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Chapter 14

Chapter 14. Acids & Bases. The Nature of Acids and Bases--Section 14.1. Arrhenius model Oldest model; only applies to compounds that contain H + or OH - ions Br ønsted-Lowry model Refers to a compound’s ability to donate or accept an H + ion Ex: HCl NH 3 H 2 O.

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Chapter 14

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  1. Chapter 14 Acids & Bases

  2. The Nature of Acids and Bases--Section 14.1 • Arrhenius model • Oldest model; only applies to compounds that contain H+ or OH- ions • Brønsted-Lowry model • Refers to a compound’s ability to donate or accept an H+ ion • Ex: HCl NH3 H2O

  3. Brønsted-Lowry Acids and Bases • Water as Brønsted-Lowry acid/base: • No such thing as H+ ion in solution (too unstable) • Only H3O+ • Proton transfer reactions:

  4. Brønsted-Lowry Example NH3(aq) + H2O(l) NH4+(aq) + OH-(aq) • Which acts as Brønsted-Lowry base? acid? • H2O behaves as an amphoteric compound • Capable of accepting OR donating H+ ions

  5. Conjugate Acids/Bases • The reactants and products associated with a proton transfer reaction are known as a conjugate acid-base pair:

  6. Relative Strength of Acids and Bases • The strength of an acid depends primarily on the willingness to donate or accept electrons • i.e. strength of conjugate acid/base • Equilibrium for strong acids lies heavily on the side of the deprotonated form and vice versa

  7. The Autoionization of Water • Water has a very interesting property due to its amphoterism • Capable of autoionization:

  8. An acid is added to water so that the hydrogen ion concentration is 0.25 M. Calculate the hydroxide ion concentration. See Interactive Example14.5 (Pg. 558) Calculating the [H3O+]

  9. The pH ScaleSection 14.3 • Concentrations of either H3O+ or OH- are typically very small and therefore cumbersome • The pH scale is a logarithmic scale and is much more convenient: pH = -log[H3O+] pOH = -log[OH-] pH + pOH = 14

  10. pH of Common Substances

  11. Strong Acids and BasesSection 14.4 • For a compound to be classified as a strong acid or base it must completely dissociate into ions when placed into aqueous solution • Very weak conjugate bases • No equilibrium • Ex: HCl(g) + H2O(l)  H3O+(aq) + NO3-(aq) • There are 7 strong acids which you will have to remember: • HCl, HBr, HI, HNO3, HClO3, HClO4, H2SO4

  12. Weak AcidsSection 14.5 • Weak acids have a relatively strong conjugate base (compared to strong acids) and therefore some competition exists for the proton HF(g) + H2O(l) H3O+(aq) + F-(aq) • As a result, the equilibrium between conjugate acid/base is treated as if it were any other equilibrium

  13. Calculating Ka for a Weak Acid Picric acid, a weak acid, is dissolved in water to prepare a 0.100 M solution. The pH of the solution was found to be 1.09. Calculate Ka for picric acid at this temperature.

  14. Calculate the pH in a 0.100 M solution of the weak acid naphthol, for which Ka is 1.7  10-10 See Interactive Example 14.8 (Pg. 564) Using Ka to Calculate pH and the 5% Rule

  15. Calculate the pH of 0.017 M C6H5COOH. The Ka of C6H5COOH is 6.3 x 10-5. See Interactive Example14.8 (Pg. 564) Using Ka to Calculate pH

  16. Percent Ionization • Weak acids, by definition, do not ionize 100% when placed in aqueous solution • It is therefore possible to calculate the extent of ionization (percent ionization)

  17. Calculate the percent of HF molecules ionized in a 0.010 M HF solution. See Interactive Example 14.10 (Pg. 568) Calculating Percent Ionization

  18. Effect of Concentration on Percent Ionization • As the concentration of a weak acid increases, the equilibrium concentration of H3O+(aq) increases • However, the percent ionization decreases as concentration increases

  19. Polyprotic Acids • Acids that have more than one acidic proton are referred to as polyprotic acids H2C2O4 + H2O HC2O4- + H3O+ HC2O4- + H2O C2O42- + H3O+ = 1.6 x 10-5 = 5.6 x 10-2

  20. Calculating pH for Polyprotic Acid Solutions • Because the acidic protons of a polyprotic acid are removed in discreet steps, we must calculate the [H3O+] for each step • However, because Ka1 is typically very large compared to Ka2 or Ka3, we may initially assume that the [H3O+] is a result of the first ionization

  21. Calculate the pH of a 0.0037 M H2CO3 solution. For super duper fun, calculate the [CO32-] in solution. Calculating pH and Concentration of All Species for a Polyprotic Acid

  22. Weak BasesSection 14.6 • There are fewer weak bases compared to the number of weak acids • Equilbria are the same (only involve OH- as product as opposed to H3O+)

  23. What is the pH of a 1.44 M (concentration of household ammonia) solution of NH3? Kb = 1.8 x 10-5 See Interactive Example 14.13 (Pg. 574) Calculating pH of Weak Base Solutions

  24. Relationship Between Ka and Kb • Examine the following equilibria: NH4+(aq) NH3(aq) + H+(aq) NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)

  25. Converting Between Ka and Kb • The product of the dissociation constants for any conjugate acid/base pair is always equal to the dissociation constant for H2O: • Ka x Kb = Kw = 1.0 x 10-14 • Likewise: pKa + pKb = pKw = 14.00

  26. Calculating pH of a Weak Base • Calculate the pH of a solution that is 0.0500 M in ammonium ion. Kb = 1.8 x 10-5

  27. Acid-Base Properties of Salt Solutions Section 14.8 • Hydrolysis reactions • The conjugate base of weak acids are capable of producing hydroxide ions in solution • Raises pH • Conjugate acid of weak bases are capable of producing hydronium ions in solution • Lowers pH

  28. Examples • Predict whether any of the following salts can alter the pH of pure water • NaCl • KF • NH4Cl • BaBr2 • LiC2H3O2

  29. Acid-Base Behavior and Chemical Structure Section 14.9 • The chemical structure of a compound is what ultimately determines acid/base behavior • Ex: Why does NaOH act as a base whereas CH3OH acts as a weak acid? Why does KH act as a strong base? • Factors that affect? • Polarization of H—X bond (NaH vs. CH4) • Bond strength (HF vs. HCl) • Stability of conjugate base (HNO3 vs. HNO2)

  30. Oxyacids • Special attention is paid to oxyacids because the majority of weak acids fall into this category • The strength of these acids is governed by one overriding principle: charge stabilization • Either through electronegativity or resonance

  31. Effect of Additional Oxygens • Because oxygen is a very electronegative element, the addition of more oxygen leads to a stronger acid:

  32. Carboxylic Acids • A second major class of weak acids are the carboxylic acids • Characterized by the presence of an COOH group

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