1. 1 Acids and Bases Chapter 17

2. 2 Why are lemons sour?

3. 3 What about other acids?

4. 4 What about bases?

5. 5 Definitions Brønsted-Lowry: Brønsted-Lowry: An acid is anything that can donate a proton An acid is anything that can donate a proton HCl (aq) + H 2 O (l)  Cl - (aq) + H 3 O + (aq) A base is anything that can accept a proton A base is anything that can accept a proton –Water (above) acts as a base H 3 O + = hydronium ion H 3 O + = hydronium ion HCl = acid, Cl - = conjugate base HCl = acid, Cl - = conjugate base –Conjugate acid-base pair H 2 O = base, H 3 O + = conjugate acid H 2 O = base, H 3 O + = conjugate acid –Conjugate acid-base pair

6. 6 A problem Write the conjugate bases for the acids and the conjugate acids for the bases Acids Bases HIO 3 S 2- HCO 3 - CN - CH 3 COOH NH 3

7. 7 Some more traits Species donates 1 proton = monoprotic acid Species donates 1 proton = monoprotic acid –2 protons = diprotic acid Accepts 1 proton = monoprotic base Accepts 1 proton = monoprotic base –Accepts 2 protons = diprotic base Give a triprotic acid Give a triprotic acid

8. 8 Lewis acids and bases The same dude who gave us Lewis structures The same dude who gave us Lewis structures Lewis acid = lone pair acceptor Lewis acid = lone pair acceptor Lewis base = lone pair donor Lewis base = lone pair donor Form coordinate covalent bonds or adducts Form coordinate covalent bonds or adducts –NH 3 + BH 3

9. 9 Coordination complexes = adducts = adducts Water, ammonia, ions, molecules, etc. (ligands) give lone pairs to (usually transition) metal cations in soln Water, ammonia, ions, molecules, etc. (ligands) give lone pairs to (usually transition) metal cations in soln Ex: Cu 2+ (aq) + 4NH 3(aq)  [Cu(NH 3 ) 4 ] 2+ (aq) Ex: Cu 2+ (aq) + 4NH 3(aq)  [Cu(NH 3 ) 4 ] 2+ (aq) Very colorful solns Very colorful solns Chem 133 Chem 133

10. 10 Water’s uniqueness Can act as both a base and an acid Can act as both a base and an acid –Amphiprotic H 2 O (l) + H 2 O (l)  H 3 O + (aq) + OH - (aq) Water autoionizes Water autoionizes

11. 11 pH = power of hydrogen = power of hydrogen Measurement of protons/hydronium ions in soln Measurement of protons/hydronium ions in soln Based on -logarithm (log 10 ) Based on -logarithm (log 10 ) –  pH = -log[H 3 O + ] Likewise Likewise –pOH = -log[OH - ] (10 -pH = [H 3 O + ] & 10 -pOH = [OH - ]) (10 -pH = [H 3 O + ] & 10 -pOH = [OH - ])

12. 12

13. 13 pH scale So pH + pOH = 14.00 = pK w So pH + pOH = 14.00 = pK w Where K w = [H 3 O + ][OH - ] = 1.0 x 10 -14 Where K w = [H 3 O + ][OH - ] = 1.0 x 10 -14 7.00 = neutrality 7.00 = neutrality <7.00  acidity (increase going to 0) <7.00  acidity (increase going to 0) >7.00  basicity (increases going to 14) >7.00  basicity (increases going to 14) At neutrality, [H 3 O + ] = [OH - ] = 1.0 x 10 -7 At neutrality, [H 3 O + ] = [OH - ] = 1.0 x 10 -7 Is [OH - ] more concentrated under acidic or basic conditions? Is [OH - ] more concentrated under acidic or basic conditions?

14. 14 Before you work on next slide! Sig figs in logs: Sig figs in logs: –Result has the same number of decimal places in the input has sig figs: Log (1.00  10 –5 ) = -5.000 3 sig figs 3 dec place 3 sig figs 3 dec place Sig figs in antilogs: Sig figs in antilogs: –Result has the same number of sig figs as the number of decimal places in the input: 10 –6.00 = 1.0  10 –6 2 dec place 2 sig figs 2 dec place 2 sig figs

15. 15 pH calculation problems

16. 16 Equilibria for acids: HA (aq) + H 2 O (l)  H 3 O + (aq) + A - (aq) As K a increases what happens to the acid dissociation? Does it increase or decrease? As K a increases what happens to the acid dissociation? Does it increase or decrease? –It increases A stronger acid A stronger acid If K a < 1.0, weak acid If K a < 1.0, weak acid

17. 17 Strong acids HCl HCl HBr HBr HI HI H 2 SO 4 H 2 SO 4 HNO 3 HNO 3 HClO 4 HClO 4 HClO 3 HClO 3

18. 18 Equilibria for bases: B (aq) + H 2 O (l)  BH + (aq) + OH - (aq) As K b increases so does base strength As K b increases so does base strength If K b < 1.0 If K b < 1.0 –Weak base

19. 19 Strong bases Metal (I) hydroxides Metal (I) hydroxides Ba(OH) 2 Ba(OH) 2

20. 20

21. 21 Comparing acid/base strengths Can use a logarithmic scale Can use a logarithmic scale pK a = -log K a As K a increases acid strength increases As K a increases acid strength increases But what about pK a ? Does it increase or decrease as K a increases? But what about pK a ? Does it increase or decrease as K a increases? What can we say about acid strength and pK a ? What can we say about acid strength and pK a ?

22. 22 Conversely pK b = -log K b Likewise, as pK b decreases base strength increases Likewise, as pK b decreases base strength increases

23. 23 The relation between K a and K b HCN (aq) + H 2 O (l)  H 3 O + (aq) + CN - (aq) K a = 4.0 x 10 -10 CN - (aq) + H 2 O (l)  HCN (aq) + OH - (aq) K b = 2.5 x 10 -5 Net rxn: 2H 2 O (l)  H 3 O + (aq) + OH - (aq) K net = 1.0 x 10 -14 = K w

24. 24 Hence And pK w = pK a + pK b = 14.00 And pK w = pK a + pK b = 14.00

25. 25 Problem What is the K a and pK a of a 0.10 M soln of chloroacetic acid with a pH of 1.95?

26. 26 Solution

27. 27 Problem What is the pH of a 0.237 M solution of benzoic acid? The pK a is 4.19.

28. 28 Solution

29. 29 Problem What is the pH of a 1.00 M solution of Sodium Acetate? (pK b = 5.6 x 10 -10 ) Why pK b and not pK a ?

30. 30 Solution

31. 31 Another problem What is the pH of a solution that is made from 250.0 mL of 0.250 M KOH and 150.0 mL of 0.0125 M HBr?

32. 32 Solution

33. 33 Which side will be favored? Rxns proceed from stronger acid/base to weaker acid/base Predict the products and the direction of arrow for the following reaction: NH 3(aq) + HCO 3 -  Use table 17.3 on pg. 808

34. 34 Solution

35. 35 More problems For the following write the complete reaction, and determine the correct K value both in symbols (e.g., 1/K a ) and in number. a) Potassium bicarbonate and hydrochloric acid b) The acid dissociation of HCN c) Acetic acid and potassium hydroxide

36. 36 So what really makes it strong vs. weak? 1) Electronegativity: 1) Electronegativity: Acid strength increases as electronegativity of A increases Acid strength increases as electronegativity of A increases 2) Bond strength: 2) Bond strength: Lessens as descend GVII  stronger acid Lessens as descend GVII  stronger acid 3) Larger atomic radius: 3) Larger atomic radius: Increases as descend G Increases as descend G  easier to lose H, stronger acid  easier to lose H, stronger acid

37. 37 Continued 3) Oxyacids 3) Oxyacids The more oxygens the stronger The more oxygens the stronger Ex: in order of increasing acid strength Ex: in order of increasing acid strength HClO < HClO 2 < HClO 3 < HClO 4 HClO < HClO 2 < HClO 3 < HClO 4 O-H bond polarity increases as oxygens are added  easier to remove hydrogen (as proton) O-H bond polarity increases as oxygens are added  easier to remove hydrogen (as proton) Inductive effect = ability of atoms in a molecule to attract electrons from another part of the molecule Inductive effect = ability of atoms in a molecule to attract electrons from another part of the molecule

38. 38 More… Also, more oxygens allow for more stable (delocalized) structures Also, more oxygens allow for more stable (delocalized) structures Compare Lewis structures of deprotonated (conjugate bases) HNO 3 to HNO 2 Compare Lewis structures of deprotonated (conjugate bases) HNO 3 to HNO 2 Allows for greater acidity Allows for greater acidity

39. 39 Carboxylic acids Brønsted-Lowry acids Brønsted-Lowry acids RCO 2 -H + H 2 O  RCO 2 - + H 3 O + RCO 2 -H + H 2 O  RCO 2 - + H 3 O + Give me the resonance structures Give me the resonance structures If Electron Withdrawing Groups are substituted for hydrogens what would happen? If Electron Withdrawing Groups are substituted for hydrogens what would happen? Order of increasing K a Order of increasing K a CH 3 CO 2 H > ClCH 2 CO 2 H > Cl 2 CHCO 2 H > Cl 3 CCO 2 H CH 3 CO 2 H > ClCH 2 CO 2 H > Cl 2 CHCO 2 H > Cl 3 CCO 2 H

40. 40 Anions as Brønsted bases Basicity increased as negative charge of anion increases Basicity increased as negative charge of anion increases PO 4 3- PO 4 3- HPO 4 2- HPO 4 2- H 2 PO 4 - H 2 PO 4 - Which is most basic? Which is most basic? Which has the highest pK b ? Which has the highest pK b ?

41. 41 Salts Will a salt soln be acidic, basic, or neutral? Will a salt soln be acidic, basic, or neutral? Let’s do these: Let’s do these: HCl + NaOH  NaCl + H 2 O HCl + NH 4 OH  NH 4 Cl + H 2 O H 3 PO 4 + 3CsOH  Cs 3 PO 4 + 3H 2 O H 3 PO 4 + 3NH 4 OH  (NH 4 ) 3 PO 4 + 3H 2 O