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Understanding Acids and Bases: Sourness of Lemons and Key Concepts

This chapter explores the nature of acids and bases, starting with the sour taste of lemons. It defines acids and bases based on the Brønsted-Lowry and Lewis theories, outlining their behaviors as proton donors and acceptors, respectively. Key concepts like monoprotic and diprotic behavior, pH, pKa, and the strength of acids and bases are discussed. The chapter also addresses equilibria involving weak acids and bases, providing insight into their dissociation constants and the factors affecting acid strength, including electronegativity and atomic size.

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Understanding Acids and Bases: Sourness of Lemons and Key Concepts

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  1. Acids and Bases Chapter 17

  2. Why are lemons sour?

  3. What about other acids?

  4. What about bases?

  5. Definitions • Brønsted-Lowry: • An acid is anything that can donate a proton HCl(aq) + H2O(l) Cl-(aq) + H3O+(aq) • A base is anything that can accept a proton • Water (above) acts as a base • H3O+ = hydronium ion • HCl = acid, Cl- = conjugate base • Conjugate acid-base pair • H2O = base, H3O+ = conjugate acid • Conjugate acid-base pair

  6. A problem • Write the conjugate bases for the acids and the conjugate acids for the bases • Acids Bases • HIO3 S2- • HCO3- CN- • CH3COOH NH3

  7. Some more traits • Species donates 1 proton = monoprotic acid • 2 protons = diprotic acid • Accepts 1 proton = monoprotic base • Accepts 2 protons = diprotic base • Give a triprotic acid

  8. Lewis acids and bases • The same dude who gave us Lewis structures • Lewis acid = lone pair acceptor • Lewis base = lone pair donor • Form coordinate covalent bonds or adducts • NH3 + BH3

  9. Coordination complexes • = adducts • Water, ammonia, ions, molecules, etc. (ligands) give lone pairs to (usually transition) metal cations in soln • Ex: Cu2+(aq) + 4NH3(aq) [Cu(NH3)4]2+(aq) • Very colorful solns • Chem 133

  10. Water’s uniqueness • Can act as both a base and an acid • Amphiprotic H2O(l) + H2O(l) H3O+(aq)+ OH-(aq) • Water autoionizes

  11. pH • = power of hydrogen • Measurement of protons/hydronium ions in soln • Based on -logarithm (log10) •  pH = -log[H3O+] • Likewise • pOH = -log[OH-] • (10-pH = [H3O+] & 10-pOH = [OH-])

  12. pH scale • So pH + pOH = 14.00 = pKw • Where Kw = [H3O+][OH-] = 1.0 x 10-14 • 7.00 = neutrality • <7.00  acidity (increase going to 0) • >7.00  basicity (increases going to 14) • At neutrality, [H3O+] = [OH-] = 1.0 x 10-7 • Is [OH-] more concentrated under acidic or basic conditions?

  13. Before you work on next slide! • Sig figs in logs: • Result has the same number of decimal places in the input has sig figs: Log (1.00  10–5) = -5.000 3 sig figs 3 dec place • Sig figs in antilogs: • Result has the same number of sig figs as the number of decimal places in the input: 10–6.00 = 1.0  10–6 2 dec place 2 sig figs

  14. pH calculation problems

  15. Equilibria for acids: HA(aq) + H2O(l) H3O+(aq) + A-(aq) • As Ka increases what happens to the acid dissociation? Does it increase or decrease? • It increases • A stronger acid • If Ka < 1.0, weak acid

  16. Strong acids • HCl • HBr • HI • H2SO4 • HNO3 • HClO4 • HClO3

  17. Equilibria for bases:B(aq) + H2O(l) BH+(aq) + OH-(aq) • As Kb increases so does base strength • If Kb < 1.0 • Weak base

  18. Strong bases • Metal (I) hydroxides • Ba(OH)2

  19. Comparing acid/base strengths • Can use a logarithmic scale pKa = -log Ka • As Ka increases acid strength increases • But what about pKa? Does it increase or decrease as Ka increases? • What can we say about acid strength and pKa?

  20. Conversely pKb = -log Kb • Likewise, as pKb decreases base strength increases

  21. The relation between Ka and Kb HCN(aq) + H2O(l) H3O+(aq) + CN-(aq) Ka = 4.0 x 10-10 CN-(aq) + H2O(l)  HCN(aq) + OH-(aq) Kb = 2.5 x 10-5 Net rxn: 2H2O(l)  H3O+(aq) + OH-(aq) Knet = 1.0 x 10-14 = Kw

  22. Hence • And pKw = pKa + pKb = 14.00

  23. Problem • What is the Ka and pKa of a 0.10 M soln of chloroacetic acid with a pH of 1.95?

  24. Solution

  25. Problem • What is the pH of a 0.237 M solution of benzoic acid? The pKa is 4.19.

  26. Solution

  27. Problem • What is the pH of a 1.00 M solution of Sodium Acetate? (pKb = 5.6 x 10-10) Why pKb and not pKa?

  28. Solution

  29. Another problem • What is the pH of a solution that is made from 250.0 mL of 0.250 M KOH and 150.0 mL of 0.0125 M HBr?

  30. Solution

  31. Which side will be favored? • Rxns proceed from stronger acid/base to weaker acid/base • Predict the products and the direction of arrow for the following reaction: • NH3(aq) + HCO3- • Use table 17.3 on pg. 808

  32. Solution

  33. More problems • For the following write the complete reaction, and determine the correct K value both in symbols (e.g., 1/Ka) and in number. • a) Potassium bicarbonate and hydrochloric acid • b) The acid dissociation of HCN • c) Acetic acid and potassium hydroxide

  34. So what really makes it strong vs. weak? • 1) Electronegativity: • Acid strength increases as electronegativity of A increases • 2) Bond strength: • Lessens as descend GVII  stronger acid • 3) Larger atomic radius: • Increases as descend G •  easier to lose H, stronger acid

  35. Continued • 3) Oxyacids • The more oxygens the stronger • Ex: in order of increasing acid strength • HClO < HClO2 < HClO3 < HClO4 • O-H bond polarity increases as oxygens are added  easier to remove hydrogen (as proton) • Inductive effect = ability of atoms in a molecule to attract electrons from another part of the molecule

  36. More… • Also, more oxygens allow for more stable (delocalized) structures • Compare Lewis structures of deprotonated (conjugate bases) HNO3 to HNO2 • Allows for greater acidity

  37. Carboxylic acids • Brønsted-Lowry acids • RCO2-H + H2O  RCO2- + H3O+ • Give me the resonance structures • If Electron Withdrawing Groups are substituted for hydrogens what would happen? • Order of increasing Ka • CH3CO2H > ClCH2CO2H > Cl2CHCO2H > Cl3CCO2H

  38. Anions as Brønsted bases • Basicity increased as negative charge of anion increases • PO43- • HPO42- • H2PO4- • Which is most basic? • Which has the highest pKb?

  39. Salts • Will a salt soln be acidic, basic, or neutral? • Let’s do these: HCl + NaOH  NaCl + H2O HCl + NH4OH  NH4Cl + H2O H3PO4 + 3CsOH  Cs3PO4 + 3H2O H3PO4 + 3NH4OH  (NH4)3PO4 + 3H2O

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