Contents and Concepts

Transcription

1 Learning Objectives Chapter 15 Acid Base Concepts Arrhenius Concept of Acids and Base a. Define and according to the Arrhenius concept. Brønsted Lowry Concept of Acids and Bases a. Define and according to the Brønsted Lowry concept. b. Define the term conjugate pair. c. Identify and species. d. Define amphiprotic species. Contents and Concepts Acid Base Concepts 1. Arrhenius Concept of Acids and Bases. 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 3. Lewis Concept of Acids and Bases a. Define Lewis and Lewis. b. Identify Lewis and Lewis species. Acid and Base Strengths 4. Relative Strengths of Acids and Bases a. Understand the relationship between the strength of an and that of its conjugate. b. Decide whether reactants or products are favored in an reaction. Self-Ionization of Water and p 6.Self-Ionization of Water 7.Solutions of a Strong Acid or Base 8.The p of a Solution 5. Molecular Structure and Acid Strength a. Note the two factors that determine relative strengths. b. Understand the periodic trends in the strengths of the binary s X. c. Understand the rules for determining the relative strengths of oxos. d. Understand the relative strengths of a polyprotic and its anions. 1

2 Self-Ionization of Water and p 6. Self-Ionization of Water a. Define self-ionization (or autoionization). b. Define the ion-product constant for water. 7. Solutions of a Strong Acid or Base a. Calculate the concentrations of 3 O and O - in solutions of a strong or When gaseous hydrogen chloride meets gaseous ammonia, a smoke composed of ammonium chloride is formed. Cl(g) N 3 (g) N 4 Cl(s) This is an reaction. 8. The p of a Solution 1. Define p.. Calculate the p from the hydronium-ion concentration. 3. Calculate the hydronium-ion concentration from the p. 4. Describe the determination of p by a p meter and by indicators. We will examine three ways to explain behavior: Arrhenius Concept and O - Brønsted Lowry Concept Lewis Concept electron pair = proton donor acceptor donor acceptor Note: in water is 3 O Acid-Base Concepts Antoine Lavoisier was one of the first chemists to try to explain what makes a substance ic. In 1777, he proposed that oxygen was an essential element in s. The actual cause of ity and basicity was ultimately explained in terms of the effect these compounds have on water by Svante Arrhenius in 1884.

3 Acid-Base Concepts In the first part of this chapter we will look at several concepts of - theory including: The Arrhenius concept The Bronsted Lowry concept The Lewis concept This chapter expands on what you learned in Chapter 3 about s and s. Arrhenius Concept of Acids and Bases According to the Arrhenius concept of s and s, an is a substance that, when dissolved in water, increases the concentration of hydronium ion ( 3 O ). The 3 O is shown here hydrogen bonded to three water molecules. Arrhenius Concept of Acids and Bases According to the Arrhenius concept of s and s, an is a substance that, when dissolved in water, increases the concentration of hydronium ion ( 3 O ). Arrhenius Concept of Acids and Bases A, in the Arrhenius concept, is a substance that, when dissolved in water, increases the concentration of hydroxide ion, O -. Chemists often use the notation for the 3 O ion, and call it the hydrogen ion. Remember, however, that the aqueous hydrogen ion is actually chemically bonded to water, that is, 3 O. Arrhenius Concept of Acids and Bases The Arrhenius concept limits s to compounds that contain a hydroxide ion. The Brønsted Lowry concept expands the compounds that can be considered s and s. In the Arrhenius concept, a strong is a substance that ionizes completely in aqueous solution to give 3 O and an anion. (See Animation: Acid Ionization Equilibirum) An example is perchloric, ClO 4. ClO4 O(l) 3O ClO4 Other strong s include Cl, Br, I, NO 3, and SO 4. 3

4 Arrhenius Concept of Acids and Bases In the Arrhenius concept, a strong is a substance that ionizes completely in aqueous solution to give O - and a cation. An example is sodium hydroxide, NaO. O NaO(s) Na O Other strong s include LiO, KO, Ca(O), Sr(O), and Ba(O). Brønsted-Lowry Concept of Acids and Bases According to the Brønsted-Lowry concept, an is the species donating the proton in a proton-transfer reaction. A is the species accepting the proton in a proton-transfer reaction. In any reversible - reaction, both forward and reverse reactions involve proton transfer. Arrhenius Concept of Acids and Bases Most other s and s that you encounter are weak. They are not completely ionized and exist in reversible reaction with the corresponding ions. An example is acetic, C 3 O. C 3O O(l) 3O C3O Ammonium hydroxide, N 4 O, is a weak. N4O N4 O Consider the reaction of N 3 and 0. N 3 O(l) N4 N 3 O(l) N4 O O In the forward reaction, N 3 accepts a proton from O. Thus, N 3 is a and O is an. Brønsted Lowry Concept of Acids and Bases An reaction is considered a proton ( ) transfer reaction. 4

5 Consider the reaction of N 3 and O. N3 O(l) N4 O What is the conjugate of O? What is the conjugate of O? N 3 O(l) N4 O The species N 4 and N 3 are a conjugate - pair. A conjugate - pair consists of two species in an - reaction, one and one, that differ by the loss or gain of a proton. The conjugate of O has gained a proton. It is 3 O. The conjugate of O has lost a proton. It is O -. Brønsted-Lowry Concept of Acids and Bases Consider the reaction of N 3 and O. N 3 O(l) N4 O ere N 4 is the conjugate of N 3 and N 3 is the conjugate of N 4. The Brønsted-Lowry concept defines a species as an or a according to its function in the proton-transfer reaction. Label each species as an or. Identify the conjugate - pairs. a. CO 3- F CO 3 F - Base Acid Conjugate Conjugate b. CO 3- O - CO - 3 O(l) Acid Base Conjugate Conjugate Substances in the reaction that differ by the gain or loss of a proton,, are called a conjugate pair. The is called the conjugate ; the is called a conjugate. A Brønsted Lowry is the species donating a proton in a proton-transfer reaction; it is a proton donor. Acid Base Conjugate Conjugate A Brønsted Lowry is the species accepting a proton in a proton-transfer reaction; it is a proton acceptor. 5

6 Some species can act as an or a. An amphoteric species is a species that can act either as an or a (it can gain or lose a proton). For example, CO 3- acts as a proton donor (an ) in the presence of O - O CO 3 3 O(l ) CO In the Brønsted-Lowry concept: 1. A is a species that accepts protons; O - is only one example of a.. Acids and s can be ions as well as molecular substances. 3. Acid- reactions are not restricted to aqueous solution. 4. Some species can act as either s or s depending on what the other reactant is. Look at Example 15.1 Do Exercise 15.1 See Problems An amphoteric species is a species that can act either as an or a (it can gain or lose a proton). Alternatively, CO 3- can act as a proton acceptor (a ) in the presence of F. CO F 3 CO3 F The amphoteric characteristic of water is important in the - properties of aqueous solutions. Lewis Concept of Acids and Bases The Lewis concept defines an as an electron pair acceptor and a as an electron pair donor. This concept broadened the scope of theory to include reactions that did not involve. The Lewis concept embraces many reactions that we might not think of as - reactions. Water reacts as an with the N 3. N O(l) 3 N 4 O Water can also react as a with the F. The reaction of boron trifluoride with ammonia is an example. : : F B : : : F: : F: : : N : : F B : : : F: : F: : N F O(l) F 3O Boron trifluoride accepts the electron pair, so it is a Lewis. Ammonia donates the electron pair, so it is the Lewis. Do Exercise 15. See Example 15. and Problems

7 Relative Strength of Acids and Bases Consider the equilibrium below. C 3O O(l) 3O C3O conjugate - pairs In this system we have two opposing Brønsted- Lowry - reactions. In this example, 3 O is the stronger of the two s. Consequently, the equilibrium is skewed toward reactants. Relative Strength of Acids and Bases The Brønsted-Lowry concept introduced the idea of conjugate - pairs and proton-transfer reactions. We consider such - reactions to be a competition between species for hydrogen ions. From this point of view, we can order s by their relative strength as hydrogen ion donors. Consider the equilibrium below. C 3O O(l) conjugate - pairs 3O C3O Table 15. outlines the relative strength of some common s and their conjugate s. This concept of conjugate pairs is fundamental to understanding why certain salts can act as s or s. Do Exercise 15. See Example 15.3 and Problems Relative Strength of Acids and Bases The stronger s are those that lose their hydrogen ions more easily than other s. Similarly, the stronger s are those that hold onto hydrogen ions more strongly than other s. If an loses its, the resulting anion is now in a position to reaccept a proton, making it a Brønsted-Lowry. It is logical to assume that if an is considered strong, its conjugate (that is, its anion) would be weak, since it is unlikely to accept a hydrogen ion. 7

8 Molecular Structure and Acid Strength Two factors are important in determining the relative strengths. One is the polarity of the bond to which the hydrogen atom is attached. The atom should have a partial positive charge: δ δ X The more polarized the bond, the more easily the proton is removed and the greater the strength. 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- X The larger atom X, the weaker the bond and the greater the strength. For a binary, as the size of X in X increases, going down a group, strength increases. For a binary, going across a period, as the electronegativity increases, strength increases. Acetate Ion 8

9 Which is a stronger : F or Cl? Which is a stronger : O or S? Which is a stronger : Cl or S? F and Cl These are binary s from the same group, so we compare the size of F and Cl. Because Cl is larger, Cl is the stronger. As you go across a row of elements, the polarity of the -X bond becomes the dominant factor. 3 N < O < F As electronegativity increases going to the right, the polarity of the -X bond increases and the strength increases. You can predict the following order of ic strength. 3 N < O < F O and S These are binary s from the same group, so we compare the size of O and S. Because S is larger, S is the stronger. Cl and S These are binary s from the same period, but different groups, so we compare the electronegativity of O and S. Because Cl is more electronegative, Cl is the stronger. Consider the oxos. An oxo has the structure: O Y The ic 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 oxos. This, in turn, depends on the electronegativity of the atom Y. Molecular Structure and Acid Strength Consider a series of binary s from a given column of elements. As you go down the column of elements, the radius increases markedly and the -X bond strength decreases. You can predict the following order of ic strength. F < Cl < Br < I Consider the oxos. An oxo has the structure: O Y If the electronegativity of Y is large, then the O- bond is relatively polar and the strength is greater. You can predict the following order of ic strength. OCl > OBr > OI Other groups, such as O atoms or O- groups, may be attached to Y. With each additional O atom, Y becomes effectively more electronegative. 9

10 For oxos, several factors are relevant: the number and bonding of oxygens, the central element, and the charge on the species. For a series of oxos, (O) m YO n, strength increases as n increases. (O)Cl n = 0 (O)ClO n = 1 (O)ClO n = (O)ClO 3 n = 3 Molecular Structure and Acid Strength Consider polyprotic s and their corresponding anions. Each successive atom becomes more difficult to remove. Therefore the strength of a polyprotic and its anions decreases with increasing negative charge. Weakest Strongest 4 < PO4 3PO4 PO < As a result, the atom becomes more ic. The strengths of the oxos of chlorine increase in the following order. ClO < ClO < < ClO3 ClO4 The strength of a polyprotic and its anions decreases with increasing negative charge. CO 3 is a stronger than CO 3-. SO 4 is a stronger than SO PO 4 is a stronger than PO 4-. PO 4- is a stronger than PO 4 -. A reaction will always go in the direction from stronger to weaker, and from stronger to weaker. For a series of oxos differing only in the central atom Y, the strength increases with the electronegativity of Y. Decide which species are favored at the completion of the following reaction: CN SO 3- CN - SO 3 Stronger Weaker We first identify the on each side of the reaction: CN and SO 3. Next, we compare their strength: SO 3 is stronger. This reaction will go from right to left ( ), and the reactants are favored.

11 Do Exercise 15.4 Self-ionization of Water Self-ionization is a reaction in which two like molecules react to give ions. See Problems The concentration of ions is extremely small, so the concentration of O remains essentially constant. This gives: [ O] K = c [3O ][O constant ] Self-Ionization of Water O(l) O(l) 3 O O - Base Acid Conjugate Conjugate We call the equilibrium value for the ion product [ 3 O ][O - ] the ion-product constant for water, which is written K w. = [ O K w 3 ][O At 5 o C, the value of K w is 1.0 x -14. Like any equilibrium constant, K w varies with temperature. Because we often write 3 O as, the ionproduct constant expression for water can be written: K w = [ ][O ] ] 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. O(l) O(l) 3O (aq ) O (aq ) The equilibrium-constant expression for this system is: [3O ][O ] K = c [ O] Self-ionization of Water These ions are produced in equal numbers in pure water, so if we let x = [ ] = [O - ] 1.0 x = 14 = 1.0 (x)(x) 14 at 5 = Thus, the concentrations of and O - in pure water are both 1.0 x -7 M. If you add or to water they are no longer equal but the K w expression still holds. o C 11

12 O(l) O(l) 3 O O - We call the equilibrium constant the ionproduct constant, K w. K w = [ 3 O ][O - ] At 5 C, K w = As temperature increases, the value of K w increases. Do Exercise 15.5 See Example 15.4 and Problems Solutions of Strong Acid or Base As an example, calculate the concentration of O - ion in 0. M Cl. Because you started with 0. M Cl (a strong ) the reaction will produce 0. M. Cl Cl Substituting [ ]=0. into the ion-product expression, we get: [ O ] = = M Solutions of Strong Acid or Base In a solution of a strong you can normally ignore the self-ionization of water as a source of. The concentration is usually determined by the strong concentration. owever, the self-ionization still exists and is responsible for a small concentration of O - ion. Similarly, in a solution of a strong you can normally ignore the self-ionization of water as a source of O -. The O - concentration is usually determined by the strong concentration. owever, the self-ionization still exists and is responsible for a small concentration of ion. As an example, calculate the concentration of O - ion in 0. M Cl. Because you started with 0. M Cl (a strong ) the reaction will produce 0. M. Cl Cl Substituting [ ]=0. into the ion-product expression, we get: 1.0 = (0.)[O 14 ] As an example, calculate the concentration of ion in 0.0 M NaO. Because you started with 0.0 M NaO (a strong ) the reaction will produce 0.0 M O -. NaO(s) O Na O Substituting [O - ]=0.0 into the ionproduct expression, we get: 1.0 = [ 14 ](0.0) 1

13 Because you started with 0.0 M NaO (a strong ) the reaction will produce 0.0 M O -. NaO(s) O Na O Substituting [O - ]=0.0 into the ionproduct expression, we get: [ ] = = M The p of a Solution Although you can quantitatively describe the ity of a solution by its [ ], it is often more convenient to give ity in terms of p. The p of a solution is defined as the negative logarithm of the molar hydrogenion concentration. p = log[ ] Solutions of Strong Acid or Base By dissolving substances in water, you can alter the concentrations of and O -. In a neutral solution, the concentrations of and O - are equal, as they are in pure water. In an ic solution, the concentration of is greater than that of O -. For a solution in which the hydrogen-ion concentration is 1.0 x -3, the p is: p = log(1.0 3 = 3.00 Note that the number of decimal places in the p equals the number of significant figures in the hydrogen-ion concentration. ) In a basic solution, the concentration of O - is greater than that of. At 5 C, you observe the following conditions. In an ic solution, [ ] > 1.0 x -7 M. In a neutral solution, [ ] = 1.0 x -7 M. In a basic solution, [ ] < 1.0 x -7 M. The p of a Solution In a neutral solution, whose hydrogen-ion concentration is 1.0 x -7, the p = For ic solutions, the hydrogen-ion concentration is greater than 1.0 x -7, so the p is less than Similarly, a basic solution has a p greater than Figure 15.8 shows a diagram of the p scale and the p values of some common solutions. 13

14 A Problem to Consider Figure 15.8: The p Scale The p of human arterial blood is What is the hydrogen-ion concentration? [ ] = anti log( p) [ ] = anti log( 7.40) [ ] = = 4.0 M See Example 15.5 and Problems Do Exercise 15.7 Calculate the hydronium and hydroxide ion concentration at 5 C in a. 0. M Cl b M Mg(O) a. When Cl ionizes, it gives and Cl -. So [ ] = [Cl - ] = [Cl] = 0. M. a. When Mg(O) ionizes, it gives Mg and O -. So [O - ] = [Mg ] = [Mg(O) ] =.8-4 M. A has 5 3 O and 5 O -. It is neutral. B has 7 3 O and 3 O -. It is ic. C has 3 3 O and 7 O -. It is basic. Listed from most ic to most basic: B, A, C. A Problem to Consider A sample of orange juice has a hydrogen-ion concentration of.9 x -4 M. What is the p? p = log[ p = log(.9 p = 3.54 ] 4 ) The po of a Solution A measurement of the hydroxide ion concentration, similar to p, is the po. The po of a solution is defined as the negative logarithm of the molar hydroxideion concentration. po = log[o ] 14

15 The po of a Solution A measurement of the hydroxide ion concentration, similar to p, is the po. Then because K w = [ ][O - ] = 1.0 x -14 at 5 o C, you can show that p po = The p of a Solution The p of a solution can accurately be measured using a p meter (see Figure 15.9). Although less precise, - indicators are often used to measure p because they usually change color within a narrow p range. Figure 15.8 shows the color changes of various - indicators. The p of a Solution A measurement of the hydroxide ion concentration, similar to p, is the po. Then because K w = [ ][O - ] = 1.0 x -14 at 5 o C, you can show that Figure 15.9: A digital p meter. Photo courtesy of American Color. p po = See Exercise 15.6 and Problems Do Exercise

16 Problem 15.7 Problem 15.8 Figure 15.1: Preparation of Sodium ydroxide by ydrolysis Problem

17 Problem Operational Skills Identifying and species Identifying Lewis and species Deciding whether reactants or products are favored in an - reaction Calculating the concentration of and O - in solutions of strong or Calculating the p from the hydrogen-ion concentration, and vice versa 17