Hydrogen Ions and Acidity Prentice-Hall Chapter 19.2 Dr. Yager

Objectives  Describe how [H + ] and [OH - ] are related in an aqueous solution  Classify a solution as neutral, acidic, or basic given the hydrogen-ion or hydroxide-ion concentration  Convert hydrogen-ion concentration into pH values and hydroxide-ion concentrations into pOH values  Describe the purpose of an acid-base indicator

To test a diagnosis of diabetic coma, a doctor tests for the acidity of the patient’s blood. Results from this test will be expressed in units of pH. You will learn how the pH scale is used to indicate the acidity of a solution.

Self-Ionization of Water The reaction in which water molecules produce ions is called the self-ionization of water.

In the self-ionization of water, a proton (hydrogen ion) transfers from one water molecule to another water molecule.

The self-ionization of water is a reversible reaction. The concentrations of hydronium and hydroxide ions are equal and known, that is: [H 3 O + ] = [OH - ] = 1 x M In any aqueous solution when [H 3 O + ] and [OH - ] are equal the solution is neutral.   

Ion Constant Product for Water Calculate the equilibrium constant for the reaction of self-ionization of water: H 2 O (l) + H 2 O (l) H 3 O + (aq) +OH - (aq) H 2 O (l) + H 2 O (l)  H 3 O + (aq) + OH - (aq) [H 3 O + ] [OH - ] = ( 1.0 x ) ( 1.0 x ) = 1.0  or K w = 1.0 

The product of the concentrations of hydrogen ions and hydroxide ions in water is called the ion-product constant for water (Kw).

Acidic Solution Acidic Solution An acidic solution is one in which [H + ] is greater than [OH - ].

Unrefined hydrochloric acid, commonly called muriatic acid, is used to clean stone buildings and swimming pools.

Basic Solution A basic solution is one in which [H+] is less than [OH]. Basic solutions are also known as alkaline solutions.

Sodium hydroxide, or lye, is commonly used as a drain cleaner.

The pH Concept What does pH stand for? pOWER of Hydrogen

The pH Concept The pH of a solution is the negative logarithm of the hydrogen-ion concentration.

Calculating Neutral pH A solution in which [H + ] is greater than 1  M has a pH less than 7.0 and is acidic. The pH of pure water or a neutral aqueous solution is 7.0. A solution with a pH greater than 7 is basic and has a [H + ] of less than 1  M.

pH and [H + ]

Calculating pOH

Measuring pH An indicator is a valuable tool for measuring pH because its acid form and base form have different colors in solution. Phenolphthalein changes from colorless to pink at pH 8–9.

Acid-Base Indicators

Universal Indicators

1. If the [OH - ] in a solution is 7.65  M, what is the [H + ] of this solution? a)7.65  M b)1.31  M c)2.12M d)11.88M

1. If the [OH - ] in a solution is 7.65  M, what is the [H + ] of this solution? a)7.65  M b)1.31  M c)2.12M d)11.88M

2. The [OH - ] for four solutions is given below. Which one of the solutions is basic? a)1.0 x M b)1.0 x M c)1.0 x M d)1.0 x M

2. The [OH - ] for four solutions is given below. Which one of the solutions is basic? a)1.0 x M b)1.0 x M c)1.0 x M d)1.0 x M

3. What is the pH of a solution with a hydrogen-ion concentration of 8.5 x M? a)12.93 b)8.50 c)5.50 d)1.07

3. What is the pH of a solution with a hydrogen-ion concentration of 8.5 x M? a)12.93 b)8.50 c)5.50 d)1.07