Chemistry 1011 TOPIC TEXT REFERENCE Acids and Bases Masterton and Hurley Chapter 4.2 (Review), 13, 14.1, 15.1 (page 427), 21.2 (page589) Chemistry 1011 Slot 5

13.2 Water Ionization Constant YOU ARE EXPECTED TO BE ABLE TO: Identify KW as the ionization constant for water. Define a solution as acidic, basic or neutral based upon the concentration of hydrogen ions. Given KW and one of [H+] and [OH-], calculate the other. Chemistry 1011 Slot 5

The Ionization of Water Water molecules can be amphiprotic H2O(l) + H2O(l) H3O+(aq) + OH-(aq) This can be written more simply as H2O(l) H+(aq) + OH-(aq) The equilibrium constant for this reaction is KW = [H+]x[OH-] [H2O] But [H2O] is constant, so KW = [H+]x[OH-] Chemistry 1011 Slot 5

The Value of KW KW is known as the ionization constant for water The value of KW is very small KW = 1.0 x 10-14 In a pure water, [H+] = [OH-] = 1.0 x 10-7 This is a neutral solution Chemistry 1011 Slot 5

Acidic and Basic Solutions In an acidic solution, [H+] > 1.0 x 10-7 [OH-] < 1.0 x 10-7 The product, KW is always = 1.0 x 10-14 In a basic solution, [H+] < 1.0 x 10-7 [OH-] > 1.0 x 10-7 Chemistry 1011 Slot 5

Example If a water sample has a hydrogen ion concentration of 4.0 x 10-5, what is the concentration of OH-? Is the solution acidic or basic? Remember KW = [H+]x[OH-] = 1.0 x 10-14 If [H+] = 4.0 x 10 -5, then [OH-] = 1.0 x 10-14 = 2.5 x 10-10 4.0 x 10 -5 The sample is acidic Chemistry 1011 Slot 5