19.3 Strengths of Acids and Bases > Chapter 19 Acids, Bases, and Salts Strengths of Acids and Bases

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1 Chapter 19 Acids, Bases, and Salts 19.1 Acid-Base Theories 19.2 Hydrogen Ions and Acidity 19.3 Strengths of Acids and Bases 19.4 Neutralization Reactions 19.5 Salts in Solution 1 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

2 Acids and bases are classified as strong or weak based on the degree to which they ionize in water. 2 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

3 In general, a strong acid is completely ionized in aqueous solution. Hydrochloric and sulfuric acid are examples of strong acids. HCl(g) + H 2 O(l) H 3 O + (aq) + Cl (aq) 100% 3 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

4 A weak acid ionizes only slightly in aqueous solution. The ionization of ethanoic acid (CH 3 COOH), a typical weak acid, is not complete. 4 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

5 Interpret Data 5 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

6 Interpret Graphs Dissociation of an acid (HA) in water yields H 3 O + and an anion, A. The bar graphs compare the extent of the dissociation of a strong acid and a weak acid. 6 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

7 Acid Dissociation Constant A strong acid, such as hydrochloric acid, completely dissociates in water. As a result, [H 3 O + ] is high in an aqueous solution of strong acid. 7 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

8 Acid Dissociation Constant By contrast, weak acids remain largely undissociated. In an aqueous solution of ethanoic acid, less than 1 percent of the molecules are ionized. 8 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

9 Acid Dissociation Constant You can use a balanced equation to write the equilibrium-constant expression for a reaction. The equilibrium-constant expression shown below is for ethanoic acid. K eq = [H 3O + ] [CH 3 COO ] [CH 3 COOH] [H 2 O] 9 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

10 Acid Dissociation Constant The acid dissociation constant (K a ) is the ratio of the concentration of the dissociated form of an acid to the concentration of the undissociated form. The dissociated form includes both the H 3 O + and the anion. K eq [H 2 O] = K a = [H 3 O + ] [CH 3 COO ] [CH 3 COOH] [H 2 O] 10 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

11 Acid Dissociation Constant The acid dissociation constant (K a ) reflects the fraction of an acid that is ionized. For this reason, dissociation constants are sometimes called ionization constants. 11 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

12 Acid Dissociation Constant If the degree of dissociation or ionization of the acid is small, the value of the dissociation constant will be small. Weak acids have small K a values. If the degree of ionization of an acid is more complete, the value of K a will be larger. The stronger an acid is, the larger its K a value will be. 12 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

13 Acid Dissociation Constant Nitrous acid (HNO 2 ) has a K a of , and ethanoic acid (CH 3 COOH) has a K a of Which one is stronger? 13 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

14 Answer Nitrous acid is stronger 14 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

15 Interpret Data 15 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

16 Acid Dissociation Constant Some acids have more than one dissociation constant because they have more than one ionizable hydrogen. Oxalic acid is a diprotic acid. It loses two hydrogens, one at a time. Therefore, it has two dissociation constants. Oxalic acid is found naturally in certain herbs and vegetables. 16 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

17 Acid Dissociation Constant Observe what happens to the K a with each ionization. The K a decreases from first ionization to second. It decreases again from second ionization to third. 17 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

18 Calculating Dissociation Constants To calculate the acid dissociation constant (K a ) of a weak acid, you need to know the initial molar concentration of the acid and the [H + ] (or alternatively, the ph) of the solution at equilibrium. You can use these data to find the equilibrium concentrations of the acid and the ions. These values are then substituted into the expression for K a. 18 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

19 Calculating Dissociation Constants You can find the K a of an acid in water by substituting the equilibrium concentrations of the acid, [HA], the anion from the dissociation of the acid, [A ], and the hydrogen ion, [H + ], into the equation below. K a = [H + ][A ] [HA] 19 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

20 Sample Problem 19.6 Calculating a Dissociation Constant In a M solution of ethanoic acid, [H + ] = M. Calculate the K a of this acid. Refer to the table for the ionization equation for ethanoic acid. 20 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

21 Sample Problem Analyze List the knowns and the unknown. KNOWNS [ethanoic acid] = M [H + ] = M UNKNOWN K a =? 21 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

22 Sample Problem Calculate Solve for the unknown. Start by determining the equilibrium concentration of the ions. [H + ] = [CH 3 COO ] = M Each molecule of CH 3 COOH that ionizes gives an H + ion and a CH 3 COO ion. 22 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

23 Sample Problem Calculate Solve for the unknown. Determine the equilibrium concentrations of each component. ( )M = M Concentration [CH 3 COOH] [H + ] [CH 3 COO ] Initial Change Equilibrium Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

24 Sample Problem Calculate Solve for the unknown. Substitute the equilibrium values into the expression for K a. K a = [H + ] [CH 3 COO ] [CH 3 COOH] = ( M) ( M) = Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

25 Sample Problem Evaluate Does the result make sense? The calculated value of K a is consistent with that of a weak acid. 25 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

26 Base Dissociation Constant Just as there are strong acids and weak acids, there are strong bases and weak bases. 26 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

27 Base Dissociation Constant Just as there are strong acids and weak acids, there are strong bases and weak bases. A strong base dissociates completely into metal ions and hydroxide ions in aqueous solution. 27 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

28 Base Dissociation Constant Just as there are strong acids and weak acids, there are strong bases and weak bases. A strong base dissociates completely into metal ions and hydroxide ions in aqueous solution. A weak base reacts with water to form the conjugate acid of the base and hydroxide ions. For a weak base, the amount of dissociation is relatively small. 28 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

29 Base Dissociation Constant Ammonia is an example of a weak base. Window cleaners often use a solution of ammonia in water to clean glass. NH 3 (aq) + H 2 O(l) Ammonia Water Ammonium ion NH 4+ (aq) + OH (aq) Hydroxide ion 29 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

30 Base Dissociation Constant When equilibrium is established, only about 1 percent of the ammonia is present as NH 4+. This ion is the conjugate acid of NH 3. The concentrations of NH 4 + and OH are low and equal. NH 3 (aq) + H 2 O(l) Ammonia Water Ammonium ion NH 4+ (aq) + OH (aq) Hydroxide ion 30 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

31 Base Dissociation Constant The equilibrium-constant expression for the dissociation of ammonia in water is as follows: [NH 4+ ] [OH ] K eq = [NH 3 ] [H 2 O] NH 3 (aq) + H 2 O(l) Ammonia Water Ammonium ion NH 4+ (aq) + OH (aq) Hydroxide ion 31 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

32 Base Dissociation Constant Recall that the concentration of water is constant in dilute solutions. This constant can be combined with the K eq for ammonia to give a base dissociation constant (K b ) for ammonia. K eq [H 2 O] = K b = [NH 4+ ] [OH ] [NH 3 ] 32 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

33 Base Dissociation Constant The base dissociation constant (K b ) is the ratio of the concentration of the conjugate acid times the concentration of the hydroxide ion to the concentration of the base. K b = [conjugate acid] [OH ] [base] 33 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

34 Base Dissociation Constant The magnitude of K b indicates the ability of a weak base to compete with the very strong base OH for hydrogen ions. 34 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

35 Base Dissociation Constant The magnitude of K b indicates the ability of a weak base to compete with the very strong base OH for hydrogen ions. Because bases such as ammonia are weak relative to the hydroxide ion, the K b for such a base is usually small. The K b for ammonia is Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

36 Base Dissociation Constant The magnitude of K b indicates the ability of a weak base to compete with the very strong base OH for hydrogen ions. Because bases such as ammonia are weak relative to the hydroxide ion, the K b for such a base is usually small. The K b for ammonia is The smaller the value of K b, the weaker the base. 36 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

37 Concentration Versus Strength Sometimes people confuse the concepts of concentration and strength. The words concentrated and dilute indicate how much of an acid or base is dissolved in solution. These terms refer to the number of moles of the acid or base in a given volume. The words strong and weak refer to the extent of ionization or dissociation of an acid or base. 37 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

38 Interpret Data Concentration Versus Strength The table below shows four possible combinations of concentration and strength for acids. Comparing Concentration and Strength of Acids Concentration Acidic solution Quantitative (or Molar) Relative Strength Hydrochloric acid 12M HCl Concentrated Strong Gastric juice 0.8M HCl Dilute Strong Ethanoic acid 17M CH 3 COOH Concentrated Weak Vinegar 0.2M CH 3 COOH Dilute Weak 38 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

39 Concentration Versus Strength The gastric juice in your stomach is a dilute solution of HCl. The relatively small number of HCl molecules in a given volume of gastric juice are all dissociated into ions. Even when concentrated hydrochloric acid is diluted with water, it is still a strong acid. 39 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

40 Concentration Versus Strength Conversely, ethanoic acid (acetic acid) is a weak acid because it ionizes only slightly in solution. Vinegar is a dilute solution of ethanoic acid. Even at a high concentration, ethanoic acid is still a weak acid. 40 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

41 Concentration Versus Strength The same concepts apply to bases. A solution of ammonia can be either dilute or concentrated. However, in any solution of ammonia, the relative amount of ionization will be small. Thus, ammonia is a weak base at any concentration. Likewise, sodium hydroxide is a strong base at any concentration. 41 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

42 Key Concept & Key Equation Acids and bases are classified as strong or weak based on the degree to which they ionize in water. K a = [H + ][A ] [HA] 42 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

43 Example problem 1 43 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

44 Example problem 2 44 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.