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Chapter Outline 11/16/2016 Aqueous Equilibria: Chemistry of the Water World Chapter Outline • 15.1 Acids and Bases: The BrØnsted–Lowry Model • 15.2 Acid Strength and Molecular Structure • 15.3 pH and the Autoionization of Water • 15.4 Calculations Involving pH, Ka, and Kb • 15.5 Polyprotic Acids • 15.6 pH of Salt Solutions • 15.7 The Common-Ion Effect • 15.8 pH Buffers • 15.9 pH Indicators and Acid–Base Titrations • 15.10 Solubility Equilibria 2 1 11/16/2016 Acids Have a sour taste. Vinegar owes its taste to acetic acid. Citrus fruits contain citric acid. React with certain metals to produce hydrogen gas. React with carbonates and bicarbonates to produce carbon dioxide gas Bases Have a bitter taste. Feel slippery. Many soaps contain bases. Nomenclature Review – Ch 4, Section 4.2 You are only responsible for nomenclature taught in the lab. These ions are part of many different acids and you need to know them! 3- 2- - PO4 , HPO4 , H2PO4 H3PO4 2- - SO4 , HSO4 H2SO4 2- - SO3 , HSO3 H2SO3 2- - CO3 , HCO3 H2CO3 - - HNO HNO NO3 , NO2 3, 2 2- - S , HS H2S - - C2H3O2 (CH3COO ) HC2H3O2 binary acids, oxoacids HCl, HClO4 2 11/16/2016 Strong and Weak Acids A Brønsted acid is a proton donor A Brønsted base is a proton acceptor Strong Acid: Completely ionized - + HNO3(aq) + H2O(ℓ) → NO3 (aq) + H3O (aq) (H+ donor) (H+ acceptor) Weak Acid: Partially ionized - + HNO2(aq) + H2O(ℓ) ⇌ NO2 (aq) + H3O (aq) (H+ donor) (H+ acceptor) 3 11/16/2016 Hydronium Ion Conjugate Acid-Base Pairs 4 11/16/2016 Weak Acids reordered stronger − + CH3COO (aq) [H3O (aq)] Kc = CH3COOH(aq) [H2O(l)] 5 11/16/2016 Strong and Weak Bases Strong and Weak Bases 6 11/16/2016 Weak Bases 7 11/16/2016 Relative Strengths of Acids/Bases Leveling Effect: + • H3O is the strongest H+ donor that can exist in water. • Strong acids all have the same strength in water; they are completely converted + into H3O ions. Relative Strengths of Acids/Bases Leveling Effect Bases: - + OH is the strongest H acceptor that can exist in H2O 8 11/16/2016 Chapter Outline • 15.1 Acids and Bases: The BrØnsted–Lowry Model • 15.2 Acid Strength and Molecular Structure • 15.3 pH and the Autoionization of Water • 15.4 Calculations Involving pH, Ka, and Kb • 15.5 Polyprotic Acids • 15.6 pH of Salt Solutions • 15.7 The Common-Ion Effect • 15.8 pH Buffers • 15.9 pH Indicators and Acid–Base Titrations • 15.10 Solubility Equilibria 17 Acid Strength and Molecular Structure H2SO4 is a stronger acid because – 1. The -2 charge is delocalized over 4 oxygen atoms compared to three 2. the larger number of oxygens in H2SO4 creates a greater electronegativity effect and consequent weakening of the O-H bond. 9 11/16/2016 10 11/16/2016 The Acid-Base Properties of Water Water is amphoteric - which means that it can behave either as an acid or a base + - H2O (l) H (aq) + OH (aq) autoionization of water H+ + - H O + H O [ H O H] + H O H H H conjugate base acid + - H2O + H2O H3O + OH equivalent conjugate expressions acid base 11 11/16/2016 Chapter Outline • 15.1 Acids and Bases: The BrØnsted–Lowry Model • 15.2 Acid Strength and Molecular Structure • 15.3 pH and the Autoionization of Water • 15.4 Calculations Involving pH, Ka, and Kb • 15.5 Polyprotic Acids • 15.6 pH of Salt Solutions • 15.7 The Common-Ion Effect • 15.8 pH Buffers • 15.9 pH Indicators and Acid–Base Titrations • 15.10 Solubility Equilibria 23 pH and the Autoionization of Water [H O+][OH-] 2 H O(l) = H O+ + OH- 3 2 3 Kc = 2 [H2O] + - What is the concentration of H3O and OH in pure water? Using the RICE table - 12 11/16/2016 pH - A Measure of Acidity pH = -log [H+] pH [H+] Solution 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 The pH Scale 13 11/16/2016 pH, pOH, and K pOH is defined the same way as pH - pOH = - log[OH-] “p-functions” are very common in chemistry, i.e. the negative log of any physical constant is calculated the same way. Since Ka and Kb values for weak acids and bases tend to be very small, it’s convenient to take the negative log of these values as well pKa = - log[Ka] • The smaller the Ka, the weaker the acid • The weaker the acid, the larger the pKa • The same concepts apply for weak bases pKb = - log[Kb] 14 11/16/2016 Useful Equation for Acid-Base Calculations 1. Starting with Kw + - -14 Kw = [H3O ][OH ] = 1.00 x 10 2. Taking the negative log of both sides - + - - log Kw = - log [H3O ][OH ] -14 + - - log(1.00 x 10 ) = - log [H3O ] - log[OH ] 14 = pH + pOH 15 11/16/2016 Chapter Outline • 15.1 Acids and Bases: The BrØnsted–Lowry Model • 15.2 Acid Strength and Molecular Structure • 15.3 pH and the Autoionization of Water • 15.4 Calculations Involving pH, Ka, and Kb • 15.5 Polyprotic Acids • 15.6 pH of Salt Solutions • 15.7 The Common-Ion Effect • 15.8 pH Buffers • 15.9 pH Indicators and Acid–Base Titrations • 15.10 Solubility Equilibria 31 Weak Acids • Most acids are weak. How do you know if an acid is weak? • Because it’s not one of the 6 strong ones you’ve memorized! HCl hydrochloric HBr hydrobromic HI hydroiodic HNO3 nitric HClO4 perchloric The monster in the movie “Alien’” had blood that contained H2SO4 sulfuric “molecular acid” and ate through six decks of the spaceship! 16 11/16/2016 General Weak Acid Equilibrium Equation and Ka + - HA(aq) + H2O(l) = H3O (aq) + A (aq) [H O+][A-] K = 3 a [HA] If you measure the pH of a solution containing a weak acid, you can calculate the equilibrium constant Calculating Ka for a weak acid when the pH is known, e.g 0.100 M, pH = 2.20 + - HA(aq) + H2O(l) = H3O (aq) + A (aq) 0.100 M 17 11/16/2016 Percent Ionization + [H ]equil Percent Ionization = X 100% [퐻퐴]initial 18 11/16/2016 Weak Bases Weak bases frequently contain nitrogen because the lone pair makes a good proton acceptor NH3 ammonia NH(CH3)2 C6H5NH2 dimethylamine aniline General Weak Base Equilibrium Equation and Kb + - B(aq) + H2O(l) = BH (aq) + OH (aq) [BH+][OH-] K = b [B] Ordinary bleach contains the weak base ClO- 19 11/16/2016 Relationship Between Ka and Kb + - - [H3O ][A ] [HA][OH ] K = K = a [HA] b [A-] 20 11/16/2016 Chapter Outline • 15.1 Acids and Bases: The BrØnsted–Lowry Model • 15.2 Acid Strength and Molecular Structure • 15.3 pH and the Autoionization of Water • 15.4 Calculations Involving pH, Ka, and Kb • 15.5 Polyprotic Acids • 15.6 pH of Salt Solutions • 15.7 The Common-Ion Effect • 15.8 pH Buffers • 15.9 pH Indicators and Acid–Base Titrations • 15.10 Solubility Equilibria 41 Polyprotic Acids Two or more ionizable protons Ka1 > Ka2 > Ka3 H H+ -3 Ka1 = 7.11 x 10 H H+ -8 Ka2 = 6.32 x 10 H+ -13 Ka3 = 4.5 x 10 H 21 11/16/2016 Chapter Outline • 15.1 Acids and Bases: The BrØnsted–Lowry Model • 15.2 Acid Strength and Molecular Structure • 15.3 pH and the Autoionization of Water • 15.4 Calculations Involving pH, Ka, and Kb • 15.5 Polyprotic Acids • 15.6 pH of Salt Solutions • 15.7 The Common-Ion Effect • 15.8 pH Buffers • 15.9 pH Indicators and Acid–Base Titrations • 15.10 Solubility Equilibria 44 22 11/16/2016 pH of Salt Solutions 1. Neutral Salts (pH = 7) are from strong electrolytes (100% ionization) (a)Ionic Compounds: NaCl(aq) Na+(aq) + Cl-(aq) base conj. acid + - HCl(aq) + H2O(l) H3O (aq) + Cl (aq) acid conj. base Infinitely Infinitely strong weak pH of Salt Solutions 2. Basic Salts (pH > 7) are conjugate bases of weak acids + - HClO(aq) + H2O(l) = H3O (aq) + ClO (aq) weak conj. acid base - - ClO (aq) + H2O(l) = OH (aq) + HClO(aq) conj. base pH > 7 K 1.0 x 10-14 K (HClO) = 2.9 x 10-8 K = w K = a b b -8 Ka 2.9 x 10 - -7 Kb (ClO ) = 3.4 x 10 23 11/16/2016 pH of Salt Solutions 3. Acidic Salts (pH < 7) are conjugate acids of weak bases - + NH3(aq) + H2O(l) = OH (aq) + NH4 (aq) weak conj. base acid + + NH4 (aq) + H2O(l) = H3O (aq) + NH3(aq) conj. acid pH < 7 K 1.0 x 10-14 K (NH ) = 1.8 x 10-5 K = w K = b 3 a a -5 Kb 1.8 x 10 + -10 Ka (NH4 ) = 5.6 x 10 24 11/16/2016 Calculating the pH of Solutions of Weak Acids and Bases: Use the RICE Table as Before 1. Calculating the pH of a Solution of a Basic Salt - - -8 ClO (aq) + H2O(l) = OH (aq) + HClO(aq) Ka = 2.9 x 10 0.100 M - x x x K = Kw b K 2 a -7 x Kb = 3.4 x 10 = -7 0.100 M - x Kb = 3.4 x 10 x2 -5 3.4 x 10-7 = Since Ka is < 10 , assume 0.100 M that x << 0.100 M x = (3.4 x 10-7)(0.100) x = [OH-] = 1.9 x 10-4 M Assumption OK pOH = - log (1.9 x 10-4) = 3.7 pH = 14 - 3.7 = 10.3 Calculating the pH of Solutions of Weak Acids and Bases: Use the RICE Table as Before 2.
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