Acids and Bases. Ch a pt e r Aqueous Equilibria: Chemistry 4th Edition McMurry/Fay. MOH(aq) M + (aq) + OH (aq)

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1 15 Ch a pt e r Aqueous Equilibria: Acids and Bases Chemistry th Edition McMurry/Fay Dr. Paul Charlesworth Michigan Technological University AcidBase 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) Chapter 15 Slide 2 AcidBase Concepts 02 BrønstedLowry Acid: Substance that can donate H + BrønstedLowry Base:Substance that can accept H + Chemical species whose formulas differ only by one proton are said to be conjugate acidbase pairs. Chapter 15 Slide 3 1

2 AcidBase Concepts 03 Chapter 15 Slide AcidBase Concepts 0 Chapter 15 Slide 5 AcidBase Concepts 05 Chapter 15 Slide 6 2

3 AcidBase 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. Chapter 15 Slide 7 AcidBase Concepts 06 Chapter 15 Slide 8 AcidBase Concepts 07 Write balanced equations for the dissociation of each of the following BrønstedLowry acids. (a) H 2 SO (b) HSO (c) H 3 O + Identify the Lewis acid and Lewis base in each of the following reactions: (a) SnCl (s) + 2 Cl (aq) æ SnCl 6 2 (aq) (b) Hg 2+ (aq) + CN (aq) æ Hg(CN) 2 (aq) (c) Co 3+ (aq) + 6 NH 3 (aq) æ Co(NH 3 ) 6 3+ (aq) Chapter 15 Slide 9 3

4 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) Chapter 15 Slide 10 Dissociation of Water 02 This equilibrium gives us the ion product constant for water. K w = K c = [H + ][OH ] = 1.0 x 10 1 If we know either [H + ] or [OH ] then we can determine the other quantity. Chapter 15 Slide 11 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. Chapter 15 Slide 12

5 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 + ] ph + poh = 1 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 Chapter 15 Slide 13 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? Chapter 15 Slide 1 ph A Measure of Acidity 0 Chapter 15 Slide 15 5

6 ph A Measure of Acidity 0 Chapter 15 Slide 16 Strength of Acids and Bases 01 Strong acids and bases: are strong electrolytes that are assumed to ionize completely in water. Weak acids and bases:are weak electrolytes that ionize only to a limited extent in water. Solutions of weak acids and bases contain ionized and non-ionized species. Chapter 15 Slide 17 Strength of Acids and Bases 02 If an acid is strong, its conjugate base has no measurable strength. H 3 O + is the strongest acid that can exist in aqueous solution. OH ion is the strongest base that can exist in aqueous solution. Chapter 15 Slide 18 6

7 Strength of Acids and Bases 03 Increasing Acid Strength ACID CONJ. BASE ACID CONJ. BASE HClO HI HBr HCl H 2 SO HNO 3 H 3 O + HSO ClO I Br Cl HSO NO 3 H 2 O SO 2 Increasing Acid Strength HSO HF HNO 2 HCOOH NH + HCN H 2 O NH 3 SO 2 F NO 2 HCOO NH 3 CN OH NH 2 Chapter 15 Slide 19 Strength of Acids and Bases 0 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 + (aq) Chapter 15 Slide 20 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) + [H ][A ] K a = [HA] Chapter 15 Slide 21 7

8 Acid Ionization Constants 02 ACID K a CONJ. BASE K b HF HNO 2 C 9 H 8 O (aspirin) HCO 2 H (formic) C 6 H 8 O 6 (ascorbic) C 6 H 5 CO 2 H (benzoic) CH 3 CO 2 H (acetic) HCN C 6 H 5 OH (phenol) 7.1 x 10.5 x x x x x x x x F NO 2 C 9 H 7 O HCO 2 C 6 H 7 O 6 C 6 H 5 CO 2 CH 3 CO 2 CN C 6 H 5 O 1. x x x x x x x x x 10 5 Chapter 15 Slide 22 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? Chapter 15 Slide 23 Acid Ionization Constants 0 ph of a Weak Acid: 1. Write a balanced equation and equilibrium expression. 2. Identify all species present that may affect ph. 3. Major species will be HA, H +, A. We are not concerned with H 2 O or OH at this point.. Summarize the changes in concentration of HA, H +, A using an I.C.E. table. Chapter 15 Slide 2 8

9 Acid Ionization Constants 05 Initial Change Equilibrium Table: Determine the ph of 0.50 M HA solution at 25 C. K a = 7.1 x 10. HA æ H A (aq) (aq) (aq) Initial (M): Change (M): x + x +x Equilib (M): 0.50 x x x Chapter 15 Slide 25 Acid Ionization Constants 06 ph of a Weak Acid (Cont d): 1. Substitute new values into equilibrium expression. 2. If K a is significantly (>1000 x) smaller than [HA] the expression (0.50 x) approximates to (0.50). 3. The equation can now be solved for x and ph.. If K a is not significantly smaller than [HA] the quadratic equation must be used to solve for x and ph. Chapter 15 Slide 26 Acid Ionization Constants 07 The Quadratic Equation: The expression must first be rearranged to: 2 ax + bx + c = 0 The values are substituted into the quadratic and solved for a positive solution to x and ph. b ± x = 2 b ac 2a Chapter 15 Slide 27 9

10 Acid Ionization Constants 08 Calculate the ph of a M nitrous acid (HNO 2 ) solution. What is the ph of a M monoprotic acid whose K a is 5.7 x 10? The ph of a M weak monoprotic acid is 3.. Calculate the K a of the acid. Chapter 15 Slide 28 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. Chapter 15 Slide 29 Acid Ionization Constants 10 Concentration Dependence: Chapter 15 Slide 30 10

11 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 + ]. Chapter 15 Slide 31 Base Ionization Constants 02 BASE K b CONJ. ACID K a C 2 H 5 NH 2 (ethylamine) 5.6 x 10 CH. x 10 3 NH 2 (methylamine) C.1 x 10 8 H 10 N O 2 (caffeine) NH 3 (ammonia) C 5 H 5 N (pyridine) C 6 H 5 NH 2 (aniline) NH 2 CONH 2 (urea) 1.8 x x x x 10 1 C 2 H 5 NH 3 + CH 3 NH 3 + C 8 H 11 N O 2 + NH + C 5 H 6 N + C 6 H 5 NH 3 + NH 2 CONH x x x x x x Note that the positive charge sits on the nitrogen. Chapter 15 Slide 32 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 What is the ph of a 0.0 M ammonia solution? Calculate the ph of a 0.26 M methylamine solution. Chapter 15 Slide 33 11

12 Diprotic & Polyprotic Acids 01 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. Chapter 15 Slide 3 Diprotic & Polyprotic Acids 02 ACID K a CONJ. BASE K b H 2 SO HSO C 2 H 2 O C 2 HO H 2 SO 3 HSO 3 H 2 CO 3 HCO 3 H 2 S HS H 3 PO H 2 PO HPO 2 Very Large 1.3 x x x x x x x x x x x x HSO SO 2 C 2 HO C 2 O 2 HSO 3 SO 2 3 HCO 3 CO 2 3 HS S 2 H 2 PO HPO 2 PO 3 Very Small 7.7 x x x x x x x x x x x x 10 2 Chapter 15 Slide 35 Diprotic & Polyprotic Acids 03 Calculate the concentration of all species present in a 0.10 M solution of oxalic acid (C 2 H 2 O ). Determine the ph of the solution. Calculate the concentration of all species present in a 0.20 M solution of phosphoric acid (H 3 PO ). Determine the ph of the solution. Chapter 15 Slide 36 12

13 Molecular Structure and Acid Strength 01 The strength of an acid depends on its tendency to ionize. For general acids of the type HX: 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 Chapter 15 Slide 37 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. Chapter 15 Slide 38 Molecular Structure and Acid Strength 03 For binary acids in the same group, HA bond strength decreases with increasing size of A, so acidity increases. For binary acids in the same row, HA polarity increases with increasing electronegativity of A, so acidity increases. Chapter 15 Slide 39 13

14 Molecular Structure and Acid Strength 0 For oxoacids bond polarity is more important. If we consider the main element (Y): YOH If Y is an electronegative element, or in a high oxidation state, the YO bond will be more covalent and the OH bond more polar and the acid stronger. Chapter 15 Slide 0 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. Chapter 15 Slide 1 Molecular Structure and Acid Strength 06 For oxoacids having the same central atom but different numbers of attached groups, acid strength increases with increasing central atom oxidation number. As shown on the next slide, the number of oxygen atoms increases the positive charge on the chlorine which weakens the OH bond and increases its polarity. Chapter 15 Slide 2 1

15 Molecular Structure and Acid Strength 07 Oxoacids of Chlorine: Chapter 15 Slide 3 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. Chapter 15 Slide AcidBase 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. Chapter 15 Slide 5 15

16 AcidBase Properties of Salts 02 Calculate the ph of a 0.15 M solution of sodium acetate (CH 3 COONa). What is the percent hydrolysis? Calculate the ph of a 0.2 M sodium formate solution (HCOONa). Chapter 15 Slide 6 AcidBase Properties of Salts 03 Metal Ion Hydrolysis: Chapter 15 Slide 7 AcidBase Properties of Salts 0 Calculate the ph of a M Al(NO 3 ) 3 solution. What is the ph of a M AlCl 3 solution? Predict whether the following solutions will be acidic, basic, or nearly neutral: (a) NH I (b) CaCl 2 (c) KCN (d) Fe(NO 3 ) 3 Chapter 15 Slide 8 16