Copyright©2000 by Houghton Mifflin Company. All rights reserved. 1 Models of Acids and Bases Arrhenius Concept: Acids produce H + in solution, bases produce OH  ion. Brønsted-Lowry: Acids are H + donors, bases are proton acceptors. HCl + H 2 O  Cl  + H 3 O + acid base

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 2 The Lewis Acid-Base Model A Lewis acid is an e - pair acceptor. A Lewis base is an e - pair donor. NH 3(g) + HCl (g) ⇄ NH 4 Cl (s) In this rxn, a proton is donated by the hydrogen chloride to the ammonia. See the rxn on the whiteboard please.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 3 Figure 14.1 The Reaction of HCI and H 2 O

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 4 Conjugate Acid/Base Pairs HA(aq) + H 2 O(l) ⇄ H 3 O + (aq) + A  (aq) conj conj conj conj acid 1 base 2 acid 2 base 1 conjugate base: everything that remains of the acid molecule after a proton is lost. conjugate acid: formed when the proton is transferred to the base.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 5 Figure 14.2 The Reaction of an Acid with Water

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 6 Acid Dissociation Constant (K a ) HA(aq) + H 2 O(l) ⇄ H 3 O + (aq) + A  (aq)

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 7 Acid Strength 4 Its equilibrium position lies far to the right. (HNO 3 )  Yields a weak conjugate base (compared to water) (NO 3  ) Strong Acid:

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 8 Acid Strength (continued) 4 Its equilibrium lies far to the left. (CH 3 COOH)  Yields a much stronger (it is relatively strong) conjugate base than water. (CH 3 COO  ) Weak Acid:

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 9 Figure 14.4 Graphic Representation of the Behavior of Acids of Different Strengths in Aqueous Solution

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 10 Figure 14.5 Acid Strength Versus Conjugate Base Strength

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 11 Figure 14.6 A Strong Acid (a) and a Weak Acid (b) in Water

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 12 Common Strong Acids Sulfuric (first ionization only is strong) (H 2 SO 4 ) Hydrochloric (HCl) Hydrobromic (HBr) Hydroiodic (HI) Nitric (HNO 3 ) Perchloric (HClO 4 )

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 13 Common Weak Acids All those that are not strong acids! All those with a K a < 1 See Table 14.2, p.663 (no need to memorize).

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 14 Mo’ Acid Terms Monoprotic, diprotic, triprotic Oxyacids: acidic H attached to an O atom. Organic acids: almost always contain the carboxyl group, -COOH (see the board).

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 15 Water as an Acid and a Base Water is amphoteric (it can behave either as an acid or a base). H 2 O + H 2 O ⇄ H 3 O + + OH  conj conj acid 1 base 2 acid 2 base 1 K w = 1  10  14 at 25°C

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 16 Figure 14.7 Two Water Molecules React to Form H 3 O + and OH -

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 17 The pH Scale pH   log[H + ] pH in water ranges from 0 to 14 (at 25 ˚C). K w = 1.00  10  14 = [H + ] [OH  ] pK w = = pH + pOH As pH rises, pOH falls (sum = 14.00). Strange sig fig rule: pay attention.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 18 Figure 14.8 The pH Scale (at 25 ˚C) and pH Values of Some Common Substances

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 19 Solving Weak Acid Equilibrium Problems 4 List major species in solution. 4 Choose species that can produce H + and write reactions. 4 Based on K values, decide on dominant equilibrium. 4 Write equilibrium expression for dominant equilibrium. 4 List initial concentrations in dominant equilibrium.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 20 Solving Weak Acid Equilibrium Problems (continued) 4 Define change at equilibrium (as “x”). 4 Write equilibrium concentrations in terms of x. 4 Substitute equilibrium concentrations into equilibrium expression. 4 Solve for x the “easy way.” 4 Verify assumptions using 5% rule. 4 Calculate [H + ] and pH.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 21 Percent Dissociation (Ionization)

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 22 Figure The Effect of Dilution on the Percent Dissociation and (H+) of a Weak Acid Solution

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 23 Bases “Strong” and “weak” are used in the same sense for bases as for acids. strong = complete dissociation (hydroxide ion supplied to solution) NaOH(s)  Na + (aq) + OH  (aq)

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 24 Bases (continued) weak = very little dissociation (or reaction with water) H 3 CNH 2 (aq) + H 2 O(l)  H 3 CNH 3 + (aq) + OH  (aq)

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 25 Polyprotic Acids... can furnish more than one proton (H + ) to the solution.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 26 Polyprotic Acids Always dissociate in a stepwise manner. See the carbonic and phosphoric acid examples on pp 688 – 689. Also see Table 14.4, p See S/E 14.15, pp 689 – 691

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 27 Polyprotic Acids: H 2 SO 4 Sulfuric acid is unique in that the first dissociation step is strong and the second dissociation step is weak. See S/E 14.16, pp. 691 – 692. Here, only the first dissociation is significant

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 28 Polyprotic Acids: H 2 SO 4 Be careful! Percent dissociation increases as a solution of weak acid gets more dilute. So, if [init] < 1.0M for H 2 SO 4, the second dissociation step may be significant! See S/E 14.17, pp 693 – 694. Also see the Methods Box on p. 694.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 29 Acid-Base Properties of Salts

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 30 Structure and Acid-Base Properties Two factors for acidity in binary compounds: 4 Bond Polarity (high is good) 4 Bond Strength (low is good)

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 31 Oxides Acidic Oxides (Acid Anhydrides):  O  X bond is strong and covalent. SO 2, NO 2, CrO 3 Basic Oxides (Basic Anhydrides):  O  X bond is ionic. K 2 O, CaO

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 32 Lewis Acids and Bases Lewis Acid: electron pair acceptor Lewis Base: electron pair donor

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 33 Figure The AI(H 2 O)6 3 + Ion