1 Chapter 8 Acids and Bases 8.4 Ionization of Water Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

2 In water, H + is transferred from one H 2 O molecule to another. one water molecule acts as an acid, while another acts as a base. H 2 O + H 2 O H 3 O + + OH − :O: H + H:O: H:O:H + + :O:H − H H H water water hydronium hydroxide ion (+) ion (-) Ionization of Water

3 Pure Water is Neutral In pure water, the ionization of water molecules produces small, but equal quantities of H 3 O + and OH − ions. molar concentrations are indicated in brackets as [H 3 O + ] and [OH − ]. [H 3 O + ] = 1.0 x 10 − 7 M [OH − ] = 1.0 x 10 − 7 M Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

4 Acidic Solutions Adding an acid to pure water increases the [H 3 O + ]. cause the [H 3 O + ] to exceed 1.0 x M. decreases the [OH − ].

5 Basic Solutions Adding a base to pure water increases the [OH − ]. causes the [OH − ] to exceed 1.0 x 10 − 7 M. decreases the [H 3 O + ]. Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

6 Comparison of [H 3 O + ] and [OH − ]

7 The ion product constant, K w, for water is the product of the concentrations of the hydronium and hydroxide ions. K w = [ H 3 O + ] [ OH − ] can be obtained from the concentrations in pure water. K w = [ H 3 O + ] [ OH − ] K w = [1.0 x 10 − 7 M] x [ 1.0 x 10 − 7 M] = 1.0 x 10 − 14 Ion Product of Water, K w

8 [H 3 O + ] and [OH − ] in Solutions In neutral, acidic, or basic solutions, the K w is always 1.0 x 10 −14.

9 Guide to Calculating [H 3 O + ]

10 Calculating [H 3 O + ] What is the [H 3 O + ] of a solution if [OH − ] is 5.0 x M? STEP 1: Write the K w for water. K w = [H 3 O + ][OH − ] = 1.0 x 10 −14 STEP 2: Rearrange the K w expression. [H 3 O + ] = 1.0 x [OH − ] STEP 3: Substitute [OH − ]. [H 3 O + ] = 1.0 x = 2.0 x M 5.0 x

11 If lemon juice has [H 3 O + ] of 2 x 10 −3 M, what is the [OH − ] of the solution? 1) 2 x 10 −11 M 2) 5 x 10 −11 M 3) 5 x 10 −12 M Learning Check

12 3) 5 x 10 −12 M Rearrange the K w to solve for [OH - ] K w = [H 3 O + ][OH − ] = 1.0 x 10 −14 [OH − ] = 1.0 x = 5 x 10 −12 M 2 x Solution

13 The [OH − ] of an ammonia solution is 4.0 x 10 −2 M. What is the [H 3 O + ] of the solution? 1) 2.5 x 10 − 11 M 2) 2.5 x 10 −12 M 3) 2.5 x 10 −13 M Learning Check

14 3) 2.5 x 10 −13 M [ H 3 O + ] = 1.0 x 10 −14 = 2.5 x 10 −13 M 4.0 x 10 −2 Solution