Chapter 11: Acids and Bases

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Chapter 11: Acids and Bases University Chemistry   Chapter 11: Acids and Bases Copyright © The McGraw-Hill Companies, Inc.  Permission required for reproduction or display.

Arrhenius Definition of Acids and Bases Arrhenius acid is a substance that produces H+ in water Arrhenius base is a substance that produces OH- in water

Properties of Acids Have a sour taste. Vinegar owes its taste to acetic acid. Citrus fruits contain citric acid. Acids cause color changes in plant dyes, for example, they change the color of litmus from blue to red. React with certain metals to produce hydrogen gas. 2HCl (aq) + Mg (s) MgCl2 (aq) + H2 (g) React with carbonates and bicarbonates to produce carbon dioxide gas 2HCl (aq) + CaCO3 (s) CaCl2 (aq) + CO2 (g) + H2O (l) Aqueous acid solutions conduct electricity.

Properties of Bases Have a bitter taste. Feel slippery. Many soaps contain bases. Bases cause color changes in plant dyes, for example, they change the color of litmus from red to blue. Aqueous base solutions conduct electricity.

Brønsted Definition of Acids and Bases Acid is a proton* donor Base is a proton acceptor * In solution exists as a hydrated proton (H3O+) or hydronium ion.

Hydronium ion, hydrated proton, H3O+ 4.3

Strong acids for all practical purposes are completely ionized in water. Strong bases for all practical purposes are completely ionized in water. Weak acids ionize only to a limited extent in water. Weak bases ionize only to a limited extent in water.

Monoprotic acids Diprotic acids Triprotic acids HCl H+ + Cl- Strong electrolyte, strong acid HNO3 H+ + NO3- Strong electrolyte, strong acid CH3COOH H+ + CH3COO- Weak electrolyte, weak acid Diprotic acids H2SO4 H+ + HSO4- Strong electrolyte, strong acid HSO4- H+ + SO42- Weak electrolyte, weak acid Triprotic acids H3PO4 H+ + H2PO4- Weak electrolyte, weak acid H2PO4- H+ + HPO42- Weak electrolyte, weak acid HPO42- H+ + PO43- Weak electrolyte, weak acid

Amphoteric: refers to a substances that can act as both a Brønsted acid and a Brønsted base

Neutralization Reaction An ionic compound made up of a cation other than H+ and an anion other than OH- or O2-. acid + base salt + water molecular equation HCl (aq) + NaOH (aq) NaCl (aq) + H2O H+ + Cl- + Na+ + OH- Na+ + Cl- + H2O H+ + OH- H2O net ionic equation spectator ions

Conjugate Acid-Base Pairs Defined as an acid and its conjugate base or a base and its conjugate acid. The word conjugate means “joined together”. base1 acid2 acid1 base2 base acid conjugate acid conjugate base

Conjugate acid-base pairs: The conjugate base of a strong acid has no measurable strength. H3O+ is the strongest acid that can exist in aqueous solution. The OH- ion is the strongest base that can exist in aqueous solution.

Lewis Definition of Acids and Bases A Lewis acid is a substance that can accept a pair of electrons A Lewis base is a substance that can donate a pair of electrons + OH- • H O H • H+ acid base N H • H N H H + H+ + acid base

Acid-Base Properties of Water hydroxide ion hydronium ion autoionization of water hydroxide ion hydronium ion O H + - [ ] conjugate acid base H2O + H2O H3O+ + OH- acid conjugate base

The Ion Product of Water The ion-product constant (Kw) is the product of the molar concentrations of H+ and OH- ions at a particular temperature. Solution is [H+] = [OH-] neutral At 250C Kw = [H+][OH-] = 1.0 x 10-14 [H+] > [OH-] acidic [H+] < [OH-] basic

pH – A Measure of Acidity Solution Is At 250C neutral [H+] = [OH-] [H+] = 1 x 10-7 pH = 7 acidic [H+] > [OH-] [H+] > 1 x 10-7 pH < 7 basic [H+] < [OH-] [H+] < 1 x 10-7 pH > 7 pH [H+]

[H+][OH-] = Kw = 1.0 x 10-14 -log [H+] – log [OH-] = 14.00

Temperature Dependence of Kw and pH Le Châtelier’s principle: the equilibrium of an endothermic process will shift in the direction of products when the temperature is increased, leading to a corresponding increase in the equilibrium constant.

Weak Acid: HF Strong Acid: HCl

Weak Acids (HA) and Acid Ionization Constants HA (aq) + H2O (l) H3O+ (aq) + A- (aq) HA (aq) H+ (aq) + A- (aq) Ka is the acid ionization constant weak acid strength Ka

Solvent leveling: All acids with Ka>1 will yield identical H3O+ concentrations in water, independent of their individual Ka values Proton exchange reaction: Ka = 1 The strongest acid that can exist in aqueous solution is the hydronium ion (H3O+).

Weak Bases and Base Ionization Constants Kb is the base ionization constant weak base strength Kb

Solvent leveling: All bases with Kb>1 will yield identical OH- concentrations in water, independent of their individual Kb values Proton exchange reaction: Kb = 1 The strongest base that can exist in aqueous solution is the hydroxide ion (OH-).

Ionization Constants of Conjugate Acid-Base Pairs 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 Weak Acid Conjugate Base Ka = Kw Kb Kb = Kw Ka

Ionization Constants for Diprotic and Triprotic Acids

Solving weak acid ionization problems: Identify the major species that can affect the pH. In most cases, you can ignore the autoionization of water. Ignore [OH-] because it is determined by [H+]. Use initial concentration and change to express the equilibrium concentrations in terms of single unknown x. Write Ka in terms of equilibrium concentrations. Solve for x by the approximation method. 4, If approximation is not valid, solve for x exactly. 5. Calculate concentrations of all species and/or pH of the solution.

For a monoprotic acid HA Percent Ionization For a monoprotic acid HA Percent ionization = [H+] [HA]0 x 100% [HA]0 = initial concentration

pH of Dilute Acid and Base Solutions Dilute Solutions of Weak Acids H2O(l) = H+(aq) + OH-(aq) Need two more equations: mass balance charge balance Use if H+ ion concentration is expected to be less than 10-6 M. Very Dilute Solutions of Strong Acids Only the terms containing Ka are significant. If neglect Kw

pH of Diprotic and Triprotic Acids H2A + H2O = H3O+(aq) + HA-(aq) Ka1 HA-(aq) + H2O = H3O+(aq) + A-(aq) Ka2 Concentration of H+ ions is the product of only the first stage of ionization. Concentration of the conjugate base for the second stage of ionization is numerically equal to

Molecular Structure and Acid Strength H X H+ + X- Hydrohalic Acids The stronger the bond Two competing effects: The weaker the acid The polar the bond The stronger the acid HF << HCl < HBr < HI

Oxoacids Z O H O- + H+ d- d+ The O-H bond will be more polar and easier to break if: Z is very electronegative or Z is in a high oxidation state

Acid strength increases with increasing electronegativity of Z 1. Oxoacids having different central atoms (Z) that are from the same group and that have the same oxidation number. Acid strength increases with increasing electronegativity of Z H O Cl O O • H O Br O O • Cl is more electronegative than Br HClO3 > HBrO3

Acid strength increases as the oxidation number of Z increases. 2. 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

Lewis structures can be generalized as: Carboxylic Acids Lewis structures can be generalized as: R is part of the acid molecule Carboxyl group (-COOH). Strengths of carboxylic acids depend on the nature of the R group. Electronegative groups in the R group polarize the –O—H bond thereby increasing acidity

Acid-Base Properties of Salts Salts producing neutral solutions: 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-). NaCl (s) Na+ (aq) + Cl- (aq) H2O Salts producing basic solutions: Salts derived from a strong base and a weak acid. NaCH3COOH (s) Na+ (aq) + CH3COO- (aq) H2O CH3COO- (aq) + H2O (l) CH3COOH (aq) + OH- (aq)

Salts producing acidic solutions: Salts derived from a strong acid and a weak base. NH4Cl (s) NH4+ (aq) + Cl- (aq) H2O NH4+ (aq) NH3 (aq) + H+ (aq) Salts with small, highly charged metal cations (e.g. Al3+, Cr3+, and Be2+) and the conjugate base of a strong acid. Al(H2O)6 (aq) Al(OH)(H2O)5 (aq) + H+ (aq) 3+ 2+

Acid Hydrolysis of Al3+

Salts 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

Salts of amphoteric ions:

Oxides of the Representative Elements In Their Highest Oxidation States

Antacids and the Stomach pH Balance NaHCO3 (aq) + HCl (aq) NaCl (aq) + H2O (l) + CO2 (g) Mg(OH)2 (s) + 2HCl (aq) MgCl2 (aq) + 2H2O (l)

Alkali metal hydroxides Alkaline earth metal hydroxides except Be(OH)2 Basic hydroxides: Alkali metal hydroxides Alkaline earth metal hydroxides except Be(OH)2 Amphoteric hydroxides: Be(OH)2, Al(OH)3, Sn, Pd, Cr, Cu, Zn & Cd hydroxides Al(OH)3(s) + 3H+  Al3+(aq) + 2H2O(l) Al(OH)3(s) + OH- = Al(OH)4-(aq)