1 Chapter 15 Acids and Bases Fu-Yin Hsu
2 Stomach Acid and Heartburn The cells that line your stomach produce hydrochloric acid. To kill unwanted bacteria To help break down food To activate enzymes that break down food If the stomach acid backs up into your esophagus, it irritates those tissues, resulting in heartburn( 胃 灼 熱 ). Acid reflux
3 Curing Heartburn Mild cases of heartburn can be cured by neutralizing the acid in the esophagus. Swallowing saliva, which contains bicarbonate ion Taking antacids that contain hydroxide ions and/or carbonate ions
4 heartburn Chronic heartburn is a problem for some people. GERD (gastroesophageal reflux disease) is chronic leaking of stomach acid into the esophagus. In people with GERD, the muscles separating the stomach from the esophagus do not close tightly, allowing stomach acid to leak into the esophagus.
5 15.2 The Nature of Acids and Bases Properties of Acids: Sour taste Ability to dissolve many metals Ability to neutralize bases Change blue litmus paper to red
6 TABLE 15.1 Common Acids
7 Properties of Bases Taste bitter Alkaloids( 生 物 鹼 ) = plant product that is alkaline often poisonous Feel slippery Ability to turn red litmus paper blue Ability to neutralize acids
8 TABLE 15.2 Common Bases
9 15.3 Definitions of Acids and Bases Three different definitions the Arrhenius definition the Bronsted Lowry definition the Lewis definition
10 Arrhenius theory Arrhenius theory: an acid forms H + in water; and a base forms OH in water. (acid or base?)
11 Arrhenius Theory HCl(aq) H + (aq) + Cl (aq) The H + ions produced by the acid are so reactive they cannot exist in water. H + ions are protons! Instead, they react with water molecules to produce complex ions, mainly hydronium ion, H 3 O +. H + + H 2 O H 3 O + There are also minor amounts of H + with multiple water molecules, H(H 2 O) n+.
12 Arrhenius Acid Base Reactions The H + from the acid combines with the OH from the base to make a molecule of H 2 O. The cation from the base combines with the anion from the acid to make a salt. acid + base salt + water HCl(aq) + NaOH(aq) NaCl(aq) + H 2 O(l)
13 Brønsted Lowry theory Brønsted Lowry theory defines acids and bases in terms of proton (H + ) transfer. an acid is a proton donor a base is a proton acceptor.
14 conjugate acid and base H 2 S + NH 3 NH HS acid base Conjugated acid Conjugated base conjugate pair: (H 2 S, HS ), (NH 3, NH 4 + ) OH + H 2 PO 4 H 2 O + HPO 4 2 base acid Conjugated acid Conjugated base conjugate pair: (OH, H 2 O), (H 2 PO 4, HPO 4 2 ) Summarizing the Bronsted Lowry Definition of Acids and Bases: A base accepts a proton and becomes a conjugate acid. An acid donates a proton and becomes a conjugate base.
15 Water Is Amphiprotic ( 兩 性 ) H 2 O acts as an acid when it donates H +, forming the conjugate base H 2 O acts as a base when it accepts H +, forming the conjugate acid Amphiprotic: Can act as either an acid or as a base
16 EXAMPLE 15.1 Identifying Bronsted Lowry Acids and Bases and Their Conjugates In each reaction, identify the Bronsted Lowry acid, the Bronsted Lowry base, the conjugate acid, and the conjugate base.
17 Solution (a) (b)
18 15.4 Acid Strength and the Acid Ionization Constant ( Ka ) A strong electrolyte completely dissociates into ions in solution, whereas a weak electrolyte only partially dissociates. We define strong and weak acids accordingly. A strong acid completely ionizes in solution, whereas a weak acid only partially ionizes. If the equilibrium lies far to the right, the acid is strong- it completely ionizes. If the equilibrium lies to the left, the acid is weak- only a small percentage of the acid molecules ionize.
19 Strong Acids Hydrochloric acid (HCl) is an example of a strong acid. An HCl solution contains virtually no intact HCl; the HCl has essentially all ionized to form H 3 O + (aq) and Cl - (aq)
20 Weak Acids HF is a weak acid, one that does not completely ionize in solution. An HF solution contains a large number of intact (or un-ionized) HF molecules; it also contains some H 3 O + (aq) and F - (aq)
21 Strong Acids and Weak Acids monoprotic acids : containing only one ionizable proton. diprotic acid: containing two ionizable protons. triprotic: containing three ionizable protons).
22 Ionic Attraction and Acid Strength In general, the stronger the acid, the weaker the conjugate base and vice versa.
23 The Acid Ionization Constant (Ka) We quantify the relative strength of a weak acid with the acid ionization constant (Ka), which is the equilibrium constant for the ionization reaction of the weak acid. Or:
24 TABLE 15.5 Acid Ionization Constants (Ka) for Some Monoprotic Weak Acids at 25 C
25 15.5 Autoionization of Water and ph Water is amphoteric; it can act either as an acid or a base. Therefore, there must be a few ions present.
26 autoionization water acts as an acid and a base with itself, a process called autoionization : The equilibrium constant for this process is called the ion product( 離 子 積 ) of water and is represented by the symbol Kw
27 Ion product of water All aqueous solutions contain both H 3 O + and OH. The concentration of H 3 O + and OH are equal in water. [H 3 O + ] = [OH ] = 10 7 M at 25 C
28 Ion product of water An acidic solution contains an acid that creates additional H 3 O + ions, causing [H 3 O + ] to increase. However, the ion product constant still applies : This equilibrium constant is very important because it applies to all aqueous solutions
29 Summarizing K w:
30 EXAMPLE 15.2 Using Kw in Calculations (a) (b)
31 The ph Scale: A Way to Quantify Acidity and Basicity ph is a measure of the strength of an acid; low ph = stronger acid ph = log[h 3 O + ] and [H 3 O + ] = 10 ph poh is a measure of the strength of a base; low poh = stronger base poh = log[oh ] and [OH ] = 10 poh
32 Ionization Constant Relationships pk w is the negative logarithm of Kw and at 25 is equal to pk w = ph + poh = K w = [H + ][OH ] =
33 Significant Figures we report the ph to two decimal places here. This is because only the numbers to the right of the decimal point are significant in a logarithm.
34 EXAMPLE 15.3 Calculating ph from [H 3 O + ] or [OH - ] (a) (b)
35 EXAMPLE 15.4 Calculating [H 3 O + ] from ph Calculate the [H 3 O + ] for a solution with a ph of Sol:
36 EX: a student determines the ph of milk of magnesia, a suspension of solid magnesium hydroxide in its saturated aqueous solution, and obtains a value of What is the molarity of Mg(OH) 2 in its saturated aqueous solution? The suspended, undissolved Mg(OH) 2 (s) does not affect the measurement.
37 Solution Mg(OH) 2 (aq) Mg 2+ (aq) + 2 OH (aq) poh = ph = = 3.48 log[oh ] = 3.48 [OH ] = = 3.3 x 10 4 M
38 15.6 Finding the [H 3 O + ] and ph of Strong and Weak Acid Solutions If we let HA be a strong or weak acid, the ionization equations are: strong acids completely ionize in solution, and we can ignore the contribution of the autoionization of water, the concentration of H 3 O + in a strong acid solution is equal to the concentration of the strong acid. EX:
39 Finding the [H 3 O + ] and ph of Strong and Weak Acid Solutions Weak Acids: the concentration of H 3 O + is not equal to the concentration of the weak acid. Ex:
40 EXAMPLE 15.5 Finding the [H 3 O + ] of a Weak Acid Solution Find the [H 3 O + ] of a M HCN solution. Sol:
41 EXAMPLE 15.7 Find the ph of a M HClO 2 solution. Sol:
42 Con t
43 EXAMPLE 15.8 Finding the Equilibrium Constant from ph A M weak acid (HA) solution has a ph of Find Ka for the acid. Sol:
44 Percent Ionization of a Weak Acid We define the percent ionization of a weak acid as the ratio of the ionized acid concentration to the initial acid concentration, multiplied by 100%:
45 EXAMPLE 15.9 Finding the Percent Ionization of a Weak Acid Find the percent ionization of a 2.5 M HNO 2 solution. Solution:
46 The Percent Ionization of a Weak Acid The trend you can see in the table applies to all weak acids. The equilibrium H 3 O + concentration of a weak acid increases with increasing initial concentration of the acid. The percent ionization of a weak acid decreases with increasing concentration of the acid.
47 Mixtures of Acids- A Strong Acid and a Weak Acid Consider a mixture that is 0.10 M in HCl and 0.10 M in HCHO 2.
48 Con t contributions to [H 3 O + ]:
49 EXAMPLE Mixtures of Weak Acids Find the ph of a mixture that is M in HF and M in HClO.
50 Sol: Since the equilibrium constant for the ionization of HF is larger than that for the ionization of HClO, the contribution of HF to [H3O+] is by far the greatest.
51 15.7 Base Solutions Strong base as a base that completely dissociates in solution. Most strong bases are group 1A or group 2A metal hydroxides The group 1A metal hydroxides are highly soluble in water and can form concentrated base solutions. The group 2A metal hydroxides, however, are only slightly soluble.
52 Group 2A metal hydroxides Group 2A metal hydroxides dissolve in water, they produce 2 mol of OH per mole of the base. EX: Unlike diprotic acids, which ionize in two steps, bases containing two OH ions dissociate in one step.
53 TABLE 15.7 Strong Bases Table 4.1
54 Weak base A weak base is analogous to a weak acid. Unlike strong bases that contain OH and dissociate in water. The most common weak bases produce OH by accepting a proton from water, ionizing water to form OH. General equation: Ex:
55 Base Ionization Constant, K b Base strength is measured by the size of the equilibrium constant when it reacts with H 2 O :Base + H 2 O OH + H:Base + The equilibrium constant is called the base ionization constant, K b. Larger K b = stronger base
56 TABLE 15.8 Some Common Weak Bases
57 Ammonia & Amine, Ammonia and Amine have a nitrogen atom with a lone pair This lone pair acts as the proton acceptor that makes the substance a base,
58 EXAMPLE Finding the [ OH - ] and ph of a Strong Base Solution What is the OH concentration and ph in each solution? (a) M KOH (b) M Sr(OH) 2 Sol: (a) (b)
59 EXAMPLE Finding the [ OH - ] and ph of a Weak Base Solution Find the [ OH - ] and ph of a M NH 3 solution. Sol:
60 Con t
61 15.8 The Acid Base Properties of Ions and Salts Salts are water-soluble ionic compounds Salts that contain the cation of a strong base and an anion that is the conjugate base of a weak acid are basic. NaHCO 3 solutions are basic. Na + is the cation of the strong base NaOH. HCO 3 is the conjugate base of the weak acid H 2 CO 3. Salts that contain cations that are the conjugate acid of a weak base and an anion of a strong acid are acidic. NH 4 Cl solutions are acidic. NH 4 + is the conjugate acid of the weak base NH 3. Cl is the anion of the strong acid HCl.
62 Anions as Weak Bases Every anion can be thought of as the conjugate base of an acid. Therefore, every anion can potentially be a base. A (aq) + H 2 O(l) HA(aq) + OH (aq) The stronger the acid, the weaker the conjugate base. An anion that is the conjugate base of a strong acid is ph neutral. Cl (aq) + H 2 O(l) HCl(aq) + OH (aq) An anion that is the conjugate base of a weak acid is basic. F (aq) + H 2 O(l) HF(aq) + OH (aq)
63 Strength of Conjugate Acid Base Pairs the stronger an acid, the weaker its conjugate base.
64 FIGURE Strength of Conjugate Acid Base Pairs
65 EXAMPLE Determining Whether an Anion Is Basic or ph-neutral
66 Relationship between K a of an Acid and K b of Its Conjugate Base Many reference books only give tables of K a values because K b values can be found from them.
67 EXAMPLE Determining the ph of a Solution Containing an Anion Acting as a Base Find the ph of a M NaCHO 2 solution. The salt completely dissociates into Na + (aq) and CHO 2 -(aq), and the Na + ion has no acid or base properties. (HCHO 2 K a =1.8x10-4 Sol:
68 Con t
69 Cations as Weak Acids cations can in some cases act as weak acids. Cations That Are the Counterions of Strong Base Na +, K +, and Ca 2+ are the counterions of the strong bases NaOH, KOH, and Ca(OH) 2 and are therefore themselves ph-neutral. Cations That Are the Conjugate Acids of Weak Bases A cation can be formed from any nonionic weak base by adding a proton (H + ) to its formula. a cation that is the conjugate acid of a weak base is a weak acid.
70 Con t Cations That Are Small, Highly Charged Metals Small, highly charged metal cations such s Al 3+ and Fe 3+ form weakly acidic solutions. Ex: when Al 3+ is dissolved in water, it becomes hydrated The hydrated form of the ion then acts as a Bronsted Lowry acid:
71 EXAMPLE Determining Whether a Cation Is Acidic or ph-neutral Classify each cation as a weak acid or ph-neutral. (C 5 H 5 N: Pyridine)
72 Classifying Salt Solutions as Acidic, Basic, or Neutral If the salt cation is the counterion of a strong base and the anion is the conjugate base of a strong acid, it will form a neutral solution. EX: NaCl Ca(NO 3 ) 2 KBr If the salt cation is the counterion of a strong base and the anion is the conjugate base of a weak acid, it will form a basic solution. EX: NaF Ca(C 2 H 3 O 2 ) 2 KNO 2 If the salt cation is the conjugate acid of a weak base and the anion is the conjugate base of a strong acid, it will form an acidic solution. EX: NH 4 Cl
73 Con t If the salt cation is a highly charged metal ion and the anion is the conjugate base of a strong acid, it will form an acidic solution. EX: Al(NO 3 ) 3 If the salt cation is the conjugate acid of a weak base and the anion is the conjugate base of a weak acid, the ph of the solution depends on the relative strengths of the acid and base. NH 4 F because HF is a stronger acid than NH 4+, K a of NH 4 + is larger than K b of the F ; therefore, the solution will be acidic.
74 EXAMPLE Determining the Overall Acidity or Basicity of Salt Solutions Determine if the solution formed by each salt is acidic, basic, or neutral.
75 Determine if the solution formed by each salt is acidic, basic, or neutral.
76 15.9 Polyprotic Acids Sulfurous acid (H 2 SO 3 ) is a diprotic acid containing two ionizable protons Phosphoric acid (H 3 PO 4 ) is a triprotic acid containing three ionizable protons. a polyprotic acid ionizes in successive steps, each with its own K a
77 TABLE Common Polyprotic Acids and Ionization Constants at 25 C
78 EXAMPLE Dilute H 2 SO 4 Solutions Find the ph of a M sulfuric acid (H 2 SO 4 ) solution. Sol:
79 Con t
80 Finding the ph of Polyprotic Acid Solutions EXAMPLE 15.17: Find the ph of a M ascorbic acid (H 2 C 6 H 6 O 6 ) solution. (K a1 : , K a2 : ) Sol:
81 EXAMPLE Finding the concentration of the Anions for a Weak Diprotic Acid Solution Find the [C 6 H 6 O 6 2- ] of the M ascorbic acid solution Sol: (K a2 : )
82 15.10 Acid Strength and Molecular Structure The more + H X polarized the bond, the more acidic the bond. The stronger the H X bond, the weaker the acid.
83 Relationship between Bond Strength and Acidity Binary acid strength increases to the right across a period. Acidity: H C < H N < H O < H F Binary acid strength increases down the column. Acidity: H F < H Cl < H Br < H I
84 Strengths of Oxoacids ( 含 氧 酸 ) Oxoacids: H-O-Y Acid strength increases with the electronegativity of the central atom, and with the number of terminal oxygen atoms. EX: electronegativity HOI < HOBr < HOCl Ka 2.3x x x 10-8
85 Relationship between Number of Oxygens on the Central Atom and Acidity The more oxygens attached to Y, the stronger the oxyacid.
86 Lewis Acid Base Theory Lewis acid base theory focuses on transferring an electron pair. The electron donor is called the Lewis base. Electron rich; therefore nucleophile The electron acceptor is called the Lewis acid. Electron deficient; therefore electrophile
87 Lewis Bases The Lewis base has electrons it is willing to give away to or share with another atom. The Lewis base must have a lone pair of electrons on it that it can donate. Anions are better Lewis bases than neutral atoms or molecules. N: < N:
88 Lewis Acids They are electron deficient, either from being attached to electronegative atom(s) or not having an octet. They must have an empty orbital willing to accept the electron pair. Many small, highly charged metal cations have empty orbitals they can use to accept electrons.
89 Lewis Acid Base Reactions The base donates a pair of electrons to the acid. It generally results in a covalent bond forming The product that forms is called an adduct. Arrhenius and Brønsted Lowry acid base reactions are also Lewis.
90 Examples of Lewis Acid Base Reactions Ag + (aq) + 2 :NH 3(aq) Ag(NH 3 ) 2 + (aq) Lewis Acid Lewis Base Adduct
91 What Is Acid Rain? Natural rain water has a ph of 5.6. Naturally slightly acidic due mainly to CO 2 Rain water with a ph lower than 5.6 is called acid rain. Acid rain is linked to damage in ecosystems and structures.
92 What Causes Acid Rain? Many natural and pollutant gases dissolved in the air are nonmetal oxides. CO 2, SO 2, NO 2 Nonmetal oxides are acidic. CO 2 (g) + H 2 O(l) H 2 CO 3 (aq) 2 SO 2 (g) + O 2 (g) + 2 H 2 O(l) 2 H 2 SO 4 (aq) 4 NO 2 (g) + O 2 (g) + 2 H 2 O(l) 4 HNO 3 (aq) Processes that produce nonmetal oxide gases as waste increase the acidity of the rain. Natural volcanoes and some bacterial action Man made combustion of fuel