The Chemistry of Acids and Bases Chapter 17 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

2 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved ACID-BASE THEORIES The most general theory for common aqueous acids and bases is the BRØNSTED - LOWRY theoryThe most general theory for common aqueous acids and bases is the BRØNSTED - LOWRY theory ACIDS DONATE H + IONSACIDS DONATE H + IONS BASES ACCEPT H + IONSBASES ACCEPT H + IONS

3 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Strong Acids HNO 3 + H 2 O --->H 3 O + + NO 3 - Complete reaction, completely dissociated. HNO 3 is about 100% dissociated in water.

4 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved HNO 3, HCl, HBr, HI, H 2 SO 4 and HClO 4 Strong Acids

5 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Weak acids are much less than 100% ionized in water.Weak acids are much less than 100% ionized in water. One of the best known is acetic acid = CH 3 CO 2 H = HOAc HOAc(aq) + H 2 O(liq) OAc - (aq) + H 3 O + (aq) OAc - = CH 3 CO 2 - = acetate ion Weak Acids

6 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Strong Base: 100% dissociated in water.Strong Base: 100% dissociated in water. NaOH(aq) ---> Na + (aq) + OH - (aq) NaOH(aq) ---> Na + (aq) + OH - (aq) Strong Bases LiOH(aq) ---> Na + (aq) + OH - (aq) KOH(aq) ---> Na + (aq) + OH - (aq)

7 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Weak base: less than 100% ionized in waterWeak base: less than 100% ionized in water One of the best known weak bases is ammonia NH 3 (aq) + H 2 O(liq)  NH 4 + (aq) + OH - (aq) Weak Bases

8 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved ACID-BASE THEORIES NH 3 is a BASE in water — and water is itself an ACID NH 3 / NH 4 + is a conjugate pair — related by the gain or loss of H +

9 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved ACID-BASE THEORIES Every acid has a conjugate base - and every base has a conjugate acid. Every acid has a conjugate base - and every base has a conjugate acid.

10 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved ACID-BASE THEORIES A strong acid is 100% dissociated. Therefore, a STRONG ACID—a good H + donor— must have a WEAK CONJUGATE BASE—a poor H + acceptor. HNO 3 (aq) + H 2 O(liq) H 3 O + (aq) + NO 3 - (aq) HNO 3 (aq) + H 2 O(liq) H 3 O + (aq) + NO 3 - (aq) STRONG A base acid weak B STRONG A base acid weak B

11 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved ACID-BASE THEORIES Acetic acid is only 0.42% ionized when [HOAc] = 1.0 M. It is a WEAK ACID HOAc + H 2 O H 3 O + + OAc - WEAK A base acid STRONG B

12 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved ACID-BASE THEORIES Predicting the direction of an acid-base reaction. Use Table 17.3 Reactions always go from the stronger A-B pair to the weaker A-B pair.

13 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Equilibria Involving Weak Acids and Bases AcidConjugate Base AcidConjugate Base acetic, CH 3 CO 2 HCH 3 CO 2 -, acetate ammonium, NH 4 + NH 3, ammonia bicarbonate, HCO 3 - CO 3 2-, carbonate A weak acid (or base) is one that ionizes to a VERY small extent (< 5%). HA + H 2 O  A - + H 3 O +

14 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Equilibria Involving Weak Acids and Bases Aspirin is a good example of a weak acid, K a = 3.2 x 10 -4

15 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Equilibria Involving Weak Acids and Bases Consider acetic acid HOAc + H 2 O H 3 O + + OAc - Acid Conj. base (K is designated K a for ACID) Because [H 3 O + ] and [OAc - ] are SMALL, K a << 1.

16 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Equilibria Involving Weak Acids and Bases Values of K a for acid and K b for bases are found in TABLE 17.4 — page 799 Notice the relation of TABLE 17.4 to the table of relative acid/base strengths (Table 17.3). B + H 2 O  BH + + OH -

17 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved MORE ABOUT WATER H 2 O can function as both an ACID and a BASE. In pure water there can be AUTOIONIZATION

18 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved MORE ABOUT WATER K w = [H 3 O + ] [OH - ] = 1.00 x at 25 o C In a neutral solution [H 3 O + ] = [OH - ] so K w = [H 3 O + ] 2 = [OH - ] 2 and so [H 3 O + ] = [OH - ] = 1.00 x M Autoionization

19 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Calculating [H 3 O + ] & [OH - ] You add mol of NaOH to 1.0 L of pure water. Calculate [HO + ] and [OH - ]. You add mol of NaOH to 1.0 L of pure water. Calculate [H 3 O + ] and [OH - ].Solution 2 H 2 O(liq) HO + (aq) + OH - (aq) 2 H 2 O(liq) H 3 O + (aq) + OH - (aq) Le Chatelier predicts equilibrium shifts to the ____________. [HO + ] < at equilibrium. [H 3 O + ] < at equilibrium.

20 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Calculating [H 3 O + ] & [OH - ] You add mol of NaOH to 1.0 L of pure water. Calculate [HO + ] and [OH - ]. You add mol of NaOH to 1.0 L of pure water. Calculate [H 3 O + ] and [OH - ]. NaOH  Na + + OH - Stoichiometry: 1 mole  1 mole + 1 mole So, [OH - ] = M K w = [HO + ] [OH - ] K w = [H 3 O + ] [OH - ] [HO + ] = 1.0 x / = 1.0 x M [H 3 O + ] = 1.0 x / = 1.0 x M

21 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Calculating [H 3 O + ] & [OH - ] This solution is BASIC because [HO + ] < [OH - ] This solution is BASIC because [H 3 O + ] < [OH - ] ACIDS [HO + ] > [OH - ] ACIDS [H 3 O + ] > [OH - ] NEUTRAL [HO + ] = [OH - ] NEUTRAL [H 3 O + ] = [OH - ]

22 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved [H 3 O + ], [OH - ] and pH pH = log (1/ [H 3 O + ]) = - log [H 3 O + ] In a neutral solution, [HO + ] = [OH - ] = 1.00 x at 25 o C [H 3 O + ] = [OH - ] = 1.00 x at 25 o C pH = - log (1.00 x ) = - (-7) = 7

23 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved [H 3 O + ], [OH - ] and pH What is the pH of the M NaOH solution? [HO + ] = 1.0 x M [H 3 O + ] = 1.0 x M pH = - log (1.0 x ) = General conclusion — Basic solution pH > 7 Basic solution pH > 7 Neutral pH = 7 Neutral pH = 7 Acidic solutionpH < 7 Acidic solutionpH < 7

24 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved [H 3 O + ], [OH - ] and pH If the pH of Coke is 3.12, what is the [HO + ] ? If the pH of Coke is 3.12, what is the [H 3 O + ] ? Because pH = - log [HO + ] then Because pH = - log [H 3 O + ] then 3.12 = - log [HO + ] 3.12 = - log [H 3 O + ] = log [HO + ] = log [H 3 O + ] Take antilog and get [HO + ] = = 7.6 x M [H 3 O + ] = = 7.6 x M

25 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Other pX Scales In generalpX = -log X and so pOH = - log [OH - ] K w = [HO + ] [OH - ] = 1.00 x at 25 o C K w = [H 3 O + ] [OH - ] = 1.00 x at 25 o C Take the log of both sides -log ( ) = - log [HO + ] + (-log [OH - ]) -log ( ) = - log [H 3 O + ] + (-log [OH - ]) 14 = pH + pOH

26 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 - ] initial change-x+x+x equilib xx x