Dr. Paul Charlesworth Michigan Technological University Dr. Paul Charlesworth Michigan Technological University C h a p t e rC h a p t e r C h a p t e rC h a p t e r 15 Aqueous Equilibria: Acids and Bases Chemistry 4th Edition McMurry/Fay Chemistry 4th Edition McMurry/Fay

Prentice Hall ©2004 Chapter 15Slide 2 Acid–Base Concepts01 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)

Prentice Hall ©2004 Chapter 15Slide 3 Acid–Base Concepts02 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.

Prentice Hall ©2004 Chapter 15Slide 4 Acid–Base Concepts03

Prentice Hall ©2004 Chapter 15Slide 5 Acid–Base Concepts04

Prentice Hall ©2004 Chapter 15Slide 6 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. Acid–Base Concepts05

Prentice Hall ©2004 Chapter 15Slide 7 Acid–Base Concepts06

Prentice Hall ©2004 Chapter 15Slide 8 Acid–Base Concepts07 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 6 2– (aq) (b) Hg 2+ (aq) + 4 CN – (aq) æ Hg(CN) 4 2– (aq) (c) Co 3+ (aq) + 6 NH 3 (aq) æ Co(NH 3 ) 6 3+ (aq)

Prentice Hall ©2004 Chapter 15Slide 9 Dissociation of Water01 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)

Prentice Hall ©2004 Chapter 15Slide 10 Dissociation of Water02 This equilibrium gives us the ion product constant for water. K w = K c = [H + ][OH – ] = 1.0 x 10 –14 If we know either [H + ] or [OH – ] then we can determine the other quantity.

Prentice Hall ©2004 Chapter 15Slide 11 Dissociation of Water03 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.

Prentice Hall ©2004 Chapter 15Slide 12 pH – A Measure of Acidity01 The pH of a solution is the negative logarithm of the hydrogen ion concentration (in mol/L). pH = –log [H + ] pH + pOH = 14 Acidic solutions:[H + ] > 1.0 x 10 –7 M, pH 7.00 Neutral solutions:[H + ] = 1.0 x 10 –7 M, pH = 7.00

Prentice Hall ©2004 Chapter 15Slide 13 pH – A Measure of Acidity02 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?

Prentice Hall ©2004 Chapter 15Slide 14 pH – A Measure of Acidity03

Prentice Hall ©2004 Chapter 15Slide 15 Strength of Acids and Bases01 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.

Prentice Hall ©2004 Chapter 15Slide 16 Strength of Acids and Bases02 HClO 4 HI HBr HCl H 2 SO 4 HNO 3 H 3 O + HSO 4 – HF HNO 2 HCOOH NH 4 + HCN H 2 O NH 3 ClO 4 – I – Br – Cl – HSO 4 – NO 3 – H 2 O SO 4 2– F – NO 2 – HCOO – NH 3 CN – OH – NH 2 – ACID CONJ. BASE Increasing Acid Strength

Prentice Hall ©2004 Chapter 15Slide 17 Strength of Acids and Bases03 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)

Prentice Hall ©2004 Chapter 15Slide 18 Acid Ionization Constants01 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)

Prentice Hall ©2004 Chapter 15Slide 19 Acid Ionization Constants x 10 –4 4.5 x 10 –4 3.0 x 10 –4 1.7 x 10 –4 8.0 x 10 –5 6.5 x 10 –5 1.8 x 10 –5 4.9 x 10 – x 10 –10 HF HNO 2 C 9 H 8 O 4 (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) F – NO 2 – C 9 H 7 O 4 – HCO 2 – C 6 H 7 O 6 – C 6 H 5 CO 2 – CH 3 CO 2 – CN – C 6 H 5 O – ACID K a CONJ. BASE K b 1.4 x 10 – x 10 – x 10 – x 10 – x 10 – x 10 – x 10 – x 10 –5 7.7 x 10 –5

Prentice Hall ©2004 Chapter 15Slide 20 Strength of Acids and Bases03 (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?

Prentice Hall ©2004 Chapter 15Slide 21 HA æ H + +A (M): (M):–x+x+x Equilib (M): 0.50–xxx Acid Ionization Constants04 I nitial C hange E quilibrium T able : Determine the pH of 0.50 M HA solution at 25°C. K a = 7.1 x 10 –4. Initial Change (aq) -

Prentice Hall ©2004 Chapter 15Slide 22 Acid Ionization Constants05 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. 4. If K a is not significantly smaller than [HA] the quadratic equation must be used to solve for x and pH.

Prentice Hall ©2004 Chapter 15Slide 23 Acid Ionization Constants06 The Quadratic Equation: The expression must first be rearranged to: The values are substituted into the quadratic and solved for a positive solution to x and pH.

Prentice Hall ©2004 Chapter 15Slide 24 Acid Ionization Constants07 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 –4 ? The pH of a M weak monoprotic acid is Calculate the K a of the acid.

Prentice Hall ©2004 Chapter 15Slide 25 Acid Ionization Constants08 Percent Dissociation: A measure of the strength of an acid. Stronger acids have higher percent dissociation. Percent dissociation of a weak acid decreases as its concentration increases.

Prentice Hall ©2004 Chapter 15Slide 26 Base Ionization Constants01 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 + ].

Prentice Hall ©2004 Chapter 15Slide 27 Base Ionization Constants x 10 –4 4.4 x 10 –4 4.1 x 10 –4 1.8 x 10 –5 1.7 x 10 –9 3.8 x 10 – x 10 –14 C 2 H 5 NH 2 (ethylamine) CH 3 NH 2 (methylamine) C 8 H 10 N 4 O 2 (caffeine) NH 3 (ammonia) C 5 H 5 N (pyridine) C 6 H 5 NH 2 (aniline) NH 2 CONH 2 (urea) C 2 H 5 NH 3 + CH 3 NH 3 + C 8 H 11 N 4 O 2 + NH 4 + C 5 H 6 N + C 6 H 5 NH 3 + NH 2 CONH 3 + BASE K b CONJ. ACID K a 1.8 x 10 – x 10 – x 10 – x 10 – x 10 –6 2.6 x 10 – Note that the positive charge sits on the nitrogen.

Prentice Hall ©2004 Chapter 15Slide 28 Diprotic & Polyprotic Acids01 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.

Prentice Hall ©2004 Chapter 15Slide 29 Diprotic & Polyprotic Acids02 Very Large 1.3 x 10 –2 6.5 x 10 –2 6.1 x 10 –5 1.3 x 10 –2 6.3 x 10 –8 4.2 x 10 –7 4.8 x 10 – x 10 –8 1 x 10 – x 10 –3 6.2 x 10 –8 4.8 x 10 –13 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– ACID K a CONJ. BASE K b 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 10 – x 10 – x 10 – x 10 – x 10 –7 2.4 x 10 –8 2.1 x 10 –4 1.1 x 10 –7 1 x 10 –5 1.3 x 10 – x 10 –7 2.1 x 10 –2

Prentice Hall ©2004 Chapter 15Slide 30 Molecular Structure and Acid Strength01 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

Prentice Hall ©2004 Chapter 15Slide 31 Molecular Structure and Acid Strength02 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.

Prentice Hall ©2004 Chapter 15Slide 32 Molecular Structure and Acid Strength03 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.

Prentice Hall ©2004 Chapter 15Slide 33 Molecular Structure and Acid Strength04 For oxoacids bond polarity is more important. If we consider the main element (Y): Y–O–H If Y is an electronegative element, or in a high oxidation state, the Y–O bond will be more covalent and the O–H bond more polar and the acid stronger.

Prentice Hall ©2004 Chapter 15Slide 34 Molecular Structure and Acid Strength05 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.

Prentice Hall ©2004 Chapter 15Slide 35 Molecular Structure and Acid Strength06 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 O–H bond and increases its polarity.

Prentice Hall ©2004 Chapter 15Slide 36 Molecular Structure and Acid Strength07 Oxoacids of Chlorine:

Prentice Hall ©2004 Chapter 15Slide 37 Molecular Structure and Acid Strength08 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.

Prentice Hall ©2004 Chapter 15Slide 38 Acid–Base Properties of Salts01 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.

Prentice Hall ©2004 Chapter 15Slide 39 Acid–Base Properties of Salts02 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 0.24 M sodium formate solution (HCOONa).