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Acids & Bases CHAPTER 16 (& part of CHAPTER 17)

Acids & Bases CHAPTER 16 (& part of CHAPTER 17) Chemistry: The Molecular Nature of Matter, 6 th edition By Jesperson , Brady, & Hyslop. CHAPTER 16: Acids & Bases. Learning Objectives: Define Brønsted -Lowry Acid/Base Define Lewis Acid/Base Evaluate the strength of acids/bases

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Acids & Bases CHAPTER 16 (& part of CHAPTER 17)

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  1. Acids & Bases CHAPTER 16 (& part of CHAPTER 17) Chemistry: The Molecular Nature of Matter, 6th edition By Jesperson, Brady, & Hyslop

  2. CHAPTER 16: Acids & Bases • Learning Objectives: • Define Brønsted-Lowry Acid/Base • Define Lewis Acid/Base • Evaluate the strength of acids/bases • Strong vs weak acids/bases • Periodic trends • Conjugate acids/bases • Identify likely compounds that will form acids and bases from the periodic table • Acidic metal ions • Acid/Base equilibrium: • pH, pOH • Ka, Kb, pKa, pKb • Kw of water Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  3. CHAPTER 16: Acids & Bases Lecture Road Map: Brønsted-Lowry Acids/Bases Trends in acid strength Lewis Acids & Bases Acidity of hydrated metal ions Acid/Base equilibrium Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  4. CHAPTER 16 Acids & Bases Brønsted-Lowry Acid/Base Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  5. Arrhenius Acid/Base Definition Acid produces H3O+ in water Basegives OH– Acid-base neutralization • Acid and base combine to produce water and a salt. e.g. HCl(aq) + NaOH(aq) H2O + NaCl(aq) H3O+(aq) + Cl–(aq) + Na+(aq) + OH–(aq) 2H2O +Cl–(aq) + Na+(aq) • Many reactions resemble this without forming H3O+ or OH– in solution Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  6. Arrhenius Acid/Base Definition Gas Phase Acid/Base chemistry not covered by Arrhenius definition e.g. NH3(g) + HCl(g)  NH4Cl(s) Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  7. Brønsted-Lowry Definition • Acid = proton donor • Base = proton acceptor • Allows for gas phase acid-base reactions e.g. HCl + H2O  H3O+ + Cl– • HCl = acid • Donates H+ • Water = base • Accepts H+ Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  8. Brønsted-Lowry Conjugate Acid-Base Pair • Species that differ by H+ e.g. HCl + H2O  H3O+ + Cl– • HCl = acid • Water = base • H3O+ • Conjugate acid of H2O • Cl– • Conjugate base of HCl Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  9. Brønsted-Lowry Example: Formic Acid • Formic acid (HCHO2) is a weak acid • Must consider equilibrium • HCHO2(aq) + H2O CHO2–(aq) + H3O+(aq) • Focus on forward reaction Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  10. Brønsted-Lowry Example: Formic Acid Now consider reverse reaction: • Hydronium ion transfers H+ to CHO2– • Formate Ion is the Brønsted Base Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  11. Group Problem • Identify the conjugate partner for each Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  12. Group Problem Write a reaction that shows that HCO3– is a Brønstedacidwhen reacted with OH– Write a reaction that shows that HCO3– is a Brønstedbasewhen reacted with H3O+(aq) Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  13. Group Problem In the following reaction, identify the acid/base conjugate pairs. (CH3)2NH + H2SO4 → (CH3)2NH+ + HSO4– A. (CH3)2NH / H2SO4 (CH3)2NH+ / HSO4– B. (CH3)2NH / (CH3)2NH+ H2SO4 / HSO4– C. H2SO4 / HSO4– (CH3)2NH+ / (CH3)2NH D. H2SO4 / (CH3)2NH (CH3)2NH+ / HSO4– Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  14. Brønsted-Lowry Amphoteric Substances • Can act as either acid or base • Can be either molecules or ions e.g. Hydrogen carbonate ion: • Acid HCO3–(aq) + OH–(aq)  CO32–(aq) + H2O • Base HCO3–(aq) + H3O+(aq)  H2CO3(aq) + H2O [Amphiproticsubstances can donate or accept a proton. This is a subtle but important difference from the word amphoteric] Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  15. Group Problem Which of the following can act as an amphoteric substance? A. CH3COOH B. HCl C. NO2– D. HPO42– Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  16. CHAPTER 16 Acids & Bases Trends in Acid/Base Strength Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  17. Acid/Base Trends Strengths of Acids & Bases Strength of Acid • Measure of its ability to transfer H+ • Strongacids • React completely with water e.g.HCl and HNO3 • Weak acids • Less than completely ionized e.g.CH3COOH and CHOOH Strength of Baseclassified in similar fashion: • Strong bases • React completely with water e.g.Oxide ion (O2–) and OH– • Weak bases • Undergo incomplete reactions e.g.NH3 and NRH2 (NH2CH3, methylamine) Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  18. Acid/Base Trends Strength in Water • Strongest acid= hydronium ion, H3O+ • If more powerful H+ donor added to H2O • Reacts with H2O to produce H3O+ Similarly, • Strongest baseis hydroxide ion (OH–) • More powerful H+ acceptors • React with H2O to produce OH– Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  19. Acid/Base Trends Acid/Base Equilibrium • Acetic acid (HC2H3O2) is weak acid • Ionizes only slightly in water HC2H3O2(aq) + H2O H3O+(aq) + C2H3O2–(aq) weaker acidweaker basestronger acidstronger base • Hydronium ion • Better H+ donor than acetic acid • Stronger acid • Acetate ion • Better H+ acceptor than water • Stronger base • Position of equilibrium favors weakeracid and base Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  20. Group Problem In the reaction: HCl + H2O → H3O+ + Cl– which species is the weakest base ? A. HCl B. H2O C. H3O+ D. Cl– Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  21. Group Problem Group Problem Identify the preferred direction of the following reactions: H3O+(aq)+ CO32–(aq) HCO3–(aq)+ H2O Cl–(aq)+ HCN(aq)HCl(aq)+ CN–(aq) Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  22. Acid/Base Trends General Trends • Stronger acids and bases tend to react with each other to produce their weaker conjugates • Stronger Brønsted acid has weaker conjugate base • Weaker Brønsted acid has stronger conjugate base • Can be applied to binary acids (acids made from hydrogen and one other element) Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  23. Acid/Base Trends Binary Acid Trends Binary Acids = HnX X = Cl, Br, P, As, S, Se, etc. • Acid strength increases from left to right within same period (across row) • Acid strength increases as electronegativity of Xincreases e.g.HCl is stronger acid than H2S which is stronger acid than PH3 • or PH3 < H2S < HCl Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  24. Acid/Base Trends Binary Acid Trends Binary Acids = HnX X = Cl, Br, P, As, S, Se, etc. 2. Acid strength increase from top to bottom within group • Acid strength increases as size of Xand bond length increases e.g. HCl is weaker acid than HBr which is weaker acid than HI • or HCl < HBr < HI Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  25. Group Problem Which is stronger? • H2S or H2O • CH4 or NH3 • HF or HI

  26. Acid/Base Trends Oxoacid Trends Oxoacids (HnX Om) • Acids of H, O, and one other element • HClO, HIO4, H2SO3, H2SO4, etc. • Acids with same number of oxygen atoms and differing X • Acid strength increasesfrom bottom to top within group • HIO4 < HBrO4 < HClO4 • Acid strength increases from left to right within period as the electronegativity of the central atom increases H3PO4 < H2SO4 < HClO4 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  27. Acid/Base Trends Definition Oxoacids (HnXOm) • For same X • Acid strength increases with number of oxygen atoms • H2SO3 < H2SO4 • More oxygens, remove more electron density from central atom, weakening O—H bond make H more acidic Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  28. Group Problem Which is the stronger acid in each pair? • H2SO4 or H3PO4 • HNO3 or H3PO3 • H2SO4 or H2SO3 • HNO3 or HNO2 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  29. Group Problem Which corresponds to the correct order of acidity from weakest to strongest acid ? A. HBrO3, HBrO, HBrO2 B. HBrO, HBrO2, HBrO3 C. HBrO, HBrO3, HBrO2 D. HBrO3, HBrO2, HBrO

  30. Acid/Base Trends Basicity • Acid strength can be analyzed in terms of basicity of anion formed during ionization • Basicity • Willingness of anion to accept H+ from H3O+ • Consider HClO3 and HClO4: Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  31. Acid/Base Trends Basicity • Loneoxygens carry most of the negative charge • ClO4– has 4 O atoms, so each has –¼ charge • ClO3– has 3 O atoms, so each has –1/3 charge • ClO4–weaker base than ClO3– • Thus conjugate acid, HClO4, is stronger acid • HClO4 stronger acid as more fully ionized Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  32. Group Problem

  33. Acid/Base Trends Organic Acid Trends • Organic acid —COOH • Presence of electronegative atoms (halide, nitrogen or other oxygen) near —COOH group • Withdraws electron density from O—H bond • Makes organic acid, stronger acids e.g.CH3CO2H < CH2ClCO2H < CHCl2CO2H < CCl3CO2H Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  34. Group Problem Which of the following is the strongest organic acid? A B C D E

  35. CHAPTER 16 Acids & Bases Lewis Acid/Base Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  36. Lewis Acid/Base Definition • Broadest definition of species that can be classified as either acid or base • Definitions based on electron pairs • Lewis acid • Any ionic or molecular species that can acceptpair of electrons • Formation of coordinate covalent bond • Lewis base • Any ionic or molecular species that can donatepair of electrons • Formation of coordinate covalent bond Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  37. Lewis Acid/Base Lewis Neutralization • Formation of coordinate covalent bond between electron pair donor and electron pair acceptor • NH3BF3 = addition compound • Made by joining two smaller molecules Addition Compound Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  38. Lewis Acid/Base Lewis Acid-Base Reaction Electrons in coordinate covalent bond come from O in hydroxide ion Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  39. Lewis Acid/Base Lewis Acids • Molecules or ions with incomplete valence shells e.g. BF3 or H+ • Molecules or ions with complete valence shells, but with multiple bonds that can be shifted to make room for more electrons e.g. CO2 • Molecules or ions that have central atoms that can expand their octets • Capable of holding additional electrons • Usually, atoms of elements in Period 3 and below e.g. SO2 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  40. Lewis Acid/Base Lewis Acid Example: SO2 O2– Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  41. Lewis Acid/Base Lewis Bases • Molecules or ions that have unshared electron pairs and that have complete shells • e.g. O2– or NH3 Lewis Definition is Most General • All Brønsted acids and bases are Lewis acids and bases • All Arrhenius acids and bases are Brønsted acids and bases Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  42. Lewis Acid/Base Proton (H+) Transfer H2O—H+ + NH3 H2O + H+—NH3 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  43. Group Problem Identify the Lewis acid and base in the following: • NH3 + H+NH4+ • F– + BF3BF4– • SeO3 + O2– SeO42–

  44. Group Problem Which of the following species can act as a Lewis base ? A. Cl– B. Fe2+ C. NO2– D. O2–

  45. CHAPTER 16 Acids & Bases Acidity of Oxides & Hydrates Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  46. Acidic Metal Ions Acid-Base Properties of Elements & their Oxides Nonmetal oxides • React with H2O to form acids • Upper right hand corner of periodic table • Acidic Anhydrides • Neutralize bases • Aqueous solutions redto litmus • SO3(g) + H2O  H2SO4(aq) • N2O5(g) + H2O  2HNO3(aq) • CO2(g) + H2O  H2CO3(aq) Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  47. Acidic Metal Ions Acid-Base Properties of Elements & their Oxides Metal oxides • React with H2O to form hydroxide (Base) • Group 1A and 2A metals (left hand side of periodic table) • BasicAnydrides • Neutralize acids • Aqueous solutions blue to litmus • Na2O(s) + H2O  2NaOH(aq) • CaO(s) + H2O  Ca(OH)2(aq) Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  48. Acidic Metal Ions Metal Oxides • Solids at room temperature • Many insoluble in H2O • Why? • Too tightly bound in crystal • Can't remove H+ from H2O • Do dissolve in solution of strong acid • Now H+ free, can bind to O2– and remove from crystal Fe2O3(s) + 6H+(aq)  2Fe3+(aq) + 3H2O Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  49. Group Problem What is the acid formed by P2O3 when it reacts with water ? A. H2PO4 B. H2PO2 C. H3PO4 D. H3PO3

  50. Acidic Metal Ions Metal Ions in Solution • Exist with sphere of water molecules with their negative poles directed toward Mn+ • Mn+(aq) + mH2O M(H2O)mn+(aq) Lewis AcidLewis Basehydrated metal ion = addition compound • n= charge on metal ion = 1, 2, or 3 depending on metal atom • For now assume m = 1 (monohydrate) Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

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