1 / 26

Chapter 2: Water

Chapter 2: Water. WHY DO WE NEED TO DO THIS AGAIN!!!. Properties of water. Very polar Oxygen is highly electronegative H-bond donor and acceptor High b.p., m.p., heat of vaporization, surface tension. Water dissolves polar compounds. solvation shell or hydration shell.

nora
Download Presentation

Chapter 2: Water

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 2: Water

  2. WHY DO WE NEED TO DO THIS AGAIN!!!

  3. Properties of water • Very polar • Oxygen is highly electronegative • H-bond donor and acceptor • High b.p., m.p., heat of vaporization, surface tension

  4. Water dissolves polar compounds solvation shell or hydration shell

  5. Non-polar substances are insoluble in water Many lipids are amphipathic

  6. How detergents work?

  7. Hydrogen Bonding of Water One H2O molecule can associate with 4 other H20 molecules • Ice: 4 H-bonds per water molecule • Water: 2.3 H-bonds per water molecule Crystal lattice of ice

  8. Biological Hydrogen Bonds

  9. non-covalent interactions

  10. Relative Bond Strengths Bond typekJ/mole H3C-CH3 88 H-H 104 Ionic 40 to 200 H-bond 2 - 20 Hydrophobic interaction 3 -10 van der Waals 0.4 - 4

  11. Ionization of Water

  12. H20 + H20 H3O+ + OH- Ionization of Water H20 H+ + OH- Keq=1.8 X 10-16M Keq= [H+] [OH-] [H2O] [H2O] = 55.5 M [H2O] Keq = [H+] [OH-] (1.8 X 10-16M)(55.5 M ) = [H+] [OH-] 1.0 X 10-14 M2 = [H+] [OH-] = Kw If [H+]=[OH-] then [H+] = 1.0 X 10-7

  13. pH Scale • Devised by Sorenson (1902) • [H+] can range from 1M and 1 X 10-14M • using a log scale simplifies notation • pH = -log [H+] • Neutral pH = 7.0

  14. Weak Acids and Bases Equilibria • Strong acids / bases – disassociate completely • Weak acids / bases – disassociate only partially • Enzyme activity sensitive to pH • weak acid/bases play important role in • protein structure/function

  15. Acid/conjugate base pairs HA + H2O A- + H3O+ HA A- + H+ HA = acid ( donates H+)(Bronstad Acid) A- = Conjugate base (accepts H+)(Bronstad Base) Ka & pKa value describe tendency to loose H+ large Ka = stronger acid small Ka = weaker acid Ka = [H+][A-] [HA] pKa = - log Ka

  16. pKa values determined by titration

  17. Phosphate has three ionizable H+ and three pKas

  18. Buffers • Buffers are aqueous systems that resist changes in pH when small amounts of a strong acid or base are added. • A buffered system consist of a weak acid and its conjugate base. • The most effective buffering occurs at the region of minimum slope on a titration curve (i.e. around the pKa). • Buffers are effective at pHs that are within +/-1 pH unit of the pKa

  19. Henderson-Hasselbach Equation HA = weak acid A- = Conjugate base 1) Ka = [H+][A-] [HA] 2) [H+] = Ka [HA] [A-] 3) -log[H+] = -log Ka -log [HA] [A-] * H-H equation describes the relationship between pH, pKa and buffer concentration 4) -log[H+] = -log Ka +log [A-] [HA] 5) pH = pKa +log [A-] [HA]

  20. Case where 10% acetate ion 90% acetic acid • pH = pKa + log10[0.1 ] • ¯¯¯¯¯¯¯¯¯¯ • [0.9] • pH = 4.76 + (-0.95) • pH = 3.81

  21. Case where 50% acetate ion 50% acetic acid • pH = pKa + log10[0.5 ] • ¯¯¯¯¯¯¯¯¯¯ • [0.5] • pH = 4.76 + 0 • pH = 4.76 = pKa

  22. Case where 90% acetate ion 10% acetic acid • pH = pKa + log10[0.9 ] • ¯¯¯¯¯¯¯¯¯¯ • [0.1] • pH = 4.76 + 0.95 • pH = 5.71

  23. Cases when buffering fails • pH = pKa + log10[0.99 ] • ¯¯¯¯¯¯¯¯¯¯ • [0.01] • pH = 4.76 + 2.00 • pH = 6.76 • pH = pKa + log10[0.01 ] • ¯¯¯¯¯¯¯¯¯ • [0.99] • pH = 4.76 - 2.00 • pH = 2.76

More Related