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 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. 3 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. 4 Acid Strength 4 Its equilibrium position lies far to the right. (HNO 3 )  Yields a weak conjugate base. (NO 3  ) Strong Acid:

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 5 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. 6 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. 7 The pH Scale pH   log[H + ] pH in water ranges from 0 to 14. K w = 1.00  10  14 = [H + ] [OH  ] pK w = = pH + pOH As pH rises, pOH falls (sum = 14.00).

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 8 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. 9 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. 10 Percent Dissociation (Ionization)

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 11 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. 12 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. 13 Polyprotic Acids... can furnish more than one proton (H + ) to the solution.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 14 Acid-Base Properties of Salts

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 15 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. 16 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. 17 Lewis Acids and Bases Lewis Acid: electron pair acceptor Lewis Base: electron pair donor