1 / 61

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.

belden
Télécharger la présentation

Chapter 16: Acids and Bases, A Molecular Look

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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+

More Related