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Acids, Bases, and K a

Acids, Bases, and K a. From Silberberg “Chemistry” 2/e. NH 3 + H 2 O NH 4 + + OH -. NH 4 + + OH - NH 3 + H 2 O. Br ønsted-Lowry Theory of Acids and Bases. An acid is a proton donor. A base is a proton acceptor. conjugate acid. conjugate base. base. acid. base. acid.

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Acids, Bases, and K a

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  1. Acids, Bases, and Ka

  2. From Silberberg “Chemistry” 2/e

  3. NH3 + H2O NH4+ + OH- NH4+ + OH- NH3 + H2O Brønsted-Lowry Theory of Acids and Bases An acid is a proton donor. A base is a proton acceptor. conjugate acid conjugate base base acid base acid conjugate base conjugate acid

  4. CH3CO2H+ H2O CH3CO2- + H3O+ [CH3CO2-][H3O+] Ka= = 1.8 x 10-5 [CH3CO2H] Acid Ionization Constant conjugate base conjugate acid acid base

  5. Relative Strengths of Some Brønsted-Lowry Acids and Bases

  6. pH and pOH The potential of the hydrogen ion was defined in 1909 by Søren Sørensen as the negative of the logarithm of [H+]. pH = -log[H3O+] pOH = -log[OH-] KW = [H3O+][OH-]= 1.0 x 10-14 -logKW = -log[H3O+]-log[OH-]= -log(1.0 x 10-14) pKW = pH + pOH = 14

  7. pH and pOH Scales Despite stopping at 0 and 14, the scales are actually open-ended

  8. pH Scale Range Calculate the pH of a 0.020 M HCl solution pH = -log [H3O+] pH = 1.70 (Note that this must actually be measured)

  9. Neutralization Reactions and Titration Curves • Equivalence point: • The point in the reaction at which both acid and base have reacted in stoichiometric amount. • Neither acid nor base is present in excess. • End point: • The point at which the indicator changes color. • Titrant: • The solution of known concentration added to the solution of unknown concentration. • Titration Curve: • The plot of pH vs. volume.

  10. Titration of a Strong Acid with a Strong Base 10 mL of 0.10 M HCl titrated with 0.10 M NaOH

  11. Titration of a Strong Acid with a Strong Base • The pH has a low value at the beginning. • The pH changes slowly • until just before the equivalence point. • The pH rises sharply • perhaps 6 units per 0.1 mL addition of titrant. • The pH rises slowly again. • Common acid-base indicator is phenolphthalein

  12. Titration of a Weak Acid with a Strong Base

  13. Buffer Solutions • A mixture of a weak acid and its conjugate base that changes pH only slightly on addition of acid or base. • …Or, a weak base and its conjugate acid

  14. Titration of a Weak Acid with a Strong Base

  15. Titration of a Weak Acid with a Strong Base—Determination of Equivalence Point Equivalence point is ½ thedistance between tangent lines

  16. Lab This Week • Set-up LabQuest for pH determinations • Use a base of known concentration (~0.10 M NaOH) to titrate an unknown acid. Base is already standardized. • Determine the concentration and Ka of each acid • Note that in buffer regions you can use 0.5-1.0 mL increments but in non-buffer regions use 3-5 drops

  17. Proposal • How will you determine the molar concentration of the unknown acid? • How will you determine Ka?

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