Copyright © McGraw-Hill Education. Permission required for reproduction or display Chapter 13: Acids and Bases

Copyright © McGraw-Hill Education. Permission required for reproduction or display Questions for Consideration 1. How do acids and bases differ from other substances? 2. How do strong and weak acids differ? 3. How can we compare the different strengths of weak acids? 4. What causes aqueous solutions to be acidic or basic? 5. How is pH related to acidity and basicity? 6. What is a buffered solution?

Copyright © McGraw-Hill Education. Permission required for reproduction or display Chapter 13 Topics 1. What Are Acids and Bases? 2. Strong and Weak Acids and Bases 3. Relative Strengths of Weak Acids 4. Acidic, Basic, and Neutral Solutions 5. The pH Scale 6. Buffered Solutions

Copyright © McGraw-Hill Education. Permission required for reproduction or display Log and Inverse Log Functions Chapter 13 Math Toolbox:

Copyright © McGraw-Hill Education. Permission required for reproduction or display Acids and Bases Affect Our Lives Figure 13.1 Figure 13.2 Figure 13.3 Figure from p. 536

Copyright © McGraw-Hill Education. Permission required for reproduction or display What are Acids and Bases? Acids and bases have properties that differ from other substances: Acids and bases have properties that differ from other substances: Acids taste sour. Acids taste sour. Bases taste bitter and feel slippery. Bases taste bitter and feel slippery. Both change the color of some dyes. Both change the color of some dyes. Acids cause many metals to corrode. Acids cause many metals to corrode. Acids and bases combine to neutralize each other. Acids and bases combine to neutralize each other. CAUTION: Do not taste laboratory chemicals.

Copyright © McGraw-Hill Education. Permission required for reproduction or display Acid and Base Definitions 1800’s Arrhenius Model 1800’s Arrhenius Model An acid in water produces hydrogen (H + ) ions. An acid in water produces hydrogen (H + ) ions. A base in water produces hydroxide (OH  ) ions. A base in water produces hydroxide (OH  ) ions. HCl(g)  H + (aq) + Cl  (aq) HCl(g)  H + (aq) + Cl  (aq) NaOH(s)  Na + (aq) + OH  (aq) NaOH(s)  Na + (aq) + OH  (aq) Arrhenius earned the Nobel prize for his work that showed that H + (aq) and OH  (aq) ions are important in acid-base chemistry. Arrhenius earned the Nobel prize for his work that showed that H + (aq) and OH  (aq) ions are important in acid-base chemistry.

Copyright © McGraw-Hill Education. Permission required for reproduction or display The Hydronium Ion Problem with Arrhenius Model: Problem with Arrhenius Model: H + does not exist completely free in aqueous solution. It associates strongly with other water molecules. H + does not exist completely free in aqueous solution. It associates strongly with other water molecules. Chemists recognize this by representing an aqueous H + ion as H 3 O + (aq), the hydronium ion. Chemists recognize this by representing an aqueous H + ion as H 3 O + (aq), the hydronium ion. Figure 13.4

Copyright © McGraw-Hill Education. Permission required for reproduction or display Acid and Base Definitions 1923 Brønsted-Lowry definitions: 1923 Brønsted-Lowry definitions: An acid donates an H + ion to another substance. An acid donates an H + ion to another substance. A base accepts an H + from another substance. A base accepts an H + from another substance. HCl(g) + H 2 O(l)  H 3 O + (aq) + Cl  (aq) NH 3 (aq) + H 2 O(l)  NH 4 + (aq) + OH  (aq) In an acid-base reaction, the acid donates (transfers) an H + to a base. In an acid-base reaction, the acid donates (transfers) an H + to a base.

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity: Brønsted-Lowry Acids and Bases For each reaction, identify the Brønsted-Lowry acid and base reactants. For each reaction, identify the Brønsted-Lowry acid and base reactants. 1. OCl - (aq) + H 2 O(l)  HOCl(aq) + OH - (aq) 2. H 2 SO 4 (aq) + F - (aq) → HSO 4 - (aq) + HF(aq) 3. NH 4 + (aq) + H 2 O(l)  NH 3 (aq) + H 3 O + (aq)

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity Solutions: Brønsted-Lowry Acids and Bases For each reaction, identify the Brønsted-Lowry acid and base reactants. For each reaction, identify the Brønsted-Lowry acid and base reactants. 1. OCl - (aq) + H 2 O(l)  HOCl(aq) + OH - (aq) base acid base acid The water (H 2 O) donates an H + to the OCl - to form HOCl. The water is an acid. The OCl - accepts an H + from the water (H 2 O), therefore it is the base.

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity Solutions: Brønsted-Lowry Acids and Bases 2. H 2 SO 4 (aq) + F - (aq) → HSO 4 - (aq) + HF(aq) acid base acid base H 2 SO 4, sulfuric acid, is the acid and it donates an H + to the F -. F - accepts H + from the sulfuric acid and is therefore the base. 3. NH 4 + (aq) + H 2 O(l)  NH 3 (aq) + H 3 O + (aq) acid base NH 4 + donates an H + to the water and thus is the acid. H 2 O accepts the H + and is the base.

Copyright © McGraw-Hill Education. Permission required for reproduction or display Acid-base reactions can take place without water: NH 3 (g) + HCl(g)  NH 4 + (g) + Cl  (g) NH 3 (g) + HCl(g)  NH 4 + (g) + Cl  (g) NH 4 + (g) + Cl  (g)  NH 4 Cl(s) NH 4 + (g) + Cl  (g)  NH 4 Cl(s) Figure from p. 532

Copyright © McGraw-Hill Education. Permission required for reproduction or display Brønsted-Lowry Acids and Bases Many anions from dissolved ionic compounds act as bases: Many anions from dissolved ionic compounds act as bases: Na 2 CO 3 (s)  2Na + (aq) + CO 3 2  (aq) The carbonate ion acts as a base in water: The carbonate ion acts as a base in water: CO 3 2  (aq) + H 2 O(l)  HCO 3  (aq) + OH  (aq) base acid base acid and with other acids: and with other acids: HF(aq) + CO 3 2  (aq)  F  (aq) + HCO 3  (aq) acid base acid base

Copyright © McGraw-Hill Education. Permission required for reproduction or display Conjugate Acid-Base Pairs Imagine the following reaction going in the reverse direction. What would be the acid and what would be the base? Imagine the following reaction going in the reverse direction. What would be the acid and what would be the base? HF(aq) + CO 3 2  (aq)  F  (aq) + HCO 3  (aq) We call the acid and base products the conjugates of the base and acid that formed them.

Copyright © McGraw-Hill Education. Permission required for reproduction or display Conjugate Acid-Base Pairs HF(aq) + CO 3 2  (aq)  F  (aq) + HCO 3  (aq) acid base conjugate conjugate acid base conjugate conjugate base acid base acid A conjugate acid-base pair differs only by one proton. A conjugate acid-base pair differs only by one proton. The conjugate base of H 3 PO 4 is H 2 PO 4 -, not PO The conjugate base of H 3 PO 4 is H 2 PO 4 -, not PO 4 3-.

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity: Conjugate Acid-Base Pairs Determine the formula of the conjugate base of each acid. Determine the formula of the conjugate base of each acid. Acid Conjugate Base Acid Conjugate Base H 2 CO 3 H 3 PO 4 HPO 4 2- NH 4 + H 2 O HCO 3  H 2 PO 4 - PO 4 3  NH 3 OH 

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity: Conjugate Acid-Base Pairs Determine the formula of the conjugate acid of each base. Determine the formula of the conjugate acid of each base. Base Conjugate Acid Base Conjugate Acid SO 4 2  HCO 3  NH 2  NH 2  ClO 2  H 2 PO 4  HSO 4  H 2 CO 3 NH 3 HClO 2 H 3 PO 4

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity: Conjugate Acid-Base Pairs Identify the acid and base reactants and their conjugate acid and base: Identify the acid and base reactants and their conjugate acid and base: HCO H 3 PO 4  H 2 CO 3 + H 2 PO 4 - HCO H 3 PO 4  H 2 CO 3 + H 2 PO 4 - base acid conjugate conjugate acid base

Copyright © McGraw-Hill Education. Permission required for reproduction or display Amphoteric Substances A substance that can act as either an acid or a base A substance that can act as either an acid or a base Water is the most common amphoteric substance. Another common amphoteric substance is the bicarbonate ion, HCO 3 - : Water is the most common amphoteric substance. Another common amphoteric substance is the bicarbonate ion, HCO 3 - : HCO 3 - (aq) + OH - (aq) → CO (aq) + H 2 O(l) acidbase conjugate conjugate acidbase conjugate conjugate base acid base acid HCO 3 - (aq) + H 3 O + (aq) → H 2 CO 3 (aq) + H 2 O(l) baseacid conjugate conjugate baseacid conjugate conjugate acid base acid base

Copyright © McGraw-Hill Education. Permission required for reproduction or display Acidic Hydrogen Atoms If an acid has more than one hydrogen atom, we need to determine which hydrogen atoms are acidic. If an acid has more than one hydrogen atom, we need to determine which hydrogen atoms are acidic. In oxoacids, the acidic hydrogen atoms are bonded to hydrogen atoms:. In oxoacids, the acidic hydrogen atoms are bonded to hydrogen atoms:. Figure 13.5

Copyright © McGraw-Hill Education. Permission required for reproduction or display Strong and Weak Acids and Bases Strong and weak acids and bases differ in the extent of ionization. Strong and weak acids and bases differ in the extent of ionization. Strong acids ionize completely. Strong acids ionize completely. Weak acids and bases ionize to only a small extent – a small fraction of the molecules ionize. Weak acids and bases ionize to only a small extent – a small fraction of the molecules ionize.

Copyright © McGraw-Hill Education. Permission required for reproduction or display Strong Acids Strong acids are strong electrolytes. Strong acids are strong electrolytes. Figure 13.6

Copyright © McGraw-Hill Education. Permission required for reproduction or display Strong Acids Table 13.1

Copyright © McGraw-Hill Education. Permission required for reproduction or display Strong Bases Strong bases are strong electrolytes. Strong bases are strong electrolytes. Figure 13.7

Copyright © McGraw-Hill Education. Permission required for reproduction or display Strong Bases *Although the group IIA (2) metal hydroxides are not completely water soluble, they are strong bases because the amount that dissolves dissociates almost completely. Table 13.2

Copyright © McGraw-Hill Education. Permission required for reproduction or display Weak Acids Weak acids are weak electrolytes Weak acids are weak electrolytes Figure 13.8

Copyright © McGraw-Hill Education. Permission required for reproduction or display Common Weak Acids Table 13.3

Copyright © McGraw-Hill Education. Permission required for reproduction or display Weak Bases Weak bases are weak electrolytes Weak bases are weak electrolytes Figure 13.9

Copyright © McGraw-Hill Education. Permission required for reproduction or display Common Weak Bases Table 13.4

Copyright © McGraw-Hill Education. Permission required for reproduction or display Strong and Weak Acids and Bases HCl(aq) HF(aq)

Copyright © McGraw-Hill Education. Permission required for reproduction or display Identify each as a strong or weak acid or base: Figure from p. 563 A. weak base B. weak acid C. strong acid

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity: Acids and Bases on the Molecular Level One of the diagrams below represents HClO 4, and the other represents an aqueous solution of HSO 4 -. Which is which? Explain your reasoning. One of the diagrams below represents HClO 4, and the other represents an aqueous solution of HSO 4 -. Which is which? Explain your reasoning. Figure from p. 540

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity Solutions: Acids and Bases on the Microscopic Level The picture on the left (A) shows no acid molecules and therefore shows an acid that has completely dissociated. The picture on the left (A) shows no acid molecules and therefore shows an acid that has completely dissociated. Diagram A would then be a strong acid and of the two choices, perchloric acid (HClO 4 ) is the strong one. Diagram A would then be a strong acid and of the two choices, perchloric acid (HClO 4 ) is the strong one. Diagram B (the picture on the right) shows acid molecules that have not completely dissociated and are therefore the weak acid, HSO 4 -. Diagram B (the picture on the right) shows acid molecules that have not completely dissociated and are therefore the weak acid, HSO 4 -.

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity: Strong vs. Weak Acids and Bases Identify each of the following as a strong acid, weak acid, strong base, or weak base. Write an equation to describe its reaction in water. Identify each of the following as a strong acid, weak acid, strong base, or weak base. Write an equation to describe its reaction in water. 1. HI(aq) 2. NaCH 3 CO 2 (aq) 3. NH 4 + (aq) 4. NH 3 (aq)

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity Solutions: Strong vs. Weak Acids and Bases 1. HI(aq) Hydroiodic acid is a strong acid (it is on the list of strong acids in Table 13.1). Therefore, it completely dissociates in water: HI(aq) + H 2 O(l) → H 3 O + (aq) + I - (aq) 2. NaCH 3 CO 2 (aq) Sodium acetate is an ionic compound that partially dissociates in water to form Na + (aq) and CH 3 CO 2 - (aq). The CH 3 CO 2 - ion is the conjugate base of the weak acid, HC 2 H 3 O 2, so it is a weak base and does not completely dissociate in water: CH 3 CO 2 - (aq) + H 2 O(l)  CH 3 CO 2 H(aq) + OH - (aq) CH 3 CO 2 - (aq) + H 2 O(l)  CH 3 CO 2 H(aq) + OH - (aq)

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity Solutions: Strong vs. Weak Acids and Bases 3. NH 4 + (aq) Ammonium ion is the conjugate acid of the weak base, ammonia (NH 3 ), so ammonium is therefore a weak acid. It does not completely dissociate in water. NH 4 + (aq) + H 2 O(l)  NH 3 (aq) + H 3 O + (aq) 4. NH 3 (aq) Ammonia is a weak base, shown in Table 13.4, and does not dissociate completely in water. NH 3 (aq) + H 2 O(l)  NH 4 + (aq) + OH - (aq)

Copyright © McGraw-Hill Education. Permission required for reproduction or display Relative Strengths of Weak Acids Acid strength depends on the degree of ionization, which is a measure of the relative number of acid molecules that ionize to hydronium ion and conjugate base when dissolved in water. Acid strength depends on the degree of ionization, which is a measure of the relative number of acid molecules that ionize to hydronium ion and conjugate base when dissolved in water. Remember that K eq describes the relative amounts of products to reactants. Remember that K eq describes the relative amounts of products to reactants. The greater the degree of ionization, the greater the ratio of products to reactants, and therefore the greater the value of K eq for the acid ionization reaction. The greater the degree of ionization, the greater the ratio of products to reactants, and therefore the greater the value of K eq for the acid ionization reaction.

Copyright © McGraw-Hill Education. Permission required for reproduction or display Relative Strengths of Weak Acids The strengths of weak acids can be compared by examining the K eq values for their reactions with water, which we label K a, the acid ionization constant. The strengths of weak acids can be compared by examining the K eq values for their reactions with water, which we label K a, the acid ionization constant. HCN(aq) + H 2 O(l)  H 3 O + (aq) + CN - (aq) HCN(aq) + H 2 O(l)  H 3 O + (aq) + CN - (aq) CH 3 CO 2 H(aq) + H 2 O(l)  H 3 O + (aq) + CH 3 CO 2 - (aq) CH 3 CO 2 H(aq) + H 2 O(l)  H 3 O + (aq) + CH 3 CO 2 - (aq) Which of these acids is stronger? Which of these acids is stronger? Which forms the greater concentration of H 3 O + (aq) when equal concentrations of acids are added to water? Which forms the greater concentration of H 3 O + (aq) when equal concentrations of acids are added to water?