 (OH) + (aq)

16. Copyright © 2001 by Harcourt, Inc. All rights reserved. Weak Acids Equilibrium constants Calculation of K a : the weaker the acid, the smaller the value of K a Calculation of [H + ] in solution : by known amount of the weak acid to form a given volume of solution

17. Copyright © 2001 by Harcourt, Inc. All rights reserved. Weak Acids Equilibrium constants HB(aq) H + (aq) + B – (aq) pK a = -log 10 K a Weak acid  a  : pKa  HNO 2 pKa = 3.22, HCN pKa=9.24

18. Copyright © 2001 by Harcourt, Inc. All rights reserved. Percent Ionization HA + H 2 O H 3 O + + A - Degree of ionization = [H 3 O + ] from HA [HA] originally Percent ionization = [H 3 O + ] from HA [HA] originally X 100%

19. Copyright © 2001 by Harcourt, Inc. All rights reserved. Polyprotic Weak Acid H 3 PO 4 H + + H 2 PO 4 - K a1 = 7.1  10 -3 H 2 PO 4 - H + + HPO 4 2- K a2 = 6.2  10 -8 HPO 4 2- H + + PO 4 3- K a3 = 4.5  10 -13 K a1 >> K a2 >> K a3 All H 3 O + is formed in the first ionization step. H 2 PO 4 - essentially does not ionize further. Assume [H 2 PO 4 - ] = [H 3 O + ]. ex2) H 2 CO 3, H 2 SO 3, H 2 S

20. Copyright © 2001 by Harcourt, Inc. All rights reserved. Strengths of Binary Acids HIHBrHClHF 160.9>141.4>127.4>91.7 pm 297<368<431<569 kJ/mol Bond length Bond energy 10 9 >10 8 >1.3 x 10 6 >> 6.6 x 10 -4 Acid strength HF + H 2 O → [F - ·····H 3 O + ] F - + H 3 O + ion pair H-bonding free ions Tips )Structure & Acidity

21. Copyright © 2001 by Harcourt, Inc. All rights reserved. Strengths of Oxoacids Factors promoting electron withdrawal from the OH bond to the oxygen atom: High electronegativity (EN) of the central atom. A large number of terminal O atoms in the molecule. H-O-ClH-O-Br EN Cl = 3.0EN Br = 2.8 K a = 2.9 x 10 -8 K a = 2.1 x 10 -9 Tips1)Structure & Acidity

22. Copyright © 2001 by Harcourt, Inc. All rights reserved. Calculation of K a example pH of 0.100 M HC 2 H 3 O 2 solution is 2.87; K a =? [H + ] = [C 2 H 3 O 2 – ]=1.3  10 –3 M [HC 2 H 3 O 2 ] = 0.100 M – 0.0013 M = 0.099 M K a == 1.7  10 –5 (1.3  10 –3 ) 2 0.099

23. Copyright © 2001 by Harcourt, Inc. All rights reserved. Calculation of [H + ] in solution example Find [H + ] in 0.200 M HC 2 H 3 O 2, K a =1.8  10 –5 Let [H + ] = x, then [C 2 H 3 O 2 – ] = x, [HC 2 H 3 O 2 ] = 0.200 – x = 1.8  10 –5 Assume 0.200 - x  0.200 ; x = 1.9  10 –3 M In this case, % ionization =  100 = 1.0% x 2 0.200 – x 1.9  10 –3 0.200

24. Copyright © 2001 by Harcourt, Inc. All rights reserved. Calculation of [H + ] in solution example Find [H + ] in 0.100 M HF (K a = 6.9  10 –4 ) = 6.9  10 –4 Set 0.100 – x = 0.100, solve: x = 8.3  10 –3 > 5% Make second approximation: = 6.9  10 –4 ; x = 8.0  10 –3 x 2 0.100 – x x 2 0.100 – 0.008

25. Copyright © 2001 by Harcourt, Inc. All rights reserved. Weak Bases Molecules : there are many molecular weak bases, including the organic compounds known as amines Anions derived from weak acids

26. Copyright © 2001 by Harcourt, Inc. All rights reserved. Molecules NH 3 (aq) + H 2 O OH – (aq) + NH 4 + (aq) The simplest weak base is ammonia.

27. Copyright © 2001 by Harcourt, Inc. All rights reserved. Anions derived from weak acids F – (aq) + H 2 O HF(aq) + OH – (aq) F - is conjugate base of the weak acid HF.

28. Copyright © 2001 by Harcourt, Inc. All rights reserved. Weak Bases Equilibrium constant Expression for K b : omitting the term for solvent water Relation between K a and K b : the larger the value of K a, the Smaller the value of K b and vise versa Calculation of [OH – ] in solution of weak base

29. Copyright © 2001 by Harcourt, Inc. All rights reserved. Expression for K b NH 3 : K b = [NH 4 + ]  [OH – ] [NH 3 ]

30. Copyright © 2001 by Harcourt, Inc. All rights reserved. Relation between K a and K b K a  K b = K W = 1.0  10 –14 K a K b HF6.9  10 –4 F – 1.4  10 –11 HAc1.8  10 –5 Ac – 5.6  10 –10 NH 4 + 5.6  10 –10 NH 3 1.8  10 –5 Strength of base is inversely related to that of conjugate weak acid.

31. Copyright © 2001 by Harcourt, Inc. All rights reserved. Calculation of [OH – ] in solution of weak base = 1.4  10 –11 ; = 1.4  10 –11 [OH – ] = 1.2  10 –6 M ; pH = 8.08 [HF][OH – ] [F – ] [OH – ] 2 0.10

32. Copyright © 2001 by Harcourt, Inc. All rights reserved. Acid-Base Properties of Salt Solutions Cations : weak acids or spectator ions Anions : weak bases or spectator ions Overall Results

33. Copyright © 2001 by Harcourt, Inc. All rights reserved. Cations Spectator: derived from strong bases Li +, Na +, K + ; Ca 2+, Sr 2+, Ba 2+ Acidic: all other cations, including those of the transition metals.

34. Copyright © 2001 by Harcourt, Inc. All rights reserved. Anions Spectator: derived from strong acids C1 –, Br –, I –, NO 3 –, SO 4 2– Basic: anions derived from weak acids, such as F –, NO 2 –

35. Copyright © 2001 by Harcourt, Inc. All rights reserved.

36. Overall Results SaltCationAnion NaNO 3 Na + (Sp.)NO 3 – (Sp.)neutral KFK + (Sp.)F – (Ba.)basic FeCl 2 Fe 2+ (Ac.)Cl – (Sp.)acidic If cation is acidic, anion basic, compare K a, K b values NH 4 F: K a = 5.6  10 –10, K b = 1.4  10 –11 ; acidic

37. Copyright © 2001 by Harcourt, Inc. All rights reserved. Home work 3, 9,15,31,35,45,49,55,65,71