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Slide 1 of 27 Chemistry 19.3

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© Copyright Pearson Prentice Hall Slide 2 of 27 Strengths of Acids and Bases Lemons and grapefruits have a sour taste because they contain citric acid. Sulfuric acid is a widely used industrial chemical that can quickly cause severe burns if it comes into contact with skin. You will learn why some acids are weak and some acids are strong. 19.3

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© Copyright Pearson Prentice Hall Strength of Acids and Bases > Slide 3 of 27 Strong and Weak Acids and Bases How does the value of an acid dissociation constant relate to the strength of an acid? 19.3

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Slide 4 of 27 © Copyright Pearson Prentice Hall Strength of Acids and Bases > Strong and Weak Acids and Bases An acid dissociation constant (K a ) is the ratio of the concentration of the dissociated (or ionized) form of an acid to the concentration of the undissociated (nonionized) form. 19.3

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© Copyright Pearson Prentice Hall Slide 5 of 27 Strength of Acids and Bases > Strong and Weak Acids and Bases Weak acids have small K a values. The stronger an acid is, the larger is its K a value. 19.3

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Slide 6 of 27 © Copyright Pearson Prentice Hall Strength of Acids and Bases > Strong and Weak Acids and Bases Strong acids are completely ionized in aqueous solution. Weak acids ionize only slightly in aqueous solution. 19.3

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Slide 7 of 27 © Copyright Pearson Prentice Hall Strength of Acids and Bases > Strong and Weak Acids and Bases In general, 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. 19.3

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Slide 8 of 27 © Copyright Pearson Prentice Hall Strength of Acids and Bases > Strong and Weak Acids and Bases Strong bases dissociate completely into metal ions and hydroxide ions in aqueous solution. Weak bases react with water to form the hydroxide ion and the conjugate acid of the base. 19.3

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Slide 9 of 27 © Copyright Pearson Prentice Hall Strength of Acids and Bases > Strong and Weak Acids and Bases 19.3

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© Copyright Pearson Prentice Hall Strength of Acids and Bases > Slide 10 of 27 Calculating Dissociation Constants How can you calculate an acid dissociation constant (K a ) of a weak acid? 19.3

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© Copyright Pearson Prentice Hall Slide 11 of 27 Strength of Acids and Bases > Calculating Dissociation Constants To find the K a of a weak acid or the K b of a weak base, substitute the measured concentrations of all the substances present at equilibrium into the expression for K a or K b. 19.3

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Slide 12 of 27 © Copyright Pearson Prentice Hall Strength of Acids and Bases > Calculating Dissociation Constants Acid Dissociation Constant The dissociation constant, K a, of ethanoic acid is calculated from the equilibrium concentrations of all of the molecules and ions in the solution. 19.3

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Slide 13 of 27 © Copyright Pearson Prentice Hall Strength of Acids and Bases > Calculating Dissociation Constants 19.3

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Slide 14 of 27 © Copyright Pearson Prentice Hall Strength of Acids and Bases > Calculating Dissociation Constants 19.3

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Slide 15 of 27 © Copyright Pearson Prentice Hall Strength of Acids and Bases > Calculating Dissociation Constants Base Dissociation Constant The dissociation constant, K b, of ammonia is calculated from the equilibrium concentrations of all of the molecules and ions in the solution. 19.3

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Slide 16 of 27 © Copyright Pearson Prentice Hall Strength of Acids and Bases > Calculating Dissociation Constants Concentration and Strength 19.3

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© Copyright Pearson Prentice Hall SAMPLE PROBLEM Slide 17 of 27 19.5

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© Copyright Pearson Prentice Hall SAMPLE PROBLEM Slide 18 of 27 19.5

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© Copyright Pearson Prentice Hall SAMPLE PROBLEM Slide 19 of 27 19.5

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© Copyright Pearson Prentice Hall SAMPLE PROBLEM Slide 20 of 27 19.5

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© Copyright Pearson Prentice Hall Slide 21 of 27 Practice Problems for Sample Problem 19.5 Problem Solving 19.23 Solve Problem 23 with the help of an interactive guided tutorial.

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© Copyright Pearson Prentice Hall Slide 22 of 27 Section Quiz -or- Continue to: Launch: Assess students’ understanding of the concepts in Section 19.3 Section Quiz. 19.3.

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© Copyright Pearson Prentice Hall Slide 23 of 27 19.3 Section Quiz. 1. H 2 S is considered to be a weak acid because it a.is insoluble in water. b.ionizes only slightly. c.is completely ionized. d.is dilute.

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© Copyright Pearson Prentice Hall Slide 24 of 27 19.3 Section Quiz. 2. Calcium hydroxide, Ca(OH) 2, is a strong base because it a.has a large K b. b.has a small K b. c.forms concentrated solutions. d.is highly soluble in water.

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© Copyright Pearson Prentice Hall Slide 25 of 27 19.3 Section Quiz. 3. If the [H + ] of a 0.205M solution of phenol (C 6 H 5 OH) at 25ºC is 2.340 10 -6, what is the K a for phenol? Phenol is monoprotic. a. K a = 2.67 x 10 -11 b. K a = 1.14 x 10 -5 c. K a = 5.48 x 10 -12 d. K a = 1.53 x 10 -3

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© Copyright Pearson Prentice Hall Slide 26 of 27 19.3 Section Quiz. 4. The K a of three acids is given below. (1) 5.1 10 –3 (2) 4.8 10 –11 (3) 6.3 10 –5 Put the acids in order from the strongest acid to the weakest acid. a.1, 3, 2 b.2, 3, 1 c.3, 1, 2 d.2, 1, 3

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© Copyright Pearson Prentice Hall Slide 27 of 27 5. The K b of four bases is given below. (1) 7.41 x 10 -5 (2) 1.78 x 10 -5 (3) 4.27 x 10 -4 (4) 4.79 x 10 -4 Put the bases in order from the strongest base to the weakest base. a.2, 3, 4, 1 b.2, 1, 3, 4 c.4, 3, 1, 2 d.1, 4, 3, 2 19.3 Section Quiz.

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