Aqueous Equilibria: Acids and Bases

Transcription

1 Slide 1 Chapter 14 Aqueous Equilibria: Acids and Bases

2 Slide 2 Acid Base Concepts 01 Arrhenius Acid: A substance which dissociates to form hydrogen ions (H + ) in solution. HA(aq) H + (aq) + A (aq) Arrhenius Base: A substance that dissociates in, or reacts with water to form hydroxide ions (OH ). MOH(aq) M + (aq) + OH (aq)

3 Slide 3 Acid Base Concepts 02 Brønsted Lowry Acid: Substance that can donate H + Brønsted Lowry Base: Substance that can accept H + Chemical species whose formulas differ only by one proton are said to be conjugate acid base pairs.

4 Acid Base Concepts 03 Slide 4

5 Slide 5 Hydrated Protons and Hydronium Ions HA(aq) H 1+ (aq) + A 1- (aq) Due to high reactivity of the hydrogen ion, it is actually hydrated by one or more water molecules. n = 1 H 3 O 1+ [H(H 2 O) n ] 1+ n = 2 H 5 O 2 1+ n = 3 H 7 O 3 1+ n = 4 H 9 O 4 1+ For our purposes, H 1+ is equivalent to H 3 O 1+.

6 Acid Base Concepts (1-Acid Dissociation) Slide 6

7 Lewis Acid Base Concepts Slide 7

8 Slide 8 Acid Base Concepts 05 A Lewis Acid is an electron-pair acceptor. These are generally cations and neutral molecules with vacant valence orbitals, such as Al 3+, Cu 2+, H +, BF 3. A Lewis Base is an electron-pair donor. These are generally anions and neutral molecules with available pairs of electrons, such as H 2 O, NH 3, O 2. The bond formed is called a coordinate bond.

9 Slide 9 Acid Base Concepts

10 Slide 10 Lewis Acids and Bases Lewis Acid: An electron-pair acceptor. Lewis Base: An electron-pair donor.

11 Slide 11 Lewis Acids and Bases Lewis Acid: An electron-pair acceptor. Lewis Base: An electron-pair donor.

12 Slide 12 Acid Base Concepts 07 Write balanced equations for the dissociation of each of the following Brønsted Lowry acids. (a) H 2 SO 4 (b) HSO 4 (c) H 3 O + Identify the Lewis acid and Lewis base in each of the following reactions: (a) SnCl 4 (s) + 2 Cl (aq) æ SnCl 2 6 (aq) (b) Hg 2+ (aq) + 4 CN (aq) æ Hg(CN) 2 4 (aq) (c) Co 3+ (aq) + 6 NH 3 (aq) æ Co(NH 3 ) 3+ 6 (aq)

13 Slide 13 Dissociation of Water 01 Water can act as an acid or as a base. H 2 O(l) æ H + (aq) + OH (aq) This is called the autoionization of water. H 2 O(l) + H 2 O(l) æ H 3 O + (aq) + OH (aq)

14 Slide 14 Dissociation of Water 02 This equilibrium gives us the ion product constant for water. K w = K c = [H + ][OH ] = 1.0 x If we know either [H + ] or [OH ] then we can determine the other quantity.

15 Slide 15 Dissociation of Water 03 The concentration of OH ions in a certain household ammonia cleaning solution is M. Calculate the concentration of H + ions. Calculate the concentration of OH ions in a HCl solution whose hydrogen ion concentration is 1.3 M.

16 ph A Measure of Acidity 01 The ph of a solution is the negative logarithm of the hydrogen ion concentration (in mol/l). ph = log [H + ], [H + ] = 10 -ph ph + poh = 14 Acidic solutions: [H + ] > 1.0 x 10 7 M, ph < 7.00 Basic solutions: [H + ] < 1.0 x 10 7 M, ph > 7.00 Neutral solutions: [H + ] = 1.0 x 10 7 M, ph = 7.00 Slide 16

17 Slide 17 ph A Measure of Acidity 02 Nitric acid (HNO 3 ) is used in the production of fertilizer, dyes, drugs, and explosives. Calculate the ph of a HNO 3 solution having a hydrogen ion concentration of 0.76 M. The ph of a certain orange juice is Calculate the H + ion concentration. The OH ion concentration of a blood sample is 2.5 x 10 7 M. What is the ph of the blood?

18 ph A Measure of Acidity 04 Slide 18

19 ph A Measure of Acidity 04 Slide 19

20 Slide 20 Strength of Acids and Bases 03 ACID CONJ. BASE ACID CONJ. BASE Increasing Acid Strength HClO 4 HI HBr HCl H 2 SO 4 HNO 3 H 3 O + HSO 4 ClO 4 I Br Cl HSO 4 NO 3 H 2 O SO 4 2 Increasing Acid Strength HSO 4 HF HNO 2 HCOOH NH 4 + HCN H 2 O NH 3 SO 2 4 F NO 2 HCOO NH 3 CN OH NH 2

21 Slide 21 Strength of Acids and Bases 04 Stronger acid + stronger base weaker acid + weaker base Predict the direction of the following: HNO 2 (aq) + CN (aq) æ HCN(aq) + NO 2 (aq) HF(aq) + NH 3 (aq) æ F (aq) + NH 4+ (aq)

22 Slide 22 Acid Ionization Constants 01 Acid Ionization Constant: the equilibrium constant for the ionization of an acid. HA(aq) + H 2 O(l) æ H 3 O + (aq) + A (aq) Or simply: HA(aq) æ H + (aq) + A (aq) K a = [H + ][A [HA] ]

23 Slide 23 Conjugate Base Ionization Const A - + H 2 O(l) HA(aq) + OH (aq) K a K b = K b = [HA] [OH ] [A - ] [HA] [OH ] [A - ] + [H ][A [HA] = K w ] K a K b = K w

24 Acid Ionization Constants 02 ACID K a CONJ. BASE K b HF 7.1 x 10 4 F 1.4 x HNO 2 C 9 H 8 O 4 (aspirin) 4.5 x x 10 4 NO 2 C 9 H 7 O x x HCO 2 H (formic) 1.7 x 10 4 HCO x C 6 H 8 O 6 (ascorbic) 8.0 x 10 5 C 6 H 7 O x C 6 H 5 CO 2 H (benzoic) 6.5 x 10 5 C 6 H 5 CO x CH 3 CO 2 H (acetic) 1.8 x 10 5 CH 3 CO x HCN 4.9 x CN 2.0 x 10 5 C 6 H 5 OH (phenol) 1.3 x C 6 H 5 O 7.7 x 10 5 Slide 24

25 Slide 25 Strength of Acids and Bases 03 (a) Arrange the three acids in order of increasing value of K a. (b) Which acid, if any, is a strong acid? (c) Which solution has the highest ph, and which has the lowest?

26 Slide 26 Acid Ionization Constants Determine the ph of 0.50 M HA solution at 25 C. K a = 7.1 x Initial Change Equilibrium Table:. HA æ H + + A (aq) (aq) - (aq) Initial (M): Change (M): x + x + x Equilib (M): 0.50 x x x

27 What is the ph of a 0.5 M HF solution (at 25 0 C)? HF (aq) H + (aq) + F - (aq) K a = [H+ ][F - ] [HF] = 7.1 x 10-4 HF (aq) H + (aq) + F - (aq) Initial (M) Change (M) x +x x Equilibrium (M) x x x K a = K a x x x = 7.1 x 10-4 = 7.1 x K a < C o? 100 x 7.1 x 10-4 = < 0.5 x 2 = 3.55 x x 0.50 x = M [H + ] = [F - ] = M ph = -log [H + ] = 1.72 [HF] = 0.50 x = 0.48 M Slide 27

28 Slide 28 Acid Ionization Constants 06 ph of a Weak Acid (Cont d): 1. Substitute equilibrium concentrations into equilibrium expression. 2. If 100 K a < C o then (C 0 x) approximates to (C 0 ). 3. The equation can now be solved for x and ph. 4. If 100 K a is not significantly smaller than C o the quadratic equation must be used to solve for x and ph.

29 Slide 29 Acid Ionization Constants 07 The Quadratic Equation: The expression must first be rearranged to: ax 2 + bx + c The values are substituted into the quadratic and solved for a positive solution to x and ph. = 0 x = b ± 2 b 4 ac 2a

30 Slide 30 Acid Ionization Constants 09 Percent Dissociation: A measure of the strength of an acid. [H ] % Dissociation = 100 [HA] Stronger acids have higher percent dissociation. Percent dissociation of a weak acid decreases as its concentration increases. +

31 Slide 31 Percent dissociation of a weak acid decreases as its concentration increases Concentration Dependence:

32 Slide 32 Weak Bases: Base Ionization Constants 01 Base Ionization Constant: The equilibrium constant for the ionization of a base. The ionization of weak bases is treated in the same way as the ionization of weak acids. B(aq) + H 2 O(l) æ BH + (aq) + OH (aq) Calculations follow the same procedure as used for a weak acid but [OH ] is calculated, not [H + ].

33 Base Ionization Constants 02 BASE K b CONJ. ACID K a C 2 H 5 NH 2 (ethylamine) 5.6 x 10 4 C 2 H 5 NH x CH 3 NH 2 (methylamine) 4.4 x 10 4 CH 3 NH x C 8 H 10 N 4 O 2 (caffeine) 4.1 x 10 4 C 8 H 11 N 4 O x NH 3 (ammonia) 1.8 x 10 5 NH x C 5 H 5 N (pyridine) 1.7 x 10 9 C 5 H 6 N x 10 6 C 6 H 5 NH 2 (aniline) 3.8 x C 6 H 5 NH x 10 5 NH 2 CONH 2 (urea) 1.5 x NH 2 CONH Note that the positive charge sits on the nitrogen. Slide 33

34 Slide 34 Base Ionization Constants 03 Product of K a and K b : multiplying out the expressions for K a and K b equals K w. K a K b = K w

35 Slide 35 ph of Basic Solutions What is the ph of a 0.15 M solution of NH 3? NH 3 (aq) + H 2 O(l) NH 4 + (aq) + OH (aq) K b = [NH 4+ ] [OH ] [NH 3 ] =

36 Slide 36 ph of Basic Solutions Tabulate the data. [NH 3 ], M [NH 4+ ], M [OH ], M Initially At Equilibrium x x x

37 Slide 37 ph of Basic Solutions = (x) 2 ( x ) 100 x K b < C 0? < x = 0.15 ( ) (0.15) = x = x = x 2

38 Slide 38 ph of Basic Solutions Therefore, X = [OH ] = M poh = log ( ) poh = 2.80 ph = ph = 11.20

39 Diprotic & Polyprotic Acids 01 Slide 39 H 2 SO 4 H 3 PO 4 Diprotic and polyprotic acids yield more than one hydrogen ion per molecule. One proton is lost at a time. Conjugate base of first step is acid of second step. Ionization constants decrease as protons are removed.

40 Slide 40 Diprotic & Polyprotic Acids 02 ACID K a CONJ. BASE K b H 2 SO 4 HSO 4 C 2 H 2 O 4 C 2 HO 4 H 2 SO 3 HSO 3 H 2 CO 3 HCO 3 H 2 S HS H 3 PO 4 H 2 PO 4 HPO 4 2 Very Large 1.3 x x x x x x x x x x x x HSO 4 SO 4 2 C 2 HO 4 C 2 O 4 2 HSO 3 SO 3 2 HCO 3 CO 3 2 HS S 2 H 2 PO 4 HPO 4 2 PO 4 3 Very Small 7.7 x x x x x x x x x x x x 10 2

41 Molecular Structure and Acid Strength 01 The strength of an acid depends on its tendency to ionize. For general acids of the type H X: 1. The stronger the bond, the weaker the acid. 2. The more polar the bond, the stronger the acid. For the hydrohalic acids, bond strength plays the key role giving: HF < HCl < HBr < HI 567 kj/mol for HF 299 kj/mol for HI Slide 41

42 Slide 42 Molecular Structure and Acid Strength 02 The electrostatic potential maps show all the hydrohalic acids are polar. The variation in polarity is less significant than the bond strength which decreases from 567 kj/mol for HF to 299 kj/mol for HI.

43 (pm) Slide 43

44 Slide 44 Molecular Structure and Acid Strength 03 For binary acids in the same group, H A bond strength decreases with increasing size of A, so acidity increases. For binary acids in the same row, H A polarity increases with increasing electronegativity of A, so acidity increases.

45 Slide 45 Molecular Structure and Acid Strength 04 For oxoacids bond polarity is more important. If we consider the main element (Y): Y O H If Y is an electronegative element, the Y O bond will pull more electrons, the O H bond will be more polar and the acid will be stronger.

46 Slide 46 Molecular Structure and Acid Strength 05 For oxoacids with different central atoms that are from the same group of the periodic table and that have the same oxidation number, acid strength increases with increasing electronegativity.

47 Polar Covalent Bonds 02 Pauling Electronegativities Detailed List of Electronegativity; Slide 47

48 Slide 48 Molecular Structure and Acid Strength 07 Oxoacids of Chlorine:

49 Slide 49 Molecular Structure and Acid Strength 08 Predict the relative strengths of the following groups of oxoacids: a) HClO, HBrO, and HIO. b) HNO 3 and HNO 2. c) H 3 PO 3 and H 3 PO 4.

50 Acid-Base Properties of Salts Slide 50

51 Slide 51 Acid Base Properties of Salts 01 Salts that produce neutral solutions are those formed from strong acids and strong bases. Salts that produce basic solutions are those formed from weak acids and strong bases. Salts that produce acidic solutions are those formed from strong acids and weak bases.

52 Slide 52 Salts That Contain Cation from a Weak Base and anion from a Weak Base The ph of an ammonium carbonate solution, (NH 4 ) 2 CO 3, depends on the relative acid strength of the cation and the relative base strength of the anion. Is it acidic or basic?

53 Slide 53 Acid-Base Properties of Salts Salts That Contain Acidic Cations and Basic Anions (NH 4 ) 2 CO 3 : NH 4 1+ (aq) + H 2 O(l) CO 3 2- (aq) + H 2 O(l) H 3 O 1+ (aq) + NH 3 (aq) HCO 3 1- (aq) + OH 1- (aq) Three possibilities: K a > K b : The solution will contain an excess of H 3 O 1+ ions, Acidic solution, (ph < 7). K a < K b : The solution will contain an excess of OH 1- ions, Basic solutions, (ph > 7). K a K b : The solution will contain approximately equal concentrations of H 3 O 1+ and OH 1- ions (ph 7). K a K b

54 Slide 54 Salts That Contain Cation from a Weak Base and anion from a Weak Base (NH 4 ) 2 CO 3 : NH 4 1+ (aq) + H 2 O(l) CO 3 2- (aq) + H 2 O(l) H 3 O 1+ (aq) + NH 3 (aq) HCO 3 1- (aq) + OH 1- (aq) K a K b K w = K a for NH 4 1+ = Kb for NH x x 10-5 = 5.6 x K w K b for CO 2-3 = = Ka for HCO x x = 1.8 x 10-4 Basic, K a < K b

55 Acid-Base Properties of Salts Slide 55

56 Hydrated Cation of Al 3+ Slide 56

57 Slide 57 Acid Base Properties of Salts 03 Metal Ion Hydrolysis:

58 Slide 58 Acid Base Properties of Salts 04 Calculate the ph of a M Al(NO 3 ) 3 solution K a = 1.4 x Predict whether the following solutions will be acidic, basic, or nearly neutral: (a) NH 4 I (b) CaCl 2 (c) KCN (d) Fe(NO 3 ) 3