<p> Chapter 15 Aqueous Equilibria: Acids and Bases 15.1 -15.3: Definition of Acids and Bases Properties of Base</p><p>Properties of Acid • Taste bitter • Sour taste alkaloids = plant product that is alkaline • Ability to dissolve many metals often poisonous • Ability to neutralize bases • Feel slippery • Change blue litmus paper to red • Ability to turn red litmus paper blue</p><p>• Ability to neutralize acids</p><p>Definition of Arhenius acids and bases Acid: any compounds that produce H+ when dissolve in water Base: any compounds that produce –OH when dissolve in water</p><p>Base: any compounds that produce –OH when dissolve in water Problems:</p><p>• It does not explain why molecular substances, such as NH3, dissolve in water to form basic solutions, even though they do not contain OH– ions.</p><p>• It does not explain how some ionic compounds, such as Na2CO3 or Na2O, dissolve in water to form basic solutions, even though they do not contain OH– ions.</p><p>• It does not explain why molecular substances, such as CO2, dissolve in water to form acidic solutions, even though they do not contain H+ ions. • It does not explain acid–base reactions that take place outside aqueous solution. This theory is limited to only “those compounds”</p><p>Dang1 The Brønsted-Lowry Theory: a better definition for acids and bases in aqueous solution Brønsted-Lowry Acid: A substance that can transfer hydrogen ions, H+. In other words, a proton donor loses a proton Conjugate Acid-Base Pairs: Chemical + - HA(aq) + H2O(l) H3O (aq) + A (aq) species whose formulas differ only by one hydrogen ion, H+ gains a proton HA and A-</p><p>+ H2O and H3O</p><p>+ In the acid example, notice the formation of H3O -- this species is named the hydronium ion. It lets you know that the solution is acidic!</p><p>+ - HCl(aq) + H2O(l)  H3O (aq) + Cl (aq) (neutral compound)</p><p>Brønsted-Lowry Base: A substance that can accept hydrogen ions, H+ . In other words, a proton acceptor Loses a proton Conjugate Acid-Base pairs - + B(aq) + H2O(l) OH(aq) + BH (aq) B and BH+</p><p> gains a proton - H2O and OH</p><p>- + NH3(aq) + H2O(l) OH(aq) + NH4 (aq)</p><p>Amphoteric Substances</p><p>• Amphoteric substances can act as either an acid or a base because they have both a transferable H+ and an atom with lone pair electrons.</p><p>• Water acts as base, accepting H+ from HCl.</p><p>– + HCl(aq) + H2O(l) → Cl (aq) + H3O (aq)</p><p>+ • Water acts as acid, donating H to NH3.</p><p>+ – NH3(aq) + H2O(l) NH4 (aq) + OH (aq)</p><p>Examples: Write a balanced equation for the dissociation of each of the following Bronsted-Lowry acids in water </p><p>H2CO3(aq)</p><p>Dang2 - HSO3 (aq)</p><p>Examples: Write a balanced equation for the dissociation of each of the following Bronsted-Lowry bases in water</p><p>2- CO3 (aq)</p><p>CH3NH2(aq)</p><p>Example: In the following equation, label each species as an acid (A) or a base (B) on the left. Identify the conjugated acid (CA) or base on the right (CB) - - a. NO2 (aq) + HF(aq) HNO2(aq) + F (aq)</p><p>+ - b. HClO4(aq) + H2O(l) H3O (aq) + ClO4 (aq)</p><p>15.4 Acid Strength and the Acid Ionization Constant, Ka Strength is determined by the position of the "dissociation" equilibrium</p><p>Acid Strength and Base Strength</p><p>Strong acids/strong bases/strong electrolyte 1. dissociates completely in water 2. all the base molecules form H+ or OH– ions, either Dangthrough3 dissociation or reaction with water,  3. have very large dissociation or K values • Strong acids donate practically all their H’s. 100% ionized in water Strong electrolyte + • [H3O ] = [strong acid]ini • Has no Ka values (why?)</p><p> Other acids are considered “weaker acids”</p><p>Weak acids donate a small fraction of their H’s.</p><p>Most of the weak acid molecules do not donate H to water.</p><p>Much less than 1% ionized in water</p><p>+ [H3O ] << [weak acid]</p><p>1. dissociate only to a slight extent in water 2. only a small percentage of the base molecules form OH– ions, either through dissociation or reaction with water 2. dissociation constant is very small Do Not confuse concentration with strength!</p><p>Strengths of Acids and Bases</p><p>Dang4 • Commonly, acid (HA) or base (B) strength is measured by determining the equilibrium constant of a substance’s reaction with water. - + HA + H2O A + H3O + − B + H2O HB + OH</p><p>The stronger the acid the weaker its conjugate base, the converse is also true. • The stronger an acid is at donating H, the weaker the conjugate base is at accepting H.</p><p>• Higher oxidation number = stronger oxyacid</p><p>– H2SO4 > H2SO3; HNO3 > HNO2</p><p>• Cation stronger acid than neutral molecule; neutral stronger acid than anion</p><p>+ − + − – H3O > H2O > OH ; NH4 > NH3 > NH2</p><p>• The farther the equilibrium position lies toward the products, the stronger the acid or base. • The position of equilibrium depends on the strength of attraction between the base form and the H+. Stronger attraction means stronger base or weaker acid</p><p>Trend in base strength opposite</p><p>Examples Dang5  If you mix equal amount concentrations of reactants and products, decide which species (reactants or products) are favored at the completion of the reaction. Why?</p><p>◦ + - H2SO4(aq) + NH3(aq) NH4 (aq) + HSO4 (aq)</p><p>◦ - - HF(aq) + NO3 (aq) HNO3(aq) + F (aq)</p><p>15.5 Autoionization of water Fredrich Kohlrausch, around 1900, found that no matter how pure water is, it still conducts a minute amount of electric current. This proves that water self-ionizes. • Since the water molecule is amphoteric, it may dissociate with itself to a slight extent. • Only about 2 in a billion water molecules are ionized at any instant!</p><p>+ − H2O(l) + H2O(l) H3O (aq) + OH (aq)</p><p>The equilibrium expression used here is referred to as the autoionization constant for water, Kw o + - -14 At 25 C Kw = [H3O ] [ OH] = 1.0 x 10</p><p>+ - Knowing this value will help to calculate [H3O ] or [ OH] at various situations</p><p> Acidic: Neutral Basic +  +  +  [H3O ] > [OH ] [H3O ] = [OH ] [H3O ] < [OH ]</p><p> o −13 + - Example: At 60 C, At 60°C, the value of Kw is 1.0 × 10 . Calculate the [H3O ] and [ OH] in a neutral solution</p><p>The pH Scale Used to designate the [H+] in most aqueous solutions where [H+] is small. pH = −log [H+] pOH = − log [-OH] pH + pOH = 14.00</p><p>Acidic: pH < 7 Neutral: pH = 7 Basic: pH > 7</p><p>Dang6 The pH Scale and Significant figures + The hydronium ion concentration for lemon juice is approximately 0.0025 M. What is the pH when [H3O ] = 0.0025 M? 2 significant figures</p><p> pH = log(0.0025) = 2.60</p><p>2 decimal places</p><p>- + -5 Examples: Calculate the [ OH] in a solution with [H3O ] = 7.5 x 10 M. Is this solution basic, acidic or neutral?</p><p> Calculate the pH of a solution with [-OH] = 8.2 x 10-10M</p><p>+ –  Calculate the concentration of H3O and OH for a solution with a pH of 8.37</p><p>15.6 Measuring pH Acid-Base Indicator: A substance that changes color in a specific pH range. Indicators exhibit pH-dependent color changes because they are weak acids and have different colors in their acid (HIn) and conjugate base (In) forms.</p><p>- HIn(aq) + H2O(l) H3O+(aq) + In(aq) </p><p>• Indicators are chemicals that change color depending on the solution’s acidity or basicity.</p><p>• Many vegetable dyes are indicators.</p><p>– Anthocyanins</p><p>• Litmus Dang7 – From Spanish moss</p><p>– Red in acid, blue in base</p><p>• Phenolphthalein</p><p>– Found in laxatives</p><p>Red in base, colorless in acid</p><p> pH of strong acids and strong bases</p><p> A strong monoprotic acds – 100% dissociated in aqueous solution (HClO4, HCl, HBr, HI, H2SO4 HNO3 etc…) ◦ Contains a single dissociable proton + - HA(aq) + H2O(l)  H3O (aq) + A (aq)</p><p>◦ + pH = - log H3O</p><p>◦ + - [H3O ] = [A ] = initial concentration of the acid ◦ Undissociated [HA] = 0 Strong Bases  Alkali metal hydroxide, MOH ◦ Water-soluble ionic solids ◦ Exits in aqueous solution as alkali metal cations and hydroxide anions ◦ Calculate pH from [-OH]</p><p> Alkaline earth metal hydroxide, M(OH)2 where M= Mg, Ca, Sr, Ba ◦ Less soluble than alkali hydroxide, therefore lower [-OH]</p><p>Dang8 The pH in Solutions of Strong Acids and Strong Bases - Example: Calculate the [ OH] and pH of 0.30 M HNO3</p><p> What is the pH of a 0.0250 M solution of Ca(OH)2?</p><p> The pH of an unknown acid solution is 2.45. What is the concentration of the acid solution?</p><p>Dang9</p>