1 / 26

Zumdahl’s Chapter 14

Zumdahl’s Chapter 14. Acids Bases. Acid-Base Models Acidity and K a pH, pOH, and pK a Calculating pH Dominant Sources Polyprotic Acids % dissociation Bases and K b. Conjugate Acids and Bases Structure and Strength Oxoacids Lewis Acids Acid-Base Solution Al Gore Rhythms.

pmadson
Download Presentation

Zumdahl’s Chapter 14

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. Zumdahl’sChapter 14 Acids Bases

  2. Acid-Base Models Acidity and Ka pH, pOH, and pKa Calculating pH Dominant Sources Polyprotic Acids % dissociation Bases and Kb Conjugate Acids and Bases Structure and Strength Oxoacids Lewis Acids Acid-Base Solution Al Gore Rhythms Chapter Contents

  3. Acid-Base Models

  4. Caustic Characteristics • Greek: kaustikos kaiein“to burn” • Acids & bases corrode nearly everything • HA(aq) + H2O(l)  H3O+(aq) + A–(aq) • HA “acid” and A–“conjugate base” • H2O “base” and H3O+“conjugate acid” • H3O+“hydronium ion”no free protons! • Ka = [H+] [A–] / [HA] “acid dissociation contant”

  5. Acidity and Ka • Strong acid: Ka  means [HA]eq = 0 • Weak acid: Ka  0 or [HA]eq  [HA]0 • A–(aq) + H3O+(aq)  HA(aq) + H2O(l) • K=Ka–1 or weak acid is associated with a strong conjugate base and vice versa. • Acids HSO4–HC2H3O2HCN • Ka 1.210–2 1.810–5 6.210–10

  6. pH, pOH, and pKa • pX  – log10(X) • Reduces Kw = 1.01210–14 to pKw = 13.995 to a manageable entity for comparison • Actually pKw = 14 is usually memorized. • 2 H2O(l)  H3O+(aq) + OH–(aq) K = Kw • pH+pOH=pKw because Kw = [H+][OH–] • Acid (pH<7), Neutral (pH=7), Base (pH>7) • E.g., pH(0.1 M acetic acid) = 2.87

  7. Strong acids [H+]eq = [HA]0 pH = p[HA]0 Strong bases [OH–]eq = [BOH]0 pOH = p[BOH]0 pH = 14 – pOH Don’t be fooled by [HA]0< 10–7 Weak Acids Ka = [H+][A–] / [HA] Ka = x2 / ( [HA]0 – x) Ka x2 / [HA]0 x  ( Ka [HA]0 )½ pH = – log10(x) Weak Bases x  ( Kb [MOH]0 )½ pH = 14 - px Calculating pH

  8. Dominant Sources • If more than one H+ source (multiple acids or a polyprotic acid), pKa will usually differ by a few units, and the largest will determine [H+]eq. E.g., 0.01M H3PO4 • pKa: 2.12, 7.21, and 12.32, respectively • Use Ka for 1st H only: pH = 2.24 (quadratic formula) • Determines H3PO4, H2PO4–, and H+ • Use other two pKa to find HPO42– and PO43–

  9. % Dissociation • Weak electrolyte present in molecular form mostly with few ions & will have a small % dissociation unless diluted. • 100%  [ionic form] / [initial amount] • E.g., 0.1 M acetic acid = [initial amount] • [acetate ion]  (0.10 Ka)½ = 0.0013 M • % dissoc.  100%  (Ka/0.1)½ = 1.3% • 0.01 M gives 4.2% shows more ionization upon dilution (but pushes the “5% approx.”)

  10. Bases and Kb • Strong base: [OH–]eq = [MOH]0 • where M=metal and pH = 14 - pOH • Weak bases often amines, :NR3 • where R = H and/or organic groups • Amine steals H+ from H2O to leave OH– • Kb = [NR3H+] [OH–] / [:NR3] • For NH3(ammonia, not an amine), Kb = 1.810–5 • Same pH rules & approximations apply.

  11. Conjugate Acids and Bases • “Salt of a weak (acid/base) is itself a weak (base/acid).” • Example of a weak base, NH3 • NH4+ + OH– NH3 + H2O K = Kb–1 • H2O  H+ + OH– K = Kw(add eqns) • NH4+  NH3 + H+ Kconjugate = Kw / Kb • Kacid(NH4+) = 10–14/1.810–5 = 5.510–10 • Weaker parent yields stronger conjugate!

  12. Conjugate pH Calculations • Salts of strong parents are neutral; those of very weak bases, very acidic: • Fe(H2O)63+ Fe(H2O)5OH2+ + H+ • pKa1 = 2.2 or Ka1 = 6.310–3 (pKb = 11.8) • pH of 3 M Fe(NO3)3 ? (HNO3 strong; ignore!) • Ka1 = [FeOH2+] [H+] / [Fe3+] = x2 / (3-x) • x  [3Ka1]½ = 0.14 (5% OK)  pH = 0.86 • pKa2 = 3.5, pKa3 = 6, pKa4 = 10

  13. NON-Dominant Situations • In that Fe complex, pKa1 and pKa2 are very close (2.2 and 3.5), so the second conjugate may be important: • Fe(H2O)5OH2+ Fe(H2O)4(OH)2+ + H+ • This will certainly be the case at lower formal iron concentrations, F, since % dissociation is greater there. Try F=0.01

  14. Simplifing Approximations • Even at F=0.01, we can safely ignore pKa3, pKa4, and pKw, since they are dominated by pKa1 and pKa2. • Abbreviate those equilibria as • A3+ B2+ + H+ K1 = B H / A • B2+  C+ + H+ K2 = C H / A • But those are 2 eqns in 4 unknowns!

  15. 2 Additional Conditions! • Conservation of iron requires that • F = A + B + C • Conservation of charge must include a spectator anion for A3+ such as 3 Cl– • 3 A3+ + 2 B2+ + C+ + H+ = 3 F (conc. Cl–) • Notice that higher charges get scaled by their charge for proper balance.

  16. Solve 4 Nonlinear Equations • Original equations: • F = A + B + C • 3F = 3A + 2B + C + H • B H = K1 A • C H = K2 B • Remove A by substituting from 1st eqn • A = F – B – C

  17. Solve 3 Nonlinear Equations • Remaining after substitution • 3F = 3F – 3B – 3C + 2B + C + H • B H = K1 ( F – B – C ) • C H = K2 B • Simplify • H = B + 2C • B H = K1 F – K1 B – K1 C • C H = K2 B  substitute C = K2 B / H

  18. Solve 2 Nonlinear Equations • H = B + 2 K2 B / H • B H = K1 F – K1 B – K1 K2 B / H • Simplify • H2 = B H + 2 K2 B  B = H2 / (H + 2K2) • B H2 = K1 F H – K1 B H – K1 K2 B • Substitute for B • H3 + K1 H2 – K1 (F – K2)H – 2 K1 K2 F = 0 • Only H remains as an unknown!

  19. Solve 1 Cubic Equation • Use the dominant solution as first guess in an iteration to the non-dominant one. • A3+ B2+ + H+ K1 = B H / A • K1 = x2 / (F – x) • x2 + K1 x – F K1 = 0 • x = ( – K1 + [K12 + 4F K1]½) / 2 • x = 5.310–3  H • F = 0.01, K1 = 610–3, and K2 = 310– 4

  20. H3 + 610–3 H2 – 5.810–3 H – 3.610–8 = 0Hout = (3.6 10–8 + 5.810–3 Hin – 610–3 Hin2)1/3  [H+] = 5.5810– 3M and by B = H2 / (H + 2K2), [B2+] = 5.0410– 3M and by C = K2 B / H, [C+] = 2.7 10– 3M which makes [A3+] = 4.710– 3M

  21. More Conjugate Calculations • HCO3– H+ + CO32– pKa2 = 10.25 • pH of 0.10 M Na2CO3 ? (NaOH strong; ignore) • CO32– + H2O  HCO3– + OH– K = Kw/Ka2 • K = [HCO3–] [OH–] / [CO32–] = x2 / (0.10-x) • x  [0.10  1.810–4 ]½ = 4.210–3(5% OK) • pOH = 2.37  pH = 14.00 – 2.37 = 11.63 • Sodium carbonate is in laundry soap.

  22. Acid Structure and Strength • Higher polarity liberates protons. • While all hydrogen halides are strong acids (save HF), acidity lessens with halide size. • Even non-adjacent electronegativity drains e– density, liberating protons. • HClO is weak but HClO4 is strong. • (the H is attached to only 1 of the oxygens) • Cl3CCO2H is much stronger than CH3CO2H

  23. Oxoacids • German: O2 = sauerstoff or the stuff that makes sour oxoacids, –X–O–H • True when X is a fairly electronegative non-metal (halogen, N, P, S) or a metal in a high oxidation state (e.g., H2CrO4) • True as nonadjacent electronegativity aids polarity (e.g., CH3CO2H)

  24. Oxygen-containing Bases • M – O – H • Metals in lower oxidation states tend to give up the OH– and not the H+ • Sr(OH)2 Sr2+ + 2 OH– • Al(OH)3 is basic; Al3+ is acidic. • Water is amphoteric too, acting as a base by becoming H3O+, for example. • Brønsted definition: water accepts the proton.

  25. Lewis Acids and Bases • Water is both acid & base by Lewis too. • Base via H2O: + H+ H3O+ • donates lone pair e– to proton • Acid via H2O + S2– OH– + HS– • accepts the one of sulfide’s electrons • Lewis’ is the most general definition • E.g., Cu2+ + 6 :NH3  Cu(NH3)62+ • Cu’s the acid and :NH3’s the base.

  26. Know the significant species for the conditions. Ignore the rest. For quantitative reactions, keep only the products. Find acids & bases. Solve controlling K Write K expression. Set initial conditions for smallest change. Define all species in terms of x (change). Find x solving K exp. Check any approx. Find pH & [species] Acid-Base Algorithm

More Related