Basic Chemistry Copyright © 2011 Pearson Education, Inc. 1 Chapter 14 Acids and Bases 14.5 Ionization of Water

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 2 In the ionization of 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(l) + H 2 O(l) H 3 O + (aq) + OH − (aq) Ionization of Water

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 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 as [H 3 O + ] and [OH − ] [H 3 O + ] = 1.0 x 10 −7 M [OH − ] = 1.0 x 10 −7 M

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 4 Acidic Solutions Adding an acid to pure water increases the [H 3 O + ] causes the [H 3 O + ] to exceed 1.0 x 10 −7 M decreases the [OH − ]

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 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 + ]

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 6 Comparison of [H 3 O + ] and [OH − ]

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 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 − ] is obtained from the concentrations in pure water K w = [H 3 O + ][OH − ] K w = [1.0 x 10 −7 M][ 1.0 x 10 −7 M] = 1.0 x 10 −14 Ion Product Constant of Water, K w

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 8 K w in Acids and Bases In neutral, acidic, and basic solutions, the K w at 25 °C is equal to 1.0 x 10 −14.

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 9 Guide to Calculating [H 3 O + ] and [OH − ]

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 10 Example of Calculating [H 3 O + ] What is the [H 3 O + ] if [OH − ] is 5.0 x 10 −8 M? STEP 1 Write the K w for water. K w = [H 3 O + ][OH − ] = 1.0 x 10 −14 STEP 2 Solve the K w for the unknown [H 3 O + ]. K w = [H 3 O + ][OH − ] [OH − ] [OH − ] [H 3 O + ] = 1.0 x 10 −14 [OH − ]

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 11 Example of Calculating [H 3 O + ] (continued) STEP 3 Substitute the known [OH − ] and calculate. [H 3 O + ] = 1.0 x 10 −14 = 2.0 x 10 −7 M 5.0 x 10 −8

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 12 If lemon juice has [H 3 O + ] of 2.0 x 10 −3 M, what is the [OH − ] of the solution? 1) 2.0 x 10 −11 M 2) 5.0 x 10 −11 M 3) 5.0 x 10 −12 M Learning Check

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 13 STEP 1 Write the K w for water. K w = [H 3 O + ][OH − ] = 1.0 x 10 −14 STEP 2 Solve the K w for the unknown [OH − ]. Rearrange the K w to solve for [OH - ] [OH − ] = K w = 1.0 x 10 −14 [H 3 O + ] [H 3 O + ] STEP 3 Substitute the known [H 3 O + ] and calculate. [OH − ] = 1.0 x 10 −14 = 5.0 x 10 −12 M (3) 2.0 x 10 −3 Solution

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 14 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

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 15 STEP 1 Write the K w for water. K w = [H 3 O + ][OH − ] = 1.0 x 10 −14 STEP 2 Solve the K w for the unknown [H 3 O + ]. Rearrange the K w to solve for [H 3 O + ] [H 3 O + ] = 1.0 x 10 −14 [OH − ] STEP 3 Substitute the known [OH − ] and calculate. [H 3 O + ] = 1.0 x 10 −14 = 2.5 x 10 −13 M (3) 4.0 x 10 −2 Solution