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Chapter 2. Aqueous Chemistry
I. Weak interactions in aqueous solutions
4 Important weak interactions
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A. Hydrogen Bonds
• H-O bonds in H2O are polar.
• differing electronegativities lead to partial
charges (-O and +H) = electric dipole.
O

H
O
H
H
O
H

H
H
O
H
•Takes about 20 kJ/mol to break H-bonds.
Q: Are covalent bonds stronger than H-bonds?
H
2
• 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.
Hydrogen Bonds.
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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.
NaCl dissolving in water
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C. Van der Waals forces
1. Dipole-dipole interactions
•Electrostatic interaction between polar but
uncharged groups.
•Weaker than hydrogen bonds

C
O

C
O
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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.
H
H
C
H


H
H
C
H
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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
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water. Why?
Hydrophobic interactions of
amphipathic compounds.
Q: Is there an attraction
between hydrophobic
molecules?
Phospholipid vesicle
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Bond strength of weak interaction in water
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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 10-16 M
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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.
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III. Acid-base chemistry
Acid = H+ donor
Base = H+ acceptor
A. For each acid, HA, there is a
conjugate base, AHA  H+ + A• Acid dissociation constant describes equilibrium.
Ka = [H+][A-]/[HA]
• Strong acids have higher Ka’s
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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-]
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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.
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• 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)
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so, 15 mM/11 = 1.36 mM HA
Some molecules
have multiple
pKa’s.
E.g., Typical
amino acid.
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H+
ions dissociated/molecule
pH in the News:
Ocean Acidification
Carbon dioxide equilibrium
CO2 + H2O  H2CO3
H2CO3  HCO3- + H+
HCO3-  CO32- + H+
Article
Nature 437, 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, GianKasper 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 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.
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Laboratoire des Sciences du Climat et de l'Environnement, UMR CEA-CNRS, CEA Saclay, F-91191 Gif-surYvette, 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-CNRSIRD-UPS, F-31401 Toulouse, France (P.M.); AOS Program, Princeton University, Princeton, New Jersey 085440710, USA (K.B.R.); The Met Office, Hadley Centre, FitzRoy Road, Exeter EX1 3PB, UK (I.J.T.)
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