Chapter 15

Monoprotic Bronsted Acid and Base Bronsted Acids- able to donate protons in the form of hydrogen ions – protons – H+. AH A- + H+ HCl  Cl- + H+ C2H3O2H  C2H3O2- + H+ Bronsted Base- able to accept protons in the form of hydrogen ions or H+. B- + H+ BH NH3 + H+  NH4+ HO- + H+  H2O

Conjugate Acid-Base Pairs In the reaction of HF and H2O, HF/F− is one conjugate acid-base pair. H2O/H3O+ is the other conjugate acid-base pair. Each pair is related by a loss and gain of H+.

Autoionization of Water H2O(l) H+(aq) + OH-(aq) Kc = [H+][OH-] = Kw Kw is called the ion-product constant of water. At 250C Kw = [H+][OH-] = 1.0 x 10-14 pH = -log [H+] [H+] = [OH-] [H+] > [OH-] [H+] < [OH-] neutral acidic basic pOH = -log [OH] pH + pOH = 14

Ka and Kb Ka = [H+][A-] [HA] Kb = [AH][OH-] [A-] HA(aq) H+(aq) + A-(aq) A-(aq) + H2O(l) AH(aq) + OH-(aq) Ka = [H+][A-] [HA] Kb = [AH][OH-] [A-] For a acid-base conjugate pair in water: HA (aq) H+ (aq) + A- (aq) Ka A- (aq) + H2O (l) OH- (aq) + HA (aq) Kb H2O (l) H+ (aq) + OH- (aq) Kw KaKb = Kw

Alternative Notation HA(aq) H+(aq) + A-(aq) acid ionization constant percent ionization = Ka = [H+][A-] [HA] [H+] at equilibrium Initial concentration of [HA] x 100% acid strength Ka Percent ionization = [H+] [HA]0 x 100% Percent ionization: Strong acids- % ionization is always 100% Weak acids- % ionization decreases as concentration increases (Ka stays the same!).

Chapter 15

Polyprotic Bronsted Acid and Base May yield more than one hydrogen ion per molecule. Ionize in a stepwise manner; they lose one proton at a time. H2CO3 HCO3- + H+ acid conjugate base acid HCO3- CO32- + H+ acid conjugate base

Polyprotic Bronsted Acid and Base Ionize in a stepwise manner; they lose one proton at a time. Ka1 Ka2 An ionization constant expression can be written for each ionization stage.

Polyprotic Bronsted Acid and Base Conjugate base is the acid for the next equilibrium. The second Ka is “always” smaller than the first. The acid strength decreases as we go to the species with fewer protons. Easier to separate H+ and X- than H+ from X2-. Consequently, two or more equilibrium constant expressions must often be used to calculate the concentrations of species in the acid solution.

15.11 Oxalic acid (H2C2O4) is a poisonous substance used chiefly as a bleaching and cleansing agent (for example, to remove bathtub rings). Calculate the concentrations of all the species present at equilibrium in a 0.10 M solution. 0.10 M H2C2O4(aq) H+(aq) + HC2O4-(aq) Ka1 HC2O4-(aq) H+(aq) + C2O42-(aq) Ka2 From Table 15.5: Ka1 = 6.5 x 10-2 Ka2 = 6.1 x 10-5

15.11 Oxalic acid (H2C2O4) is a poisonous substance used chiefly as a bleaching and cleansing agent (for example, to remove bathtub rings). Calculate the concentrations of all the species present at equilibrium in a 0.10 M solution. H2C2O4(aq) H+(aq) + (aq) Initial (M): Change (M): Equilibrium (M): 0.10 0.00 0.00 -x +x +x 0.10 - x x x = 6.5 x 10-2

15.11 H2C2O4(aq) H+(aq) + (aq) Initial (M): Change (M): Equilibrium (M): 0.10 0.00 0.00 -x +x +x 0.10 - x x x = 6.5 x 10-2 x2 + 6.5 x 10-2x - 6.5 x 10-3 = 0 If > 400 Can we neglect x? [H2C2O4]0 Ka1 we can neglect x x = 0.054 M or -0.12 M

15.11 Oxalic acid (H2C2O4) is a poisonous substance used chiefly as a bleaching and cleansing agent (for example, to remove bathtub rings). Calculate the concentrations of all the species present at equilibrium in a 0.10 M solution. H2C2O4(aq) H+(aq) + (aq) Equilibrium (M): 0.10 - x x x x = 0.054 M [H+] = 0.054 M [ ] = 0.054 M [H2C2O4] = 0.046 M the major species are HC2O4-, which acts as the acid in the second stage of ionization to generate more H+, and C2O42-. HC2O4-(aq) H+(aq) + C2O42-(aq)

15.11 For equilibrium 2: [H+] = 0.054 M [ ] = 0.054 M (aq) H+(aq) + Initial (M): Change (M): Equilibrium (M): 0.054 0.054 0.00 -y +y +y 0.054 - y 0.054 + y y If > 400 [HC2O4-]0 Ka2 we can neglect y Ka2 = 6.1 x 10-5 y = 6.1 x 10-5 M

15.11 Oxalic acid (H2C2O4) is a poisonous substance used chiefly as a bleaching and cleansing agent (for example, to remove bathtub rings). Calculate the concentrations of all the species present at equilibrium in a 0.10 M solution. Ka1 = 6.5 x 10-2 0.10 M H2C2O4(aq) H+(aq) + HC2O4-(aq) HC2O4-(aq) H+(aq) + C2O42-(aq) Ka2 = 6.1 x 10-5 [H2C2O4] = 0.046 M [ ] = (0.054 - 6.1 x 10-5) M = 0.054 M [H+] = (0.054 + 6.1 x 10-5) M = 0.054 M [ ] = 6.1 x 10-5 M [OH-] = 1.0 x 10-14/0.054 = 1.9 x 10-13 M

Last Example Calculate the pH of a 0.10 M solution of H3PO4. H3PO4(aq) ⇄ H+(aq) + H2PO4-(aq) Ka1 = 7.5 x 10-3 H2PO4-(aq) ⇄ H+(aq) + HPO42-(aq) Ka2 = 6.2 x 10-8 HPO42-(aq) ⇄ H+(aq) + PO43-(aq) Ka3 = 4.8 x 10-13 Acid strength decreases in the order: H3PO4 >> H2PO4- >> HPO42- pH of solution is determined mainly by ionization of H3PO4

Conceptual Question

Polyprotic Peptides Polypeptides Proteins

Chapter 15

Acid/Base Strength Some factors that influence acid/base strength (Ka/Kb). Temperature Solvent Acid Structure Hydrohalic acids Oxoacids Carboxylic acids Ka = [H+][A-] [HA] kfwd = krev

Solvent Dependence [H+][A-] Ka = [HA] HA (aq) H+ (aq) + A- (aq) H2O = AN = DCE = GP = Gas Phase

Hydrohalic Acid Strength The strength of an acid depends on the strength of the X–H bond that is to be broken. Relative bond strength: H─F > H─Cl > H─Br > H─I Relative acid strength: HI > HBr > HCl > HF H2Te > H2Se > H2S > H2O (all very weak acids) For H-X, electronegativity does not play a big role. 7.2 x 10-4 ~109 ~107 Ka

Hydrohalic Acid Strength H X H+ + X- The stronger the bond The weaker the acid HF << HCl < HBr < HI acidity increases

Oxoacid Strength Oxoacids is an acid that contains oxygen and a central atom Z. Z O H O- + H+ d- d+ The O-H bond strength/Ka are dependent on: electronegativity of Z oxidation state of Z

Cl is more electronegative than Br Oxoacid Strength Oxoacids is an acid that contains oxygen and a central atom Z. If they are from the same group and have the same oxidation number, acid strength increases with increasing electronegativity of Z. Cl is more electronegative than Br acidity increases HClO3 > HBrO3

Acid strength increases as the oxidation number of Z increases. Oxoacid Strength Oxoacids having the same central atom (Z) but different numbers of attached groups. Acid strength increases as the oxidation number of Z increases. HClO4 > HClO3 > HClO2 > HClO

15.12 Predict the relative strengths of the oxoacids in each of the following groups: ClOH, BrOH, and IOH (b) HNO3 and HNO2 ClOH > BrOH > IOH > The O-H bond will be easier to break if: Z is very electronegative or Z is in a high oxidation state

Carboxylic Acid Strength General structure for a carboxylic acid: Where R can be anything: H, CH3, Ph, Cl… The more electrongative R is the stronger the carboxylic acid. Proximity of the electrongative atom matters.

Chapter 15

HCl(aq) + NaOH(aq) NaCl(s) + H2O Acidic and Basic Salts acid + base salt + water HCl(aq) + NaOH(aq) NaCl(s) + H2O Salts are the ionic product of an acid base neutralization reaction. They are composed of related numbers of cations (positively charged ions) and anions (negative ions) so that the product is electrically neutral (without a net charge). XY(s) X+(aq) + Y-(aq) Soluble salts dissociate completely when dissolved in water. Ions/salts may acidic, basic or neutral.

Acidic and Basic Salts Acidic Salts are formed from a strong acid and a weak base. Neutral salts are formed from a strong acid and strong base. Basic salts are formed from a strong base and a weak acid. NaCl NaC2H3O2 NH4Cl HCl + NaOH NaCl + H2O s.a. s.b. neutral salt HC2H3O2 + NaOH NaC2H3O2 + H2O w.a. basic salt s.b. NH4OH NH4Cl + H2O HCl + s.a. w.b. acidic salt

Neutral Salts Dissociation and reaction of a neutral salt: NaCl(aq)  Na+(aq) + Cl-(aq) Na+(aq) + H2O  NR Cl- (aq) + H2O  NR The concentrations of H+ and OH- in NaCl solution are the same as in pure water  solution is neutral. Neutral salts are typically formed from a strong acid and strong base (which give weak conjugate acid and bases). Salts containing an alkali metal or alkaline earth metal ion (except Be2+) and the conjugate base of a strong acid (e.g. Cl-, Br-, and NO3-).

Basic Salts Dissociation and reaction of a basic salt: NaNO2(aq)  Na+(aq) + NO2-(aq) Na+(aq) + H2O  NR NO2-(aq) + H2O  HNO2(aq) + OH-(aq) The reaction of NO2- with water causes [OH-] > [H+] and the solution becomes basic. Basic salts are typically formed from a weak acid and strong base (which gives a weak conjugate acid and a stronger conjugate base).

15.13 Calculate the pH of a 0.15 M solution of sodium acetate (CH3COONa). (Kb of sodium acetate is 5.6 x 10-10) 0.15 M 0.15 M H2O CH3COONa (s) Na+ (aq) + CH3COO- (aq) CH3COO-(aq) + H2O(l) CH3COOH(aq) + OH-(aq) 0.15 M

15.13 Calculate the pH of a 0.15 M solution of sodium acetate (CH3COONa). (Kb of sodium acetate is 5.6 x 10-10) [CH3COO-] = 0.15 M CH3COO- (aq) + H2O (l) CH3COOH(aq) + OH-(aq) Initial (M): Change (M): Equilibrium (M): 0.15 0.00 0.00 -x +x +x 0.15 - x x x If > 400 [CH3COO-]0 Kb we can neglect x

15.13 Calculate the pH of a 0.15 M solution of sodium acetate (CH3COONa). (Kb of sodium acetate is 5.6 x 10-10) [CH3COO-] = 0.15 M CH3COO- (aq) + H2O (l) CH3COOH(aq) + OH-(aq) Initial (M): Change (M): Equilibrium (M): 0.15 0.00 0.00 -x +x +x 0.15 9.2 x 10-6 9.2 x 10-6 x = 9.2 x 10-6 [OH-] = 9.2 x 10-6 M pOH = -log (9.2 x 10-6 ) pOH = 5.04 pH = 14.00 - 5.04 pH = 8.96

Acidic Salts Dissociation and reaction of a neutral salt: NH4NO3 (aq)  NH4+(aq) + NO3-(aq) NH4+ (aq) + H2O  NH3(aq) + H3O+(aq) Cl- (aq) + H2O  NR The reaction of NH4+ with water causes [H+] > [OH-], and the solution becomes acidic. Neutral salts are typically formed from a strong acid and weak base (which give weak conjugate acid and bases). Salts with the conjugate base of a strong acid and small, highly charged metal cations (e.g. Al3+, Cr3+, and Be2+).

Hydrated Metal Ions Hydrated metal ions polarize coordinated water molecules and, consequently, act as Brønsted-Lowry acids: Their solutions, therefore, are acidic. This process is known as hydrolysis. Al(H2O)6 (aq) Al(OH)(H2O)5 (aq) + H+ (aq) 3+ 2+

Acidic and Basic Salts Acidic Salts are formed from a strong acid and a weak base. Neutral salts are formed from a strong acid and strong base. Basic salts are formed from a strong base and a weak acid. NaCl NaC2H3O2 NH4Cl HCl + NaOH NaCl + H2O s.a. s.b. neutral salt HC2H3O2 + NaOH NaC2H3O2 + H2O basic salt w.a. s.b. NH4OH NH4Cl + H2O HCl + acidic salt s.a. w.b.

Acidic and Basic Salts Salts of Strong Acid-Strong Base Reactions: NaCl, NaNO3, KBr, etc.; solutions are neutral Salts of Weak Acid-Strong Base Reactions: NaF, NaNO2, NaC2H3O2, etc.; solutions are basic Salts of Strong Acid-Weak Base Reactions: NH4Cl, NH4NO3, (CH3)2NH2Cl, C5H5NHCl; Solutions of these salts are acidic Salts of Weak Acid-Weak Base Reactions: NH4C2H3O2, NH4CN, NH4NO2, etc.. These compounds can be acidic, basic, or neutral, which depends on the relative strength of the acid and the base.

Acidic and Basic Salts Basic Salt (with B) NaB(s)  Na+(aq) + B-(aq) Kb = [BH+][OH-] [B-] B-(aq) + H2O  BH+(aq) + OH-(aq) Acidic Salt (with AH+) AHCl(s)  AH+(aq) + Cl-(aq) Ka = [H+][A] [HA+] AH+(aq)  A (aq) + H+(aq) What about AHB (with AH+ and B-)? AHB(s)  AH+(aq) + B-(aq) Generates H+ Generates OH- Solutions in which both the cation and the anion hydrolyze: Kb for the anion > Ka for the cation, solution will be basic Kb for the anion < Ka for the cation, solution will be acidic Kb for the anion  Ka for the cation, solution will be neutral

Predicting Acid-Base Property of Salts Is NH4C2H3O2 acidic, basic or neutral? NH4C2H3O2(s)  NH4+(aq) + C2H3O2-(aq) NH4+(aq) + H2O  H3O+(aq) + NH3(aq) Ka = 5.6 x 10-10 C2H3O2-(aq) + H2O  HC2H3O2(aq) + OH-(aq) Kb = 5.6 x 10-10 Ka = Kb  NH4C2H3O2 is neutral

Predicting Acid-Base Property of Salts Is (NH4)2SO4 acidic, basic or neutral? (NH4)2SO4(aq)  2NH4+(aq) + SO42-(aq); NH4+(aq) + H2O  H3O+(aq) + NH3(aq) Ka = 5.6 x 10-10 SO42-(aq) + H2O  HSO4-(aq) + OH-(aq) Kb = 8.3 x 10-13 Ka > Kb  (NH4)2SO4 is acidic.

Acidic and Basic Salts Summary XY(s) X+(aq) + Y-(aq) Ka Kb

15.14 Predict whether the following solutions will be acidic, basic, or nearly neutral: NH4I (b) NaNO2 (c) FeCl3 (d) NH4F NH4I(s)  NH4+(aq) + I-(aq) Acidic Salt acid very weak base NaNO2(s)  Na+(aq) + NO2-(aq) Basic Salt neutral base FeCl3(s)  Fe+(aq) + 3Cl-(aq) Acidic Salt acid very weak base NH4F(s)  NH4+(aq) + F-(aq) Acidic Salt acid base Ka = 5.6 x 10-10 Kb = 1.4 x 10-11

Chapter 15

Oxides Oxide is a chemical compound that contains at least one oxygen atom and one other element. Of the general from: E can be any element other than O or H. Oxides can be acidic, basic amphoteric. ExOy

Most transition metal oxides are basic. Oxides of highly electropositive metals are basic. Most transition metal oxides are basic. Oxides of electronegative nonmetals are acidic.

Oxides Oxides of electronegative nonmetals are acidic. Acidity increases left to right SiO2 < P4O10 < SO3 < Cl2O7 Acidity decreases top-to-bottom N2O5 > P4O10 > As2O5 > Sb2O5 N2O5 + H2O  2HNO3 SO3 + H2O  H2SO4 Cl2O7 + H2O  2HClO4

Oxides Oxides of highly electropositive metals are basic. Na2O(s) + H2O  2NaOH(aq) MgO(aq) + HCl(aq)  MgCl2(aq) + H2O

Oxides Some oxides are amphoteric: Can behave as an acid or a base depending on the circumstance. Al2O3(s) + HCl(aq)  2AlCl3(aq) + H2O Al2O3(s) + 2NaOH(aq) + 3H2O  2NaAl(OH)4(aq)

Chapter 15

Combine unpaired electrons Lewis Acids and Bases Lewis Dot Structure: Needs 3 electrons Need 1 electron each Combine unpaired electrons Gilbert Newton Lewis (1875-1946) Also proposed a definition for acids and bases. Now known as Lewis acids.

No H+ or OH- created. No protons donated or accepted! Lewis Acids and Bases A base is any species that donates an electron pair. An acid is any species that accepts an electron pair. This definition greatly expands the classes of acids. The acid-base reaction, in the Lewis sense, is the sharing of an electron pair between an acid and a base resulting in the formation of a bond: A + :B  A–B F B F N H • H F B F N H H + acid base No H+ or OH- created. No protons donated or accepted!

15.15 Identify the Lewis acid and Lewis base in each of the following reactions: C2H5OC2H5 + AlCl3 (C2H5)2OAlCl3 (b) Hg2+(aq) + 4CN-(aq) (aq) base acid acid base Here the Hg2+ ion accepts four pairs of electrons from the CN- ions.

Example Identify the Lewis acid and Lewis base in each of the following reactions: Cl– + AlCl3  AlCl4– H2O + CO2  H2CO3 H+ + OH-  H2O Cu2+ + 4NH3  Cu(NH3)42+ AlCl3 + Cl-  AlCl4-

Acid/Base Definitions Arrhenius acid is a substance that produces H+ in water base is a substance that produces OH- in water Brønsted acid is a substance that donates H+ base is a substance that accepts H+ Lewis acid is a substance that accepts a pair of electrons base is a substance that donates a pair of electrons

Venn Diagram

Acid/Base Venn Diagram All Brønsted-Lowry acids are Arrhenius acids. Not all Arrhenius acids are Brønsted-Lowry acids. etc…

Acid/Base Side Note Mg(OH)2 (s) + 2HCl (aq) MgCl2 (aq) + 2H2O (l) NaHCO3 (aq) + HCl (aq) NaCl (aq) + H2O (l) + CO2 (g)

Chapter 15