Chapter 16: Acid-Base Equilibria • 16.1. Acids and Bases: A Brief Review • 16.2 Brønsted-Lowry Acids and Bases • 16.3 The Autoionization of Water • 16.4 The pH Scale • 16.5 Strong Acids and Bases • 16.6 Weak Acids • 16.7 Weak Bases • 16.8 Relationship Between Ka and Kb • 16.9 Acid-Base Properties of Salt Solutions • 16.10 Acid-Base Behavior and Chemical Structure • 16.11 Lewis Acids and Bases
Arrhenius defined an acid as a substance that produces H+ ions in water; he defined a base as a substance that produces OH– ions in water. HCl—one of the strong acids—is an Arrhenius acid. Potassium hydroxide—one of the strong bases—is an Arrhenius base.
16.2 Brønsted-Lowry Acids and Bases • According to Brønsted-Lowry, an acid is a substance that donates an H+ ion to another substance; a base is a substance that accepts an H+ ion Conjugate Base Bronsted Acid Conjugate Acid Bronsted Base
Classifications of Acids • Any species that contains hydrogen can be classified as one of three types of acids. • The strong acids are those that completely transfer their protons to water, leaving no undissociated molecules in solution (see eChapter 4.3). Their conjugate bases have a negligible tendency to be protonated (to combine with a proton) in aqueous solution. • The weak acids are those that only partly dissociate in aqueous solution and therefore exist in the solution as a mixture of acid molecules and component ions. Their conjugate bases are weak bases, showing a slight ability to remove protons from water. • The substances with negligible acidity are those such as CH4, that contain hydrogen but do not demonstrate any acidic behavior in water. Their conjugate bases are strong bases, reacting completely with water, abstracting a proton to form OH– ions.
16.3 The Autoionization of Water • Pure water has a very small tendency to ionize, acting both as an acid (donating a proton) and as a base (accepting a proton). • At 25°C, the Kc for this process is 1.0 x 10–14, which means that only about one molecule per billion undergoes this autoionization. The equilibrium expression for the autoionization of water is • (Recall that a liquid does not appear in the equilibrium expression.) • Because the autoionization of water is a very important equilibrium, its equilibrium constant is given a special subscript, w. For any aqueous solution at 25°C, the product of hydronium and hydroxide ion concentrations is equal to Kw. In neutral water, where the only source of either ion is the autoionization, the hydronium and hydroxide ion concentrations are equal. • So the concentrations of both hydronium ion and hydroxide ion in neutral water is 1.0 x 10–7M.
16.4 The pH Scale • Figure 16.5. Values of pH for some common solutions. The pH scale is shown to extend from 0 to 14 because nearly all solutions commonly encountered have pH values in that range. In principle, however, the pH values for strongly acidic solutions can be less than 0, and for strongly basic solutions can be greater than 14.
16.5 Strong Acids and Bases • Strong acids are those that ionize completely in water. Strong acids are also strong electrolytes • Strong bases are ionic compounds that dissociate completely in water. They include the hydroxides of group 1A metals and group 2A metals
16.6 Weak Acids • A weak acid is one that ionizes partially in water to produce hydronium ion and a conjugate base • Ex:
Weak Acid Sample Problem What is the pH of a 1.75 10–3M nitrous acid solution? The ionization equilibrium expression for nitrous acid is HNO2 H+ + NO2-
Polyprotic acids • Polyprotic acids are those that have more than one hydrogen that can be donated as a proton. Each proton on a polyatomic acid has a Ka value associated with it—except for the first proton on sulfuric acid, because it is strongly acidic
16.7 Weak Bases • A weak base is one that ionizes partially in water to produce hydroxide ion and a conjugate acid. The general form for Bronsted Base ionization is, • B(aq) + H2O BH+(aq) + OH-(aq) • Being an equilibrium, the ionization of a weak base has an equilibrium constant called the base-dissociation constantassociated with it. The equilibrium constant for a weak base ionization has the subscript b for base. The equilibrium expression for the above equation is • As with all equilibria, the larger the value of K, the further the equilibrium lies to the right. This means that the larger the value of Kb, the stronger the base. • Two major types of weak bases are amines (Contain N-C bond) and conjugate base of weak acid
Weak base sample problem Conjugate Acid Bronsted Base (Amine type)
Using pH to Determine the Concentration of a Salt Solution • A solution is made by adding solid sodium hypochlorite (NaClO, pool chlorine) to enough water to make 2.00 L of solution has a pH of 10.50. Calculate the grams of NaClO that were added to the water. The Kb of ClO- is 3.3 x 10-7.
Problem solving technique • You are given 2.00 L solution of NaClO, pH is 10.50, Kb = 3.3 x 10-7. • NaClO is a strong electrolyte (you know that because it is ionic [metal-nonmetal], and contains alkali metal), so [ClO-] = [NaClO] • pH can give you [OH-], then you can ICE box to calculate [ClO-]
pOH = 14.00 – pH = 14.00 – 10.50 = 3.50 • [OH-] = 10-3.50 = 3.2 x 10-4 M (at equilibrium) ClO- + H2O HClO + OH- 3.3 x 10-7 = = = 46 g NaClO
16.9 Acid-Base Properties of Salts • Characteristics to Determine Acid-Base • Anion’s ability to react with water • Anion that is conjugate base of strong acid is neutral • Anion that is conjugate base of weak acid is basic • Cation’s ability to react with water • Cation of strong base is neutral • Cation that is conjugate acid of a weak base is acidic • Metal cations are acidic • Combined effect • Depends on K values, larger K wins
16.10 Acid-Base behavior and chemical structure • Factors that affect acid strength • Strength of H-X bond, weaker bonds mean stronger acid
16.10 Acid-Base behavior and chemical structure • Oxyacids (R-O-H) • If R is a metal, substance is a base • If R is a nonmetal • Strength increases as electronegativity of R increases • Strength increases as number of oxygen atoms increases • Carboxylic acids • Could have electron-withdrawing group (like element of high electronegativity) • Could have electron-donating group (like benzene ring)
16.11 Lewis acids and bases • Lewis acid is electron-pair acceptor • Lewis base is an electron-pair donor • Classic example: