CHE 124: General Chemistry II Chapter 15: Acids & Bases CHE 124: General Chemistry II Dr. Jerome Williams, Ph.D. Saint Leo University

Overview Strong vs. Weak Acids & Bases Polyprotic Acids Strengths of Acids & Bases Acid Dissociation Constant Autoionization of Water

Arrow Conventions  in these notes Chemists commonly use two kinds of arrows in reactions to indicate the degree of completion of the reactions A single arrow indicates all the reactant molecules are converted to product molecules at the end A double arrow indicates the reaction stops when only some of the reactant molecules have been converted into products  in these notes Tro, Chemistry: A Molecular Approach, 2/e

Strong or Weak A strong acid is a strong electrolyte practically all the acid molecules ionize, → A strong base is a strong electrolyte practically all the base molecules form OH– ions, either through dissociation or reaction with water, → A weak acid is a weak electrolyte only a small percentage of the molecules ionize,  A weak base is a weak electrolyte only a small percentage of the base molecules form OH– ions, either through dissociation or reaction with water,  Tro, Chemistry: A Molecular Approach, 2/e

Strong Acids The stronger the acid, the more willing it is to donate H we use water as the standard base to donate H to Strong acids donate practically all their H’s 100% ionized in water strong electrolyte [H3O+] = [strong acid] [X] means the molarity of X HCl ® H+ + Cl− HCl + H2O ® H3O+ + Cl− 0.10 M HCl = 0.10 M H3O+ Tro, Chemistry: A Molecular Approach, 2/e

Weak Acids Weak acids donate a small fraction of their H’s most of the weak acid molecules do not donate H to water much less than 1% ionized in water [H3O+] << [weak acid] HF Û H+ + F− HF + H2O Û H3O+ + F− 0.10 M HF ≠ 0.10 M H3O+ Tro, Chemistry: A Molecular Approach, 2/e

Strong & Weak Acids Tro, Chemistry: A Molecular Approach, 2/e

Tro, Chemistry: A Molecular Approach, 2/e

Strengths of Acids & Bases Commonly, acid or base strength is measured by determining the equilibrium constant of a substance’s reaction with water HAcid + H2O  Acid− + H3O+ Base: + H2O  HBase+ + OH− 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 Tro, Chemistry: A Molecular Approach, 2/e

General Trends in Acidity The stronger an acid is at donating H, the weaker the conjugate base is at accepting H Higher oxidation number = stronger oxyacid H2SO4 > H2SO3; HNO3 > HNO2 Cation stronger acid than neutral molecule; neutral stronger acid than anion H3O+ > H2O > OH−; NH4+ > NH3 > NH2− trend in base strength opposite Tro, Chemistry: A Molecular Approach, 2/e

Acid Ionization Constant, Ka Acid strength measured by the size of the equilibrium constant when reacts with H2O HAcid + H2O  Acid− + H3O+ The equilibrium constant for this reaction is called the acid ionization constant, Ka larger Ka = stronger acid Tro, Chemistry: A Molecular Approach, 2/e

Tro, Chemistry: A Molecular Approach, 2/e

Autoionization of Water Water is actually an extremely weak electrolyte therefore there must be a few ions present About 2 out of every 1 billion water molecules form ions through a process called autoionization H2O Û H+ + OH– H2O + H2O Û H3O+ + OH– All aqueous solutions contain both H3O+ and OH– the concentration of H3O+ and OH– are equal in water [H3O+] = [OH–] = 10−7M @ 25 °C Tro, Chemistry: A Molecular Approach, 2/e

Ion Product of Water The product of the H3O+ and OH– concentrations is always the same number The number is called the Ion Product of Water and has the symbol Kw aka the Dissociation Constant of Water [H3O+] x [OH–] = Kw = 1.00 x 10−14 @ 25 °C if you measure one of the concentrations, you can calculate the other As [H3O+] increases the [OH–] must decrease so the product stays constant inversely proportional Tro, Chemistry: A Molecular Approach, 2/e

Acidic and Basic Solutions All aqueous solutions contain both H3O+ and OH– ions Neutral solutions have equal [H3O+] and [OH–] [H3O+] = [OH–] = 1.00 x 10−7 Acidic solutions have a larger [H3O+] than [OH–] [H3O+] > 1.00 x 10−7; [OH–] < 1.00 x 10−7 Basic solutions have a larger [OH–] than [H3O+] [H3O+] < 1.00 x 10−7; [OH–] > 1.00 x 10−7 Tro, Chemistry: A Molecular Approach, 2/e

Practice – Complete the table [H+] vs. [OH−] Tro, Chemistry: A Molecular Approach, 2/e

Practice – Complete the table [H+] vs. [OH−] Acid Base [H+] 100 10−1 10−3 10−5 10−7 10−9 10−11 10−13 10−14 OH− H+ [OH−]10−14 10−13 10−11 10−9 10−7 10−5 10−3 10−1 100 Even though it may look like it, neither H+ nor OH− will ever be 0 The sizes of the H+ and OH− are not to scale because the divisions are powers of 10 rather than units Tro, Chemistry: A Molecular Approach, 2/e

Example 15. 2b: Calculate the [OH] at 25 °C when the [H3O+] = 1 Example 15.2b: Calculate the [OH] at 25 °C when the [H3O+] = 1.5 x 10−9 M, and determine if the solution is acidic, basic, or neutral Given: Find: [H3O+] = 1.5 x 10−9 M [OH] Conceptual Plan: Relationships: [H3O+] [OH] Solution: Check: the units are correct; the fact that the [H3O+] < [OH] means the solution is basic Tro, Chemistry: A Molecular Approach, 2/e

Practice – Determine the [H3O+] when the [OH−] = 2.5 x 10−9 M Tro, Chemistry: A Molecular Approach, 2/e

Practice – Determine the [H3O+] when the [OH−] = 2.5 x 10−9 M Given: Find: [OH] = 2.5 x 10−9 M [H3O+] Conceptual Plan: Relationships: [OH] [H3O+] Solution: Check: the units are correct; the fact that the [H3O+] > [OH] means the solution is acidic Tro, Chemistry: A Molecular Approach, 2/e