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Lecture 26

Lecture 26. Weak Acid-Base Chemistry 10.7-10.13 5-November Assigned HW 10.34, 10.36, 10.38, 10.40, 10.42, 10.48, 10.54b, 10.56a,b, 10.58, 10.60a, 10.70 Due: Monday 15-Nov. Review. Arrhenius: Bronsted -Lowry: Lewis: Strong acids/bases – dissociate completely - Very large K

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Lecture 26

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  1. Lecture 26 Weak Acid-Base Chemistry 10.7-10.13 5-November Assigned HW 10.34, 10.36, 10.38, 10.40, 10.42, 10.48, 10.54b, 10.56a,b, 10.58, 10.60a, 10.70 Due: Monday 15-Nov

  2. Review • Arrhenius: • Bronsted-Lowry: • Lewis: • Strong acids/bases – dissociate completely - Very large K • Weak acids/bases – dissociation controlled by equilibrium constant • Self dissociation of water • pH and pOH HXaq+ H2O(l) H3O+aq + X-aq MOHaq M+aq + OH-aq

  3. Lewis Acids and Bases Given the following reaction: Identify all Bronsted Acids and Bases Identify all Lewis Acids and Bases

  4. Weak Acids and Bases Calculate the pH of 465 μM HBr HBraq + H2O  Br- + H3O+ Full dissociation

  5. Weak Acids and Bases Calculate the pH of 465 μM Acetic Acid (HX, Ka=1.8x10-5) HXaq+ H2O(l) H3O+aq + X-aq

  6. Weak Acids and Bases HXaq+ H2O(l) H3O+aq + X-aq

  7. Weak Acids and Bases Xaq+ H2O(l) OH-aq + HX+aq

  8. Weak Acids and Bases Calculate the pH of 465 μM pyridine (Kb=1.8x10-9) Xaq+ H2O(l) OH-aq + HX+aq How does pyridine react with water?

  9. Weak Acids and Bases + HXaq + H2O(l) X-aq+ H2O(l) OH-aq + HXaq H3O+aq + X-aq HXaq + 2 H2O(l) + X-aq H3O+aq + X-aq+ OH-aq + HXaq 2 H2O(l) H3O+aq + OH-aq

  10. Weak Acids and Bases What contributes to the strength of an acid? HXaq+ H2O(l) H3O+aq + X-aq Bond Strength Electronegativity

  11. Weak Acids and Bases What contributes to the strength of an acid? HXaq+ H2O(l) H3O+aq + X-aq Electronegativity – decreasing ΔEN between O and X – decreasing pull of electrons away from H

  12. Weak Acids and Bases What contributes to the strength of an acid? Stability of the conjugate base – if you can draw a resonance structure for the base, it increases the stability and therefore the strength of the acid HXaq+ H2O(l) H3O+aq + X-aq

  13. Weak Acids and Bases Consider dissolving 12 mg NaHSO4 in water. Will the resulting solution be acidic, basic, or neutral? What is the pH? NaHSO4(aq) Na+ + HSO4- pKa = 6.91 H+ + SO42-

  14. Weak Acids and Bases A solution of pyridine has a pH of 8.8, what was the initial concentration of pyridine? What is the % protonation? pKa = 5.25

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