1 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Names of Acids Perchloric Sulfuric Hydrochloric Nitric Sulfurous (H 2 SO 3 ) Phosphoric Acetic Nitrous (HNO 2 ) Dihydrogen phosphate ion H 2 PO 4 - Hydrogen phosphate ion, HPO 4 2- Ammonium,NH 4 +

2 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Weak Acids & Bases Work examples writing reactions. MUST INCLUDE CHARGES FOR IONS!! H 3 PO 4 HOCl Co(H 2 O) 6 2+ HCO 3 -

3 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved FORMULAS Ka (Kb) = Kw pH = - log [H 3 O + ] pOH = -log [OH - ] pH + pOH = 14

4 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Equilibria Involving A Weak Acid You have 1.00 M HOAc. Calc. the equilibrium concs. of HOAc, H 3 O +, OAc -, and the pH. Step 1. Define equilibrium concs. [HOAc][H 3 O + ][OAc - ] [HOAc][H 3 O + ][OAc - ]initialchangeequilib

5 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Equilibria Involving A Weak Acid Step 2. Write K a expression You have 1.00 M HOAc. Calc. the equilibrium concs. of HOAc, H 3 O +, OAc -, and the pH. This is a quadratic. Solve using quadratic formula or method of approximations (see Appendix A).

6 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Useful Rule of Thumb: If 100 K a < C o, then okay to assume not much dissociated

7 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Calculate the pH of a M solution of formic acid, HCO 2 H. HCOOH + H 2 O  HCOO - + H 3 O + HCOOH + H 2 O  HCOO - + H 3 O + K a = 1.8 x Equilibria Involving A Weak Acid

8 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Equilibria Involving A Weak Base You have M NH 3. Calc. the pH. NH 3 + H 2 O NH OH - NH 3 + H 2 O NH OH - K b = 1.8 x Step 1. Define equilibrium concs. [NH 3 ][NH 4 + ][OH - ] [NH 3 ][NH 4 + ][OH - ]initialchangeequilib

9 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved NH 4 Cl(aq) ----> NH 4 + (aq) + Cl - (aq) (a)Reaction of Cl - with H 2 O Cl - + H 2 O ---->HCl + OH - Cl - + H 2 O ---->HCl + OH - baseacidacidbase baseacidacidbase Cl - ion is a VERY weak base because its conjugate acid is strong. Kb Cl - (very small) Acid-Base Properties of Salts

10 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved NH 4 Cl(aq) ----> NH 4 + (aq) + Cl - (aq) (b)Reaction of NH 4 + with H 2 O NH H 2 O ---->NH 3 + H 3 O + NH H 2 O ---->NH 3 + H 3 O + acidbasebaseacid acidbasebaseacid NH 4 + ion is a moderate acid because its conjugate base is weak. Ka NH 4 + = 5.6 x Acid-Base Properties of Salts

11 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Calculate the pH of a 0.10 M solution of Na 2 CO 3. Na + + H 2 O ---> neutral CO H 2 O HCO 3 - +OH - baseacidacidbase K b = 2.1 x K b = 2.1 x Step 1.Set up concentration table [CO 3 2- ][HCO 3 - ][OH - ] [CO 3 2- ][HCO 3 - ][OH - ] initial initial change equilib equilib Acid-Base Properties of Salts

12 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Exercise NaCl FeCl 3 NH 4 NO 3 Na 2 HPO 4

Why?Why? Why do acids and bases vary in strength?Why do acids and bases vary in strength? Can we predict variations in acidity or basicity?Can we predict variations in acidity or basicity? Why do acids and bases vary in strength?Why do acids and bases vary in strength? Can we predict variations in acidity or basicity?Can we predict variations in acidity or basicity? Copyright © 1999 by Harcourt Brace & Company All rights reserved. Requests for permission to make copies of any part of the work should be mailed to: Permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida

14 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved 1. Electronegativity of O 2.Polarity of O—H bond 3.The H atom of O—H is readily attracted to polar H 2 O. Acetic acid Partialcharges (-) (+) (+) (-)

15 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved High electronegativity of Cl withdraws electrons from the rest of the molecule. This makes the O—H bond highly polar. The H of O—H is very positive. Acetic acid Trichloroacetic acid K a = 1.8 x K a = 0.3

16 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved These ions are BASES. As the charge goes up, they interact more strongly with polar water molecules & become more and more basic. NO 3 - CO 3 2- Basicity of Oxoanions PO 4 3-

17 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Lewis acid = electron pair acceptor (BF 3 )Lewis acid = electron pair acceptor (BF 3 ) Lewis base = electron pair donor (NH 3 )Lewis base = electron pair donor (NH 3 ) Lewis Acids & Bases

18 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved A Lewis acid and base can interact by sharing an electron pair. Lewis Acids & Bases

19 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved A Lewis acid and base can interact by sharing an electron pair. Formation of hydronium ion is an excellent example. Lewis Acids & Bases +

20 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Other good examples involve metal ions. Lewis Acids & Bases

21 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Other good examples involve metal ions. COORDINATE COVALENT BONDS because both electrons are supplied by one of the atoms of the bond. Lewis Acids & Bases

22 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved The combination of metal ions (Lewis acids) with Lewis bases such as H 2 O and NH > COMPLEX IONS All metal ions form complex ions with water —and are of the type [M(H 2 O) x ] n+ where x = 4 and 6. Lewis Acids & Bases [Cu(NH 3 ) 4 ] 2+ [Cu(NH 3 ) 4 ] 2+

23 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved [Ni(H 2 O) 6 ] NH 3 ---> [Ni(NH 3 ) 6 ] 2+ [Ni(H 2 O) 6 ] NH 3 ---> [Ni(NH 3 ) 6 ] 2+ + DMG Lewis Acids & Bases

24 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved The Fe 2+ in heme can interact with O 2 or CO in a Lewis acid-base reaction. Lewis Acids & Bases

25 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Many complex ions containing water undergo HYDROLYSIS to give acidic solutions. Lewis Acids & Bases

26 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Many complex ions containing water undergo HYDROLYSIS to give acidic solutions. [Cu(H 2 O) 4 ] 2+ + H 2 O ---> [Cu(H 2 O) 3 (OH)] + + H 3 O + Lewis Acids & Bases

27 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Many complex ions containing water undergo HYDROLYSIS to give acidic solutions. This explains why water solutions of Fe 3+, Al 3+, Cu 2+, Pb 2+, etc. are acidic. Lewis Acids & Bases

28 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved AMPHOTERIC ( some metal hydroxides) Al(OH) 3 (s) + 3 H + --> Al H 2 O Here Al(OH) 3 is a Brønsted base. Al(OH) 3 (s) + OH - --> Al(OH) 4 - Here Al(OH) 3 is a Lewis acid. Lewis Acids & Bases

29 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Neutral Lewis Acid Carbon dioxide is a neutral Lewis acid

30 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Many complex ions are very stable. Cu NH 3 [Cu(NH 3 ) 4 ] 2+ K for the reaction is called K formation or a “formation constant” Here K = 6.8 x Reaction is strongly product-favored. Lewis Acids & Bases

31 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Formation of complex ions explains why you can dissolve a ppt. by forming a complex ion. AgCl(s) + 2 NH 3 Ag(NH 3 ) Cl - Ag(NH 3 ) Cl - Lewis Acids & Bases AgCl(s)

32 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Formation of complex ions explains why you can dissolve a ppt. by forming a complex ion. AgCl(s) Ag + + Cl - K sp = 1.8 x Ag NH 3 --> Ag(NH 3 ) 2 + K form = 1.6 x AgCl(s) + 2 NH 3 Ag(NH 3 ) Cl - K net = K sp K form = 2.9 x K net = K sp K form = 2.9 x Lewis Acids & Bases

33 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Exercise Lewis acid or base? PH 3 BCl 3 H 2 S HS -