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Chapter 16: Acids and Bases, A Molecular Look

Chapter 16: Acids and Bases, A Molecular Look. Chemistry: The Molecular Nature of Matter, 6E Jespersen/Brady/Hyslop. Arrhenius Acids and Bases. Acid produces H 3 O + in water Base gives OH – Acid-base neutralization Acid and base combine to produce water and a salt.

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Chapter 16: Acids and Bases, A Molecular Look

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  1. Chapter 16: Acids and Bases, A Molecular Look Chemistry: The Molecular Nature of Matter, 6E Jespersen/Brady/Hyslop

  2. Arrhenius Acids and Bases 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

  3. Gas Phase Acid-Base • Not covered by Arrhenius definition e.g. NH3(g) + HCl(g) NH4Cl(s)

  4. 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+

  5. 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

  6. Formic Acid is Bronsted Acid • Formic acid (HCHO2) is a weak acid • Must consider equilibrium • HCHO2(aq) + H2O CHO2–(aq) + H3O+(aq) • Focus on forward reaction

  7. Formate Ion is Bronsted Base • Now consider reverse reaction • Hydronium ion transfers H+ to CHO2–

  8. Identify the conjugate partner for each Learning Check Cl– NH4+ C2H3O2– HCN F–

  9. Learning Check • Write a reaction that shows that HCO3– is a Brønsted acidwhen reacted with OH– • HCO3–(aq)+ OH–(aq) • Write a reaction that shows that HCO3– is a Brønsted basewhen reacted with H3O+(aq) • HCO3–(aq) + H3O+(aq) H2O + CO32–(aq) H2CO3(aq) + H2O

  10. Your Turn! 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–

  11. 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 [Amphiprotic substances can donate or accept a proton. This is a subtle but important difference from the word amphoteric]

  12. Your Turn! Which of the following can act as an amphoteric substance? A. CH3COOH B. HCl C. NO2– D. HPO42–

  13. Strengths of Acids and 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)

  14. Reactions of Strong Acids and Bases 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–

  15. Position of 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

  16. Your Turn! In the reaction: HCl + H2O → H3O+ + Cl– which species is the weakest base ? A. HCl B. H2O C. H3O+ D. Cl–

  17. In General • 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)

  18. Learning Check Identify the preferred direction of the following reactions: H3O+(aq)+ CO32–(aq) HCO3–(aq)+ H2O Cl–(aq)+ HCN(aq) HCl(aq)+ CN–(aq)

  19. Trends in Binary Acid Strength 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

  20. Trends in Binary Acid Strength 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

  21. Learning Check • Which is stronger? • H2S or H2O • CH4 or NH3 • HF or HI • H2S • NH3 • HI

  22. Trends in Oxoacid Strength 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

  23. Trends in Oxoacid Strength 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

  24. Learning Check Which is the stronger acid in each pair? • H2SO4 or H3PO4 • HNO3 or H3PO3 • H2SO4 or H2SO3 • HNO3 or HNO2 H2SO4 HNO3 H2SO4 HNO3

  25. Your Turn! 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

  26. Alternate Definition of Acid Strength • 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:

  27. Comparing Basicity • Lone oxygens 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

  28. Learning Check • Arrange the following in order of increasing acid strength: • HBr, AsH3, H2Se • AsH3 < H2Se < HBr • H2SeO4, H2SO4, H2TeO4 • H2TeO4 < H2SeO4 < H2SO4 • HBrO3, HBrO, HBrO4, HBrO2 • HBrO < HBrO2 < HBrO3 < HBrO4

  29. Strength of Organic Acids • 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

  30. Your Turn! Which of the following is the strongest organic acid? A B C D E

  31. Lewis Definition of Acid and Base • 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

  32. 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

  33. Lewis Acid-Base Reaction • Electrons in coordinate covalent bond come from O in hydroxide ion

  34. 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

  35. SO2 as Lewis Acid O2–

  36. 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

  37. H+ Transfer from Lewis Perspective e.g.H2O—H+ + NH3 H2O + H+—NH3

  38. Learning Check Identify the Lewis acid and base in the following: • NH3 + H+NH4+ BaseAcid • F– + BF3BF4– BaseAcid • SeO3 + O2–SeO42– AcidBase

  39. Your Turn! Which of the following species can act as a Lewis base ? A. Cl– B. Fe2+ C. NO2– D. O2–

  40. Acid-Base Properties of Elements and 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)

  41. Acid-Base Properties of Elements and 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)

  42. Metal Oxides MxOy • 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

  43. Your Turn! What is the acid formed by P2O3 when it reacts with water ? A. H2PO4 B. H2PO2 C. H3PO4 D. H3PO3 • P2O3 + 3H2O → 2H3PO3

  44. Metal Ions in Solution (Once Anion is Removed) • 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)

  45. Hydrated Metal Ions = Weak Brønsted Acids M(H2O)n+(aq) + H2OM(OH)n+(aq) + H3O+(aq)

  46. Your Turn! The following reactions: Al(OH)3 + 3H+ → Al3+ + H2O Al(OH)3 + OH– → Al(OH)4– illustrate the concept of A. neutralization B. amphoteric property of Al(OH)3 C. oxidation of Al D. reduction of OH–

  47. Hydrated Metal Ions Can Act as Weak Acids • Electron deficiency of metal cations causes them to induce electron density towards metal from water of hydration • Higher charge density = more acidic metal • Acidity increases left to right across period • Acidity decreases top to bottom down group

  48. Acidity of Hydrated Metal Ions • Degree to which M(H2O)mn+ produces acidic solutions depends on • Charge on cation • Cation's size 1. As charge increases on Mn+, acidity increases • Increases metal ion’s ability to draw electron density to itself and away from O—H bond

  49. Acidity of Hydrated Metal Ions 2. As size of cation decreases, acidity increases • Smaller, more concentrated charge • Means greater pull of electron density from O—H bond • Net result • Very small, highly charged cations are very acidic [Al(H2O)6]3+(aq) + H2O [Al(H2O)5(OH)]2+(aq) + H3O+(aq)

  50. Your Turn! In the following list of pairs of ions, which is the more acidic ? Fe2+ or Fe3+; Cu2+ or Cu+; Co2+ or Co3+ A. Fe3+, Cu+, Co2+ B. Fe2+, Cu2+, Co3+ C. Fe3+, Cu2+, Co3+ D. Fe2+, Cu2+, Co2+

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