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Bases

Bases. Graphics Source: Wikipedia. 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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Bases

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  1. Bases Graphics Source: Wikipedia

  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. 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

  4. 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

  5. Kb for Some Common Weak Bases

  6. 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!)

  7. 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-

  8. 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

  9. 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

  10. 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

  11. 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

  12. Soft cover book Pg. 291-294

  13. Text Pg. 713 do question 16.75 • Then you can practice for hw 16.76

  14. For conjugate acid base pairs only • Ka x Kb = Kw • pKa = -logKa • pKb = -logKb • pKa + pKb = 14 Now do question 16.78 on Pg. 713

  15. Hydrolysis of Salts • Salt of strong acid and strong base – neutral • Salt of weak acid and strong base – basic • Salt of strong base and weak acid – acidic • In general Ka>Kb acidic • Kb>Ka basic • Soft cover book Pg. 387

  16. Text Pg. 697 • Text pg. 698 sample 16.18 and 16.19

  17. X−(aq) + H2O(l) HX(aq) + OH−(aq) Reactions of Anions with Water • Anions are bases. • As such, they can react with water in a hydrolysis reaction to form OH− and the conjugate acid:

  18. Reactions of Cations with Water • Cations with acidic protons (like NH4+) will lower the pH of a solution. • Most metal cations that are hydrated in solution also lower the pH of the solution.

  19. Reactions of Cations with Water • Attraction between nonbonding electrons on oxygen and the metal causes a shift of the electron density in water. • This makes the O-H bond more polar and the water more acidic. • Greater charge and smaller size make a cation more acidic.

  20. Effect of Cations and Anions • An anion that is the conjugate base of a strong acid will not affect the pH. • An anion that is the conjugate base of a weak acid will increase the pH. • A cation that is the conjugate acid of a weak base will decrease the pH.

  21. Effect of Cations and Anions • Cations of the strong Arrhenius bases will not affect the pH. • Other metal ions will cause a decrease in pH. • When a solution contains both the conjugate base of a weak acid and the conjugate acid of a weak base, the affect on pH depends on the Kaand Kbvalues.

  22. Factors Affecting Acid Strength • The more polar the H-X bond and/or the weaker the H-X bond, the more acidic the compound. • Acidity increases from left to right across a row and from top to bottom down a group.

  23. Factors Affecting Acid Strength In oxyacids, in which an OH is bonded to another atom, Y, the more electronegative Y is, the more acidic the acid.

  24. Factors Affecting Acid Strength For a series of oxyacids, acidity increases with the number of oxygens.

  25. Factors Affecting Acid Strength Resonance in the conjugate bases of carboxylic acids stabilizes the base and makes the conjugate acid more acidic.

  26. Lewis Acids • Lewis acids are defined as electron-pair acceptors. • Atoms with an empty valence orbital can be Lewis acids.

  27. Lewis Bases • Lewis bases are defined as electron-pair donors. • Anything that could be a Brønsted–Lowry base is a Lewis base. • Lewis bases can interact with things other than protons, however.

  28. Soft covered book Pg. 288 • Example and practice

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