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Acid Equilibrium and pH

Acid Equilibrium and pH. Søren S ø rensen. Acid/Base Definitions. Arrhenius Model Acids produce hydrogen ions in aqueous solutions Bases produce hydroxide ions in aqueous solutions Bronsted-Lowry Model Acids are proton donors Bases are proton acceptors Lewis Acid Model

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Acid Equilibrium and pH

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  1. Acid Equilibrium and pH SørenSørensen

  2. Acid/Base Definitions • Arrhenius Model • Acids produce hydrogen ions in aqueous solutions • Bases produce hydroxide ions in aqueous solutions • Bronsted-Lowry Model • Acids are proton donors • Bases are proton acceptors • Lewis Acid Model • Acids are electron pair acceptors • Bases are electron pair donors

  3. Acid Dissociation Constant HA  H+ + A- AcidProtonConjugate base Alternately, H+ may be written in its hydrated form, H3O+ (hydronium ion).HA(aq) + H2O(l)  H3O+(aq) + A-(aq) acid base conj. Acid conj. base

  4. Practice: write the simple dissociation reaction for the following acids • HCl • HC2H3O2 • NH4+ • C6H5NH3+ • [Al(H2O)6]3+

  5. Dissociation of Strong Acids Strong acids are assumed to dissociate completely in solution. Large Ka or small Ka? Reactant favored or product favored?

  6. Dissociation Constants: Strong Acids

  7. Dissociation of Weak Acids Weak acids are assumed to dissociate only slightly (less than 5%) in solution. Large Ka or small Ka? Reactant favored or product favored?

  8. Dissociation Constants: Weak Acids

  9. Autoionizationof Water:Amphoteric H2O + H2O  H3O+ + OH- At 25C, [H3O+] = [OH-] = 1 x 10-7 Kw(ion-product constant) is a constant at 25 C: Kw = [H3O+][OH-] = [H+][OH-] Kw = (1 x 10-7)(1 x 10-7) = 1 x 10-14

  10. Calculating pH, pOH pH = -log10[H+] pOH = -log10[OH-] Relationship between pH and pOH pH + pOH = 14 Finding [H3O+], [OH-] from pH, pOH [H3O+] = 10-pH [OH-] = 10-pOH

  11. pH and pOH Calculations

  12. The pH Scale Graphic: Wikimedia Commons user Slower

  13. A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC2H3O2, Ka = 1.8 x 10-5 ? Step #1:Write the dissociation equation HC2H3O2 C2H3O2- + H+

  14. A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC2H3O2, Ka = 1.8 x 10-5 ? Step #2:ICE it! HC2H3O2 C2H3O2- + H+ 0.50 0 0 +x +x - x x x 0.50 - x

  15. A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC2H3O2, Ka = 1.8 x 10-5 ? Step #3:Set up the law of mass action HC2H3O2 C2H3O2- + H+ E 0.50 - x x x

  16. A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC2H3O2, Ka = 1.8 x 10-5 ? Step #4:Solve for x, which is also [H+] HC2H3O2 C2H3O2- + H+ E 0.50 - x x x [H+] = 3.0 x 10-3 M

  17. A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC2H3O2, Ka = 1.8 x 10-5 ? Step #5:Convert [H+] to pH HC2H3O2 C2H3O2- + H+ E 0.50 - x x x

  18. Your turn • P. 691 #49, 61, 63, 65 • Refer to table 14.2 for Ka’s of common monoprotic acids on page 644

  19. Bases Graphics Source: Wikipedia

  20. Acid/Base Definitions • Arrhenius Model • Acids produce hydrogen ions in aqueous solutions • Bases produce hydroxide ions in aqueous solutions • Bronsted-Lowry Model • Acids are proton donors • Bases are proton acceptors • Lewis Acid Model • Acids are electron pair acceptors • Bases are electron pair donors

  21. Dissociation of Strong Bases MOH(s)  M+(aq) + OH-(aq) • Strong bases are metallic hydroxides • Group I hydroxides (NaOH, KOH) are very soluble • Group II hydroxides (Ca, Ba, Mg, Sr) are less soluble • pH of strong bases is calculated directly from the concentration of the base in solution

  22. Reaction of Weak Bases with Water The base reacts with water, producing its conjugate acid and hydroxide ion: CH3NH2 + H2O  CH3NH3+ + OH- Kb = 4.38 x 10-4

  23. Kb for Some Common Weak Bases Many students struggle with identifying weak bases and their conjugate acids. What patterns do you see that may help you?

  24. Reaction of Weak Bases with Water The generic reaction for a base reacting with water, producing its conjugate acid and hydroxide ion: B + H2O  BH+ + OH- (Yes, all weak bases do this – DO NOT endeavor to make this complicated!)

  25. A Weak Base Equilibrium Problem What is the pH of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ? Step #1:Write the equation for the reaction NH3 + H2O  NH4+ + OH-

  26. A Weak Base Equilibrium Problem What is the pH of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ? Step #2:ICE it! NH3 + H2O  NH4+ + OH- 0.50 0 0 +x +x - x x x 0.50 - x

  27. A Weak Base Equilibrium Problem What is the pH of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ? Step #3:Set up the law of mass action NH3 + H2O  NH4+ + OH- E 0.50 - x x x

  28. A Weak Base Equilibrium Problem What is the pH of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ? Step #4:Solve for x, which is also [OH-] NH3 + H2O  NH4+ + OH- E 0.50 - x x x [OH-] = 3.0 x 10-3 M

  29. A Weak Base Equilibrium Problem What is the pH of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ? Step #5:Convert [OH-] to pH NH3 + H2O  NH4+ + OH- E 0.50 - x x x

  30. Reference Table

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