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

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. B. Ionic Bonds Q: What is the attraction between atoms?
NaCl dissolving in water
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 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. Hydrophobic interactions of amphipathic compounds
Hydrophobic interactions of amphipathic compounds. Q: Is there an attraction between hydrophobic molecules?
Phospholipid vesicle

9. Bond strength of weak interaction in water

10. II. Ionization of water and pH A. 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 M

11. B. [OH-] & [H+] <<< [H2O]
[H2O] = 55.5 M
1.8 x 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. A. For each acid, HA, there is a conjugate base, A-
III. Acid-base chemistry
Acid = H+ donor Base = H+ acceptor
A. For each acid, HA, there is a conjugate base, A-
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.
Fig pH 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

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. Nature 437, 681-686 (29 September 2005) | doi:10.1038/nature04095
pH in the News: Ocean Acidification
Article
Nature 437, (29 September 2005) | doi: /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
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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 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.
Laboratoire des Sciences du Climat et de l'Environnement, UMR CEA-CNRS, CEA Saclay, F Gif-sur-Yvette, France
Department of Biological Sciences, California State University San Marcos, San Marcos, California , USA
Laboratoire d'Océanographie et du Climat: Expérimentations et Approches Numériques (LOCEAN), Centre IRD de Bretagne, F Plouzané, France
Woods Hole Oceanographic Institution, Woods Hole, Massachusetts , USA
National Oceanic and Atmospheric Administration (NOAA)/Pacific Marine Environmental Laboratory, Seattle, Washington , USA
NOAA/Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey 08542, USA
Institute of Geophysics and Planetary Physics, UCLA, Los Angeles, California , USA
Frontier Research Center for Global Change, Yokohama , 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 , USA
National Center for Atmospheric Research, Boulder, Colorado , USA
Max Planck Institut für Meteorologie, D 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 , USA
LOCEAN, Université Pierre et Marie Curie, F Paris, France
Alfred Wegener Institute for Polar and Marine Research, D 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 Toulouse, France (P.M.); AOS Program, Princeton University, Princeton, New Jersey , 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+