CH 16 CHEM NOTES
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Date Created: 09/14/16
Chapter 16 Acids, Bases, and Salts nd ChJulia Burdge & Jason Overby The McGraw-Hill Companies, Inc. 16.1 Brønsted Acids and Bases 16.2 Molecular Structure and Acid Strength Hydrohalic Acids Oxoacids Carboxylic Acids 16.3 The Acid-Base Properties of Water 16.4 The pH and pOH Scales 16.5 Strong Acids and Bases Strong Acids Strong Bases 16.6 Weak Acids and Acid Ionization Constants The Ionization Constant, Ka Calculating pH from K a Percent Ionization Using pH to Determine K a 16.7 Weak Bases and Base Ionization Constants The Ionization Constant, K b Calculating pH from K b Using pH to Determine K b 16.8 Conjugate Acid-Base Pairs The Strength of a Conjugate Acid or Base The Relationship Between K ana K of b Conjugate Acid- Base Pair 16.9 Diprotic and Polyprotic Acids 16.10 Acid-Base Properties of Salt Solutions Basic Salt Solutions Acidic Salt Solutions Neutral Salt Solutions Salts in Which Both the Cation and the Anion Hydrolyze 16.11 Acid-Base Properties of Oxides and Hydroxides Oxides of Metals and Nonmetals Basic and Amphoteric Hydroxides 16.12 Lewis Acids and Bases Acids - Bases Brønsted definition: Acid: a proton donor must have a removable (acidic) proton HCl(g) + H O(l) ⇌ Cl (aq) + H O (aq) 2 3 acid base conjugate base conjugate acid Base: a proton acceptor must have a pair of nonbonding electrons NH 3g) + H O2l) ⇌ NH (4q) + OH (aq) base acid conjugate acid conjugate base Conjugate acid-base pair: acid & base that differ only in one proton 4 Conjugate Acids - Bases Problem 1: a. What is the conjugate base of: H O, HCO , 2H 3- 4+ Answer: OH , CO , PH 32- 3 b. What is the conjugate acid of: CN , HCO , O 3- 2- - Answer: HCN, H CO , O2 3 5 Factors Affecting Acid Strength 1. Bond strength 2. Bond polarity Hydrohalic Acids (HX) HI < HBr < HCl < HF increasing bond strength HI > HBr > HCl > HF decreasing acid strength 6 Factors Affecting Acid Strength Oxyacids Increased electronegativity of the central atom causes increased acidity . HClO v3 HBrO 3 HClO 3tronger Increased number of bonded oxygen atoms causes increased acidity. HClO v3 HClO 4 HClO 4tronger Which is the stronger acid? H SeO vs H SO stronger acid: H SO 2 3 2 3 2 3 H 2eO vs4H SeO 2 3 stronger acid: H S2O 4 7 Factors Affecting Acid Strength Carboxylic Acids RCOOH resonance Acid strength depends on the nature of the R group. Chloroacetic acid stronger acid than acetic acid. 8 Ion-Product Constant of water: K w Amphoteric: species that can behave either as a Brønsted acid or a Brønsted base (H O HC2 ), 3- Autoionization of water: H O(l) + H O(l) ⇌ H O (aq) + OH (aq)- 2 2 3 or H O(l) ⇌ H (aq) + OH (aq) 2 • The equilibrium expression: K c K =w[H O ]3[OH ] = [H ] [OH ] - • K = ion-product constant for water w • At 25C, K = 1.0 10 -14 w • In pure water [OH ] = [H O ]3= 1.0 x 10 M -7 9 Ion-Product Constant of water: K w Problem 2: Practice problem 16.4B The value of K at normal body temperature (37 C) is w 2.8 x 10 . Calculate the concentration of hydroxide ions in stomach acid at body temperature if [H ] = 0.10 M. Answer: [OH ] = 2.8 x 10 -13M 10 pH and pOH scale pH = -log[H O ] = - log[H ] and [H ] = 10 + -pH 3 Since [H ] [OH ] = 1.0 x 10 -14 -log([H ] [OH ]) = -log(1.0 x 10 -14) pH + pOH = 14.00 Other p-scale: - pOH = -log[OH ] In pure water: [OH ] = [H O ] = 1.0 x 10 M -7 3 pH = -log[H O 3 = -log(1.0 10 ) = 7.00 11 pH and pOH scale Problem 3: o + At 25 C the [H ] in a solution was measured to be -4 -14 - 3.2 x 10 M. If K =w1.0 x 10 , what is the [OH ] and what is the pH? Answer: [OH ] = 3.1 x 10 -11M pH = 3.49 b.Determine the [H ] in a solution at 25°C if the pH = 5.75. + -6 Answer: [H ] = 1.8 x 10 M 12 Strong Acids Strong acids: HCl, HBr, HI, HNO , HClO , HClO , H SO * 3 3 4 2 4 Problem 4: -4 Determine the pH of an 2.3 x 10 M nitric acid solution. Answer: pH = 3.64 13 Strong Bases Strong bases: hydroxides of alkali metals and the heaviest alkaline earth metals LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH) , 2 Sr(OH) ,2Ba(OH) 2 Problem 5: Determine the concentration of calcium hydroxide for which the pH is 12.14. -3 Answer: [Ca(OH) ] =26.9 x 10 M 14 Weak acids and K a Weak monoprotic Acid, HA: - + HA(aq) + H O(2) ⇌ A (aq) + H O 3aq) [H 3 ] [A ]- K = K = c a [HA] K a acid ionization constant Solution (at 25 °C) K pH a 0.10 M HF 7.1 x 104 2.09 0.10 M CH 3OOH 1.8 x 105 2.87 0.10 M HCl* large 1.00 15 Weak acids and K a 16 Calculating pH weak acids using K a Problem 6: Calculate the pH of a 0.30 M solution of -5 acetic acid (CH C3OH). K = 1.8ax 10 (table 16.5) Answer: pH = 2.63 + % ionization = [H eq x 100% [HA] initial If %ionization < 5%, then simplification of the quadratic equation is allowed. 17 Calculating pH weak acids using K a Problem 7: Calculate the pH of a 5.0 x 10 M HF solution. -4 Ka= 6.8 x 10 Answer: pH = 2.82 18 Weak bases and K b Weak Base, B: B(aq) + H O(l) ⇌ HB (aq) + OH (aq)- 2 [HB ] [OH ] K c K b= [B] K b base ionization constant 19 Weak bases and K b 20 Weak bases and K b Problem 8: What is the pH of a 0.15 M ammonia solution at 25 C. o K b 1.8 x 10 . -5 Answer: pH = 11.20 Problem 9: Caffeine, the stimulant in coffee and tea, is a weak base that ionizes in water according to the equation C H N O (aq) + H O(l) ⇌ HC H N O (aq) + OH (aq) - 8 10 4 2 2 8 10 4 2 o A 0.15-M solution of caffeine at 25 C has a pH of 8.45. Determine the K of caffeine. b -11 Answer: K = b.3 x 10 21 Conjugate acids and bases A strong acid has a relatively weak conjugate base. A weak acid has a relatively strong conjugate base. A strong base has a relatively weak conjugate acid. A weak base has a relatively strong conjugate acid. 22 Relationship K and a b acid-base conjugate pair • The product of the acid-dissociation constant (K ) for an acid and the base-dissociation a constant for its conjugate base (K b equals the ion-product constant for water. • K a K =bK w 23 Polyprotic Acids If Ka1is 1000 times greater then K a2 etc, the acid can be treated as monoprotic when determining the pH. 24 Acid-Base properties of salt solutions 1. Anion: conjugate base of a strong acid neutral e.g. Cl (spectator ion) 2. Anion: conjugate base of a weak acid basic e.g. F - 3. Cation: conjugate acid of a weak base + acidic e.g. NH 4 4. Metal cations of group 1A and 2A (Ca , Ba , and Sr ) 2+ neutral e.g. Ca (spectator ion) 5. Other metal cations: undergo hydrolysis acidic e.g. Fe 3+,Cr3+ Al 3+ Be 2+ , , 25 Acid-Base properties of salt solutions Figure 16.5 26 Acid-Base properties of salt solutions In general: if the salt contains an anion and cation that both hydrolyze, the pH depends on the relative strengths of the weak acid (K ) and base(K ). b If, b > K a the solution is basic If, b < K a the solution is acidic If, b ≈ K a the solution is neutral or nearly neutral Problem 10: Predict whether the following salts will form an acidic, neutral or basic solution when dissolved in water: a. SrCl 2 b. AlBr 3 c. CH CO3K 2- -13 2- -7 d. K H2O (K 4PO a 4 = 4.2 x 10 andK HPb 4 = 1.6 x 10 ) + -5 -4 e. NH NO4(K 2H a 4 = 1.8 x 10 and K HaO = 4.2 x 10 ) 27 Acid-Base properties of salt solutions Problem 10: Predict whether the following salts will form an acidic, neutral or basic solution when dissolved in water: a. SrCl neutral 2 b. AlBr 3 acidic c. CH C3OK basic 2- -13 2- -7 d. K 2PO (K 4PO a 4 = 4.2 x 10 andK HPb 4 = 1.6 x 10 ) basic e. NH NO (K NH + = 1.8 x 10 andK HNO = 4.6 x 10 ) -4 4 2 a 4 a 2 acidic 28 Oxides of Metals Metal oxides: • Metal oxide + water → metal hydroxide (basic solution) Na 2(s) + H O2l) → 2NaOH(aq) • Metal oxide + acid → salt + water Na 2(s) + 2HNO (aq3→ 2NaNO (aq) + 3 O(l) 2 • All the alkali and alkaline earth metal hydroxides, except BeO, are basic. • Many other metals oxides including BeO are amphoteric. 29 Oxides of Nonmetals Nonmetal oxides: • Nonmetal oxide + water → acid (acidic solution) CO (2) + H O2l) ⇌ H CO 2aq) 3 SO (3 SO2); N4O (H2O 5; P O 3 4 10(H 3O ),4Cl O2(H7lO ) 4 • Nonmetal oxide + base → salt + water CO (2) + 2KOH(aq)→ K CO (aq)2+ H 3(l) 2 30 Lewis acids and bases • Lewis acid: electron-pair acceptor. (Atoms with an empty valence orbital can be Lewis acids). • Lewis base: electron-pair donor (Brønsted base) empty unhybridized 2p z orbital Boron trifluoride Ammonia Lewis acid Lewis base A coordinate covalent bond 31
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