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Chapter Outline 11/6/2014 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 11/6/2014 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 11/6/2014 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 11/6/2014 Hydronium Ion Molecular Structures of Strong Acids and Their Ionization Reactions in Water 4 11/6/2014 Molecular Structures of Strong Acids and Their Ionization Reactions in Water Conjugate Acid-Base Pairs 5 11/6/2014 Weak Acids 6 11/6/2014 Strong and Weak Bases Strong and Weak Bases 7 11/6/2014 Weak Bases 8 11/6/2014 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 9 11/6/2014 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 19 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. 10 11/6/2014 11 11/6/2014 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 12 11/6/2014 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 25 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 - 13 11/6/2014 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 14 11/6/2014 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] 15 11/6/2014 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 16 .
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