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Chapter 2. Aqueous Chemistry

Chapter 2. Aqueous Chemistry. 4 Important weak interactions. I. Weak interactions in aqueous solutions. A. Hydrogen Bonds. H-O bonds in H 2 O are polar. differing electronegativities lead to partial charges (-O and +H) = electric dipole. Takes about 20 kJ/mol to break H-bonds.

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Chapter 2. Aqueous Chemistry

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  1. Chapter 2. Aqueous Chemistry 4 Important weak interactions I. Weak interactions in aqueous solutions

  2. A. Hydrogen Bonds • H-O bonds in H2O are polar. • differing electronegativities lead to partial charges (-O and +H) = electric dipole. • Takes about 20 kJ/mol to break H-bonds. • Q: Are covalent bonds stronger than H-bonds?

  3. Hydrogen Bonds. • H-bonds occur between H atom in a polar bond and any strongly electronegative atom (usually O or N). • Form between alcohols, aldehydes, ketones, and NH groups (C-H bonds do not form H-bonds) • Water dissolves other polar or charged compounds.

  4. NaCl dissolving in water B. Ionic Bonds Q: What is the attraction between atoms? • Polar water molecules screen the ions. • Ionic bonds can be strong, but are weakened in H2O. • Ionic bonds have a 10-40 nm range.

  5. C. Van der Waals forces 1. Dipole-dipole interactions • Electrostatic interaction between polar but uncharged groups. • Weaker than hydrogen bonds

  6. + - - + - + - + 2. London dispersion forces • Electron movement around nucleus creates transient dipoles. • An opposite dipole is induced in nearby atoms. • These bonds are important in stabilizing hydrophobic interactions.

  7. D. Hydrophobic effect • Nonpolar compounds do not dissolve readily in water (hydrocarbons, O2, CO2). Q: Why? • Nonpolar compounds (a) disrupt existing H-bonds and (b) decrease entropy. • So G is positive for dissolving nonpolar solvents (G= H - TS and there is a +H and -S) • Note: All solutes disrupt H-bonds (including ions like Na+ and Cl-), but they are still soluble in water. Why?

  8. Phospholipid vesicle Hydrophobic interactions of amphipathic compounds. Q: Is there an attraction between hydrophobic molecules?

  9. Bond strength of weak interaction in water

  10. II. Ionization of water and pHA. Water molecules reversibly ionize. H2O  H+ + OH- • Extent of ionization is described by Keq: Keq=[H+][OH-]/[H2O] • at 25 C Keq = 1.8 x 10-16 M

  11. B. [OH-] & [H+] <<< [H2O] • [H2O] = 55.5 M • 1.8 x 10-16 M = [OH-][H+]/[55.5M] • So, [OH-] & [H+] = 10-7 M at neutral pH • pH = -log[H+] • so pH = 7 for neutral solution. • 1 unit pH change = 10 fold change in [H+]. • [OH-] & [H+] has dramatic biochemical effects.

  12. III. Acid-base chemistry A. For each acid, HA, there is a conjugate base, A- Acid = H+ donor Base = H+ acceptor HA  H+ + A- • Acid dissociation constant describes equilibrium. Ka = [H+][A-]/[HA] • Strong acids have higher Ka’s

  13. B. pKa = -logKa • Strong acids have low pKa’s. E.g., Acetic acid is a weak acid Ka = 1.74 x 10-5 pKa = 4.76 So at pH=4.76, [HA] = [A-]

  14. C. Acid-base titration curves. Weak acids/bases moderate changes in pH. pH is buffered within +/- 0.5 pH units from the pKa Q: What does the pKa have to be for a compound to be a pH buffer in a cell? Pi is a cellular buffer - pKa=6.9, so buffers from pH 6.4-7.4 Fig. 2-17. pH titration curves.

  15. Different conjugate acid-base pairs buffer in different ranges. • Henderson-Hasselbalch eq: pH = pKa + log([A-]/[HA]) Example:for pH = 8, pKa = 7, 15 mM of compound.What is the concentration of the HA form? 8 - 7 = log([A-]/[HA]) 10(8-7) = [A-]/[HA] = 10 (10x more A- than HA) so, 15 mM/11 = 1.36 mM HA

  16. H+ ions dissociated/molecule Some molecules have multiple pKa’s. E.g., Typical amino acid.

  17. pH in the News: Ocean Acidification Article Nature437, 681-686 (29 September 2005) | doi:10.1038/nature04095 Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms James C. Orr1, Victoria J. Fabry2, Olivier Aumont3, Laurent Bopp1, Scott C. Doney4, Richard A. Feely5, Anand Gnanadesikan6, Nicolas Gruber7, Akio Ishida8, Fortunat Joos9, Robert M. Key10, Keith Lindsay11, Ernst Maier-Reimer12, Richard Matear13, Patrick Monfray1,19, Anne Mouchet14, Raymond G. Najjar15, Gian-Kasper Plattner7,9, Keith B. Rodgers1,16,19, Christopher L. Sabine5, Jorge L. Sarmiento10, Reiner Schlitzer17, Richard D. Slater10, Ian J. Totterdell18,19, Marie-France Weirig17, Yasuhiro Yamanaka8 and Andrew Yool18 Top of page Abstract Today's surface ocean is saturated with respect to calcium carbonate, but increasing atmospheric carbon dioxide concentrations are reducing ocean pH and carbonate ion concentrations, and thus the level of calcium carbonate saturation. Experimental evidence suggests that if these trends continue, key marine organisms—such as corals and some plankton—will have difficulty maintaining their external calcium carbonate skeletons. Here we use 13 models of the ocean–carbon cycle to assess calcium carbonate saturation under the IS92a 'business-as-usual' scenario for future emissions of anthropogenic carbon dioxide. In our projections, Southern Ocean surface waters will begin to become undersaturated with respect to aragonite, a metastable form of calcium carbonate, by the year 2050. By 2100, this undersaturation could extend throughout the entire Southern Ocean and into the subarctic Pacific Ocean. When live pteropods were exposed to our predicted level of undersaturation during a two-day shipboard experiment, their aragonite shells showed notable dissolution. Our findings indicate that conditions detrimental to high-latitude ecosystems could develop within decades, not centuries as suggested previously. Top of page Laboratoire des Sciences du Climat et de l'Environnement, UMR CEA-CNRS, CEA Saclay, F-91191 Gif-sur-Yvette, France Department of Biological Sciences, California State University San Marcos, San Marcos, California 92096-0001, USA Laboratoire d'Océanographie et du Climat: Expérimentations et Approches Numériques (LOCEAN), Centre IRD de Bretagne, F-29280 Plouzané, France Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543-1543, USA National Oceanic and Atmospheric Administration (NOAA)/Pacific Marine Environmental Laboratory, Seattle, Washington 98115-6349, USA NOAA/Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey 08542, USA Institute of Geophysics and Planetary Physics, UCLA, Los Angeles, California 90095-4996, USA Frontier Research Center for Global Change, Yokohama 236-0001, Japan Climate and Environmental Physics, Physics Institute, University of Bern, CH-3012 Bern, Switzerland Atmospheric and Oceanic Sciences (AOS) Program, Princeton University, Princeton, New Jersey 08544-0710, USA National Center for Atmospheric Research, Boulder, Colorado 80307-3000, USA Max Planck Institut für Meteorologie, D-20146 Hamburg, Germany CSIRO Marine Research and Antarctic Climate and Ecosystems CRC, Hobart, Tasmania 7001, Australia Astrophysics and Geophysics Institute, University of Liege, B-4000 Liege, Belgium Department of Meteorology, Pennsylvania State University, University Park, Pennsylvania 16802-5013, USA LOCEAN, Université Pierre et Marie Curie, F-75252 Paris, France Alfred Wegener Institute for Polar and Marine Research, D-27515 Bremerhaven, Germany National Oceanography Centre Southampton, Southampton SO14 3ZH, UK †Present addresses: Laboratoire d'Etudes en Géophysique et Océanographie Spatiales, UMR 5566 CNES-CNRS-IRD-UPS, F-31401 Toulouse, France (P.M.); AOS Program, Princeton University, Princeton, New Jersey 08544-0710, USA (K.B.R.); The Met Office, Hadley Centre, FitzRoy Road, Exeter EX1 3PB, UK (I.J.T.) Carbon dioxide equilibrium CO2 + H2O  H2CO3 H2CO3HCO3- + H+ HCO3-  CO32- + H+

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