Chapter 16 1 Contents and Concepts Acid–Base Concepts 1. Arrhenius Concept of Acids and Bases 2. Brønsted–Lowry Concept of Acids and Bases 3. Lewis Concept of Acids and Bases Acid and Base Strengths 4. Relative Strengths of Acids and Bases 5. Molecular Structure and Acid Strength 2 Self-Ionization of Water and pH 6.Self-Ionization of Water 7.Solutions of a Strong Acid or Base 8.The pH of a Solution 3 1 Learning Objectives Acid Base Concepts • Arrhenius Concept of Acids and Base – a. Define acid and base according to the Arrhenius concept. • Brønsted–Lowry Concept of Acids and Bases – a. Define acid and base according to the Brønsted–Lowry concept. Define the term conjugate acid–base pair. Identify acid and base species. Define amphiprotic species. – b. – c. – d. 4 3. Lewis Concept of Acids and Bases • • a. Define Lewis acid and Lewis base. b. Identify Lewis acid and Lewis base species. Acid and Base Strengths 4. Relative Strengths of Acids and Bases – a. Understand the relationship between the strength of an acid and that of its conjugate base. – b. Decide whether reactants or products are favored in an acid–base reaction. 5 5. Molecular Structure and Acid Strength • a. • b. • c. • d. Note the two factors that determine relative acid strengths. Understand the periodic trends in the strengths of the binary acids HX. Understand the rules for determining the relative strengths of oxoacids. Understand the relative acid strengths of a polyprotic acid and its anions. 6 2 Self-Ionization of Water and pH 6. Self-Ionization of Water • a. • b. Define self-ionization (or autoionization). Define the ion-product constant for water. 7. Solutions of a Strong Acid or Base – a. Calculate the concentrations of H3O+ and OH- in solutions of a strong acid or base 7 8. The pH of a Solution 1. Define pH. 2. Calculate the pH from the hydronium-ion concentration. 3. Calculate the hydronium-ion concentration from the pH. 4. Describe the determination of pH by a pH meter and by acid–base indicators. 8 9 3 • When gaseous hydrogen chloride meets gaseous ammonia, a smoke composed of ammonium chloride is formed. • HCl(g) + NH3(g) NH4Cl(s) • This is an acid–base reaction. 10 • We will examine three ways to explain acid–base behavior: H+ and OH• Arrhenius Concept donor • Brønsted–Lowry Concept H+ = proton acceptor • Lewis Concept electron pair donor acceptor Note: H+ in water is H3O+ acid base 11 Acid-Base Concepts • Antoine Lavoisier was one of the first chemists to try to explain what makes a substance acidic. – In 1777, he proposed that oxygen was an essential element in acids. – The actual cause of acidity and basicity was ultimately explained in terms of the effect these compounds have on water by Svante Arrhenius in 1884. 12 4 Acid-Base Concepts • In the first part of this chapter we will look at several concepts of acid-base theory including: – The Arrhenius concept – The Bronsted Lowry concept – The Lewis concept This chapter expands on what you learned in Chapter 3 about acids and bases. 13 Arrhenius Concept of Acids and Bases • According to the Arrhenius concept of acids and bases, an acid is a substance that, when dissolved in water, increases the concentration of hydronium ion (H3O+). – Chemists often use the notation H+(aq) for the H3O+(aq) ion, and call it the hydrogen ion. – Remember, however, that the aqueous hydrogen ion is actually chemically bonded to water, that is, H3O+. 14 • The Arrhenius concept limits bases to compounds that contain a hydroxide ion. • The Brønsted–Lowry concept expands the compounds that can be considered acids and bases. 15 5 Arrhenius Concept of Acids and Bases • According to the Arrhenius concept of acids and bases, an acid is a substance that, when dissolved in water, increases the concentration of hydronium ion (H3O+). The H3O+ is shown here hydrogen bonded to three water molecules. 16 Arrhenius Concept of Acids and Bases • A base, in the Arrhenius concept, is a substance that, when dissolved in water, increases the concentration of hydroxide ion, OH-(aq). 17 Arrhenius Concept of Acids and Bases • In the Arrhenius concept, a strong acid is a substance that ionizes completely in aqueous solution to give H3O+(aq) and an anion – An example is perchloric acid, HClO4. + − HClO4 (aq) + H 2O(l ) → H 3O (aq) + ClO4 (aq ) – Other strong acids include HCl, HBr, HI, HNO3 , and H2SO4. 18 6 Arrhenius Concept of Acids and Bases • In the Arrhenius concept, a strong base is a substance that ionizes completely in aqueous solution to give OH-(aq) and a cation. – An example is sodium hydroxide, NaOH. + − NaOH(s) HO→ Na (aq) + OH (aq) 2 – Other strong bases include LiOH, KOH, Ca(OH)2, Sr(OH)2, and Ba(OH)2. 19 Arrhenius Concept of Acids and Bases • Most other acids and bases that you encounter are weak. They are not completely ionized and exist in reversible reaction with the corresponding ions. – An example is acetic acid, HC2H3O2. HC2 H 3O 2 (aq ) + H 2O( l ) + − H 3O (aq) + C2 H 3O 2 (aq) – Ammonium hydroxide, NH4OH, is a weak base. NH 4OH(aq ) + − NH4 (aq) + OH (aq) 20 21 7 Brønsted-Lowry Concept of Acids and Bases • According to the Brønsted-Lowry concept, an acid is the species donating the proton in a proton-transfer reaction. • A base is the species accepting the proton in a proton-transfer reaction. – In any reversible acid-base reaction, both forward and reverse reactions involve proton transfer. 22 • Consider the reaction of NH3 and H20. NH 3 (aq ) + H 2O(l ) base acid NH 3 (aq ) + H 2O(l ) + − NH 4 (aq ) + OH (aq ) + − NH 4 (aq ) + OH (aq ) H+ – In the forward reaction, NH3 accepts a proton from H2O. Thus, NH3 is a base and H2O is an acid. 23 • Brønsted–Lowry Concept of Acids and Bases • An acid–base reaction is considered a proton (H+) transfer reaction. H+ H+ H+ H+ 24 8 • Consider the reaction of NH3 and H2O. acid NH3 (aq) + H2O(l) + base − NH4 (aq) + OH (aq) H+ base acid NH 3 (aq ) + H 2O(l ) NH 4 (aq ) + OH (aq ) + − – The species NH4+ and NH3 are a conjugate acid-base pair. – A conjugate acid-base pair consists of two species in an acid-base reaction, one acid and one base, that differ by the loss or gain of a 25 proton. Brønsted-Lowry Concept of Acids and Bases • Consider the reaction of NH3 and H2O. base NH 3 (aq ) + H 2O(l ) acid + − NH 4 (aq ) + OH (aq ) – Here NH4+ is the conjugate acid of NH3 and NH3 is the conjugate base of NH4+. – The Brønsted-Lowry concept defines a species as an acid or a base according to its function in the proton-transfer 26 reaction. • Substances in the acid–base reaction that differ by the gain or loss of a proton, H+, are called a conjugate acid–base pair. The acid is called the conjugate acid; the base is called a conjugate base. • Acid Base Conjugate Conjugate base acid 27 9 • What is the conjugate acid of H2O? • What is the conjugate base of H2O? The conjugate acid of H2O has gained a proton. It is H3O+. The conjugate base of H2O has lost a proton. It is OH-. 28 • Label each species as an acid or base. Identify the conjugate acid-base pairs. a. HCO3-(aq) + HF(aq) Base b. Acid HCO3-(aq) + OH-(aq) Acid Base H2CO3(aq) + F-(aq) Conjugate Conjugate acid base CO32-(aq) + H2O(l) Conjugate Conjugate base acid 29 • A Brønsted–Lowry acid is the species donating a proton in a proton-transfer reaction; it is a proton donor. • A Brønsted–Lowry base is the species accepting a proton in a proton-transfer reaction; it is a proton acceptor. 30 10 • Some species can act as an acid or a base. – An amphoteric species is a species that can act either as an acid or a base (it can gain or lose a proton). – For example, HCO3- acts as a proton donor (an acid) in the presence of OH− −2 − HCO 3 (aq ) + OH (aq ) → CO3 (aq ) + H 2O(l ) –H+ 31 – An amphoteric species is a species that can act either as an acid or a base (it can gain or lose a proton). – Alternatively, HCO3- can act as a proton acceptor (a base) in the presence of HF. − − HCO3 (aq ) + HF(aq ) → H 2CO3 (aq ) + F (aq ) H+ • The amphoteric characteristic of water is important in the acid-base properties of aqueous solutions. 32 – Water reacts as an acid with the base NH3. + − NH 3 (aq ) + H 2O (l )→ NH 4 (aq ) +OH (aq ) H+ – Water can also react as a base with the acid HF. − + HF(aq ) + H 2O(l ) → F (aq ) + H 3O (aq ) H+ 33 11 • In the Brønsted-Lowry concept: 1. A base is a species that accepts protons; OHis only one example of a base. 2. Acids and bases can be ions as well as molecular substances. 3. Acid-base reactions are not restricted to aqueous solution. 4. Some species can act as either acids or bases depending on what the other reactant is. Practice exercise 16.7 and 16.8 34 Lewis Concept of Acids and Bases • The Lewis concept defines an acid as an electron pair acceptor and a base as an electron pair donor. – This concept broadened the scope of acidbase theory to include reactions that did not involve H+. – The Lewis concept embraces many reactions that we might not think of as acid-base reactions. 35 • The reaction of boron trifluoride with ammonia is an example. : : + :N H :F: H :F :F: H B N :F: H : B : H : : : : :F :F: H – Boron trifluoride accepts the electron pair, so it is a Lewis acid. Ammonia donates the electron pair, so it is the Lewis base. 36 12 • 37 Name and define the three acid base concepts we have discussed. Give examples of each type of acid-base. 38 Relative Strength of Acids and Bases • The Brønsted-Lowry concept introduced the idea of conjugate acid-base pairs and proton-transfer reactions. – We consider such acid-base reactions to be a competition between species for hydrogen ions. – From this point of view, we can order acids by their relative strength as hydrogen ion donors. 39 13 Relative Strength of Acids and Bases – The stronger acids are those that lose their hydrogen ions more easily than other acids. – Similarly, the stronger bases are those that hold onto hydrogen ions more strongly than other bases. – If an acid loses its H+, the resulting anion is now in a position to reaccept a proton, making it a Brønsted-Lowry base. – It is logical to assume that if an acid is considered strong, its conjugate base (that is, its anion) would be weak, since it is unlikely to accept a hydrogen ion. 40 Relative Strength of Acids and Bases • Consider the equilibrium below. HC2 H 3O 2 (aq ) + H 2O( l ) acid base − + H 3O (aq) + C2 H 3O 2 (aq) acid base conjugate acid-base pairs – In this system we have two opposing BrønstedLowry acid-base reactions. – In this example, H3O+ is the stronger of the two acids. Consequently, the equilibrium is skewed toward reactants. 41 • Consider the equilibrium below. HC2 H 3O 2 (aq ) + H 2O( l ) acid base − + H 3O (aq) + C 2 H 3O 2 (aq) acid base conjugate acid-base pairs – Table 15.2 outlines the relative strength of some common acids and their conjugate bases. – This concept of conjugate pairs is fundamental to understanding why certain salts can act as acids or bases. See Exercises 11.4-7 Problems 11.46-65 42 14 Table 11.3 and 11.4 in Text 43 44 a. Acetate ion b. Formate ion Acetate Ion 45 15 Practice Exercises 16.8 – 16.19 46 1. What is the conjugate acid of NH3? a) H3O+ b) HCl c) NH4+ d) H2O Ans : c 2. Which of the following is the weakest Brønsted acid? a) NH4+ Ka = 5.6 × 10-10 b) HSO4Ka = 1.2 × 10-2 c) HF Ka = 7.2 × 10-4 d) HPO4-2 Ka = 4.2 × 10-13 Ans: d 47 Molecular Structure and Acid Strength • Two factors are important in determining the relative acid strengths. – One is the polarity of the bond to which the hydrogen atom is attached. – The H atom should have a partial positive charge: δ+ δ− H−X – The more polarized the bond, the more easily the proton is removed and the greater the acid strength. 48 16 – The second factor is the strength of the bond. Or, in other words, how tightly the proton is held. – This depends on the size of atom X. δ+ δ- H−X – The larger atom X, the weaker the bond and the greater the acid strength. 49 • For a binary acid, as the size of X in HX increases, going down a group, acid strength increases. • For a binary acid, going across a period, as the electronegativity increases, acid strength increases. 50 • Which is a stronger acid: HF or HCl? • Which is a stronger acid: H2O or H2S? • Which is a stronger acid: HCl or H2S? HF and HCl These are binary acids from the same group, so we compare the size of F and Cl. Because Cl is larger, HCl is the stronger acid. 51 17 • H2O and H2S • These are binary acids from the same group, so we compare the size of O and S. Because S is larger, H2S is the stronger acid. • HCl and H2S • These are binary acids from the same period, but different groups, so we compare the electronegativity of Cl and S. Because Cl is more electronegative, HCl is the stronger acid. 52 Molecular Structure and Acid Strength • Consider a series of binary acids from a given column of elements. – As you go down the column of elements, the radius increases markedly and the H-X bond strength decreases. – You can predict the following order of acidic strength. HF < HCl < HBr < HI 53 • As you go across a row of elements, the polarity of the H-X bond becomes the dominant factor. H 3 N < H 2O < HF – As electronegativity increases going to the right, the polarity of the H-X bond increases and the acid strength increases. – You can predict the following order of acidic strength. H 3 N < H 2O < HF 54 18 • Consider the oxoacids. An oxoacid has the structure: H−O−Y− – The acidic H atom is always attached to an O atom, which in turn is attached to another atom Y. – Bond polarity is the dominant factor in the relative strength of oxoacids. – This, in turn, depends on the electronegativity of the atom Y. 55 • Consider the oxoacids. An oxoacid has the structure: H−O−Y− – If the electronegativity of Y is large, then the O-H bond is relatively polar and the acid strength is greater. – You can predict the following order of acidic strength. HOCl > HOBr > HOI – Other groups, such as O atoms or O-H groups, may be attached to Y. – With each additional O atom, Y becomes effectively 56 more electronegative. • For oxoacids, several factors are relevant: the number and bonding of oxygens, the central element, and the charge on the species. • For a series of oxoacids, (OH)mYOn, acid strength increases as n increases. (OH)Cl n=0 Weakest (OH)ClO n=1 (OH)ClO2 (OH)ClO3 n=2 n=3 Strongest 57 19 – As a result, the H atom becomes more acidic. – The acid strengths of the oxoacids of chlorine increase in the following order. HClO < HClO 2 < HClO 3 < HClO 4 58 • For a series of oxoacids differing only in the central atom Y, the acid strength increases with the electronegativity of Y. Stronger Weaker 59 Molecular Structure and Acid Strength • Consider polyprotic acids and their corresponding anions. – Each successive H atom becomes more difficult to remove. – Therefore the acid strength of a polyprotic acid and its anions decreases with increasing negative charge. HPO 4 2− − < H 2 PO 4 − < H 3 PO 4 60 20 • The acid strength of a polyprotic acid and its anions decreases with increasing negative charge. •H2CO3 is a stronger acid than HCO3-. •H2SO4 is a stronger acid than HSO4-. •H3PO4 is a stronger acid than H2PO4-. •H2PO4- is a stronger acid than HPO42-. • A reaction will always go in the direction from stronger acid to weaker acid, and from stronger base to weaker base. 61 •Decide which species are favored at the completion of the following reaction: •HCN(aq) + HSO3-(aq) •CN-(aq) + H2SO3(aq) We first identify the acid on each side of the reaction: HCN and H2SO3. Next, we compare their acid strength: H2SO3 is stronger. This reaction will go from right to left (), and the reactants are favored. 62 . 63 21 • Self-Ionization of Water H3O+(aq) + OH-(aq) • H2O(l) + H2O(l) Base Acid Conjugate acid Conjugate base 64 Self-ionization of Water • Self-ionization is a reaction in which two like molecules react to give ions. – In the case of water, the following equilibrium is established. + H 2O (l ) + H 2O (l ) − H 3 O ( aq ) + OH ( aq ) – The equilibrium-constant expression for this system is: + − [ H O ][OH ] Kc = 3 [ H 2O]2 65 Self-ionization of Water • Self-ionization is a reaction in which two like molecules react to give ions. – The concentration of ions is extremely small, so the concentration of H2O remains essentially constant. This gives: + − [ H 2O]2 K c = [ H 3O ][OH ] constant 66 22 – We call the equilibrium value for the ion product [H3O+][OH-] the ion-product constant for water, which is written Kw. + − K w = [ H 3O ][OH ] – At 25 oC, the value of Kw is 1.0 x 10-14. – Like any equilibrium constant, Kw varies with temperature. – Because we often write H3O+ as H+, the ionproduct constant expression for water can be written: + − K w = [ H ][OH ] 67 Self-ionization of Water • These ions are produced in equal numbers in pure water, so if we let x = [H+] = [OH-] 1.0 × 10 −14 = ( x )( x ) x = 1.0 × 10 −14 at 25 oC = 1.0 × 10 −7 – Thus, the concentrations of H+ and OH- in pure water are both 1.0 x 10-7 M. – If you add acid or base to water they are no longer equal but the Kw expression still holds. 68 • H2O(l) + H2O(l) H3O+(aq) + OH-(aq) • We call the equilibrium constant the ionproduct constant, Kw. Kw = [H3O+][OH-] At 25°C, Kw = 1.0 × 10-14 • As temperature increases, the value of Kw increases. 69 23 Solutions of Strong Acid or Base • In a solution of a strong acid you can normally ignore the self-ionization of water as a source of H+(aq). – The H+(aq) concentration is usually determined by the strong acid concentration. – However, the self-ionization still exists and is responsible for a small concentration of OH- ion. 70 • As an example, calculate the concentration of OHion in 0.10 M HCl. Because you started with 0.10 M HCl (a strong acid) the reaction will produce 0.10 M H+(aq). + − HCl(aq ) → H (aq ) + Cl (aq ) – Substituting [H+]=0.10 into the ion-product expression, we get: 1.0 × 10 −14 = ( 0.10)[OH − ] 71 Solutions of Strong Acid or Base • As an example, calculate the concentration of OHion in 0.10 M HCl. Because you started with 0.10 M HCl (a strong acid) the reaction will produce 0.10 M H+(aq). + − HCl(aq ) → H (aq ) + Cl (aq ) – Substituting [H+]=0.10 into the ion-product expression, we get: − [OH ] = 1.0 × 10-14 = × -13 1.0 10 M 0.10 72 24 • Similarly, in a solution of a strong base you can normally ignore the self-ionization of water as a source of OH-(aq). – The OH-(aq) concentration is usually determined by the strong base concentration. – However, the self-ionization still exists and is responsible for a small concentration of H+ ion. 73 • As an example, calculate the concentration of H+ ion in 0.010 M NaOH. Because you started with 0.010 M NaOH (a strong base) the reaction will produce 0.010 M OH-(aq). H 2O + − → Na (aq ) + OH (aq ) NaOH(s ) – Substituting [OH-]=0.010 into the ionproduct expression, we get: 1.0 × 10 −14 = [H + ](0.010) 74 Because you started with 0.010 M NaOH (a strong base) the reaction will produce 0.010 M OH-(aq). H 2O + − → Na (aq ) + OH (aq ) NaOH(s ) – Substituting [OH-]=0.010 into the ionproduct expression, we get: + [H ] = 1.0 × 10-14 = × -12 1.0 10 M 0.010 75 25 Solutions of Strong Acid or Base • By dissolving substances in water, you can alter the concentrations of H+(aq) and OH(aq). – In a neutral solution, the concentrations of H+(aq) and OH-(aq) are equal, as they are in pure water. – In an acidic solution, the concentration of H+(aq) is greater than that of OH-(aq). – In a basic solution, the concentration of OH-(aq) is greater than that of H+(aq). 76 • At 25°C, you observe the following conditions. – In an acidic solution, [H+] > 1.0 x 10-7 M. – In a neutral solution, [H+] = 1.0 x 10-7 M. – In a basic solution, [H+] < 1.0 x 10-7 M. 77 The pH of a Solution • Although you can quantitatively describe the acidity of a solution by its [H+], it is often more convenient to give acidity in terms of pH. – The pH of a solution is defined as the negative logarithm of the molar hydrogenion concentration. + pH = − log[H ] 78 26 • For a solution in which the hydrogen-ion concentration is 1.0 x 10-3, the pH is: − pH = − log(1.0 ×10 3 ) = 3.00 – Note that the number of decimal places in the pH equals the number of significant figures in the hydrogen-ion concentration. 79 The pH of a Solution • In a neutral solution, whose hydrogen-ion concentration is 1.0 x 10-7, the pH = 7.00. • For acidic solutions, the hydrogen-ion concentration is greater than 1.0 x 10-7, so the pH is less than 7.00 • . • Similarly, a basic solution has a pH greater than 7.00. • Figure 15.8 shows a diagram of the pH scale and the pH values of some common solutions. 80 Figure 15.8: The pH Scale 81 27 •Calculate the hydronium and hydroxide ion concentration at 25°C in • • a. b. 0.10 M HCl 1.4 × 10-4 M Mg(OH)2 a. When HCl ionizes, it gives H+ and Cl-. So [H+] = [Cl-] = [HCl] = 0.10 M. a. When Mg(OH)2 ionizes, it gives Mg2+ and 2 OH-. So [OH-] = 2[Mg2+] = 2[Mg(OH)2] = 2.8 × 10-4 M. 82 A Problem to Consider • A sample of orange juice has a hydrogen-ion concentration of 2.9 x 10-4 M. What is the pH? + pH = − log[H ] − pH = − log(2.9 × 10 4 ) pH = 3.54 83 A Problem to Consider • The pH of human arterial blood is 7.40. What is the hydrogen-ion concentration? + [H ] = anti log(−pH ) + [H ] = anti log(−7.40) + [H ] = 10 See Exercises 11.8-10 − 7.40 = 4.0 × 10−8 M Problems 74-109 84 28 • A has 5 H3O+ and 5 OH-. It is neutral. • B has 7 H3O+ and 3 OH-. It is acidic. • C has 3 H3O+ and 7 OH-. It is basic. Listed from most acidic to most basic: B, A, C 85 The pOH of a Solution • A measurement of the hydroxide ion concentration, similar to pH, is the pOH. – The pOH of a solution is defined as the negative logarithm of the molar hydroxideion concentration. − pOH = − log[OH ] 86 The pOH of a Solution • A measurement of the hydroxide ion concentration, similar to pH, is the pOH. – Then because Kw = [H+][OH-] = 1.0 x 10-14 at 25 oC, you can show that pH + pOH = 14.00 87 29 The pH of a Solution • A measurement of the hydroxide ion concentration, similar to pH, is the pOH. – Then because Kw = [H+][OH-] = 1.0 x 10-14 at 25 oC, you can show that pH + pOH = 14.00 See Exercise 11.8-10 and Problems 11.75-109 88 http://www.quia.com/rr/4051.html 89 The pH of a Solution • The pH of a solution can accurately be measured using a pH meter (see Figure 15.9). – Although less precise, acid-base indicators are often used to measure pH because they usually change color within a narrow pH range. – Figure 15.8 shows the color changes of various acid-base indicators. 90 30 Figure 15.9: A digital pH meter. Photo courtesy of American Color. 91 92 93 31 94 Operational Skills 1 Identify acid and base species. 2 Identify Lewis acids and bases. 3 Decide whether reactants or products are favored in an Acid-base reaction. 4 Calculate Concentrations of H3O+ and OH-. 5 Calculate the pH from the hydronium concentration and vise versa. 95 End of This part of Chapter 16 96 32 Figure 15.12: Preparation of Sodium Hydroxide by Hydrolysis 97 Problem 15.27 98 Problem 15.28 99 33 Problem 15.37 100 Problem 15.38 101 Operational Skills • Identifying acid and base species • Identifying Lewis acid and base species • Deciding whether reactants or products are favored in an acid-base reaction • Calculating the concentration of H+ and OH- in solutions of strong acid or base • Calculating the pH from the hydrogen-ion concentration, and vice versa 102 34 103 35