MadSci Network: Chemistry

Re: What's the relationship between gases and dissolved gases?

Date: Mon Apr 22 11:38:30 2002
Posted By: In Koo Kim, Grad student, Physical Chemistry, Harvard
Area of science: Chemistry
ID: 1018888260.Ch

You are right that Henry's Law constants are empirically determined, but
they don't have to be if you're willing to go through the complete
thermochemical evaluation, and if you have all the information needed. 
Unfortunately, this is usually way too complicated.  But let me explain
basically what's involved.  The Henry's Law constant is actually an
equilibrium constant.  Ultimately, equilibrium constants describe the
change in enthalpy and entropy between two states.  It is Le Chatelier's
principle that "drives" the eventual approach to equilibrium in the system.
 But saying this is just hiding one mystery behind another.  What's
actually going on is the redistribution of energy into the lowest overall
configuration.  Nature likes to spread itself, make itself even.  It's what
drives diffusion and chemical reactions.  Energy is bouncing around until a
stable configuration is found.  An interesting point here is that this
stable configuration depends on knowing every single aspect of the system,
including the temperature.  Another important point is that the equilibrium
constant has almost nothing to do with the rate at which the process takes
place.  It just tells you what the final states will be if conditions don't
change, eventually.

Determining the change in free energy of a molecule, say carbon dioxide,
from air to dissolved liquid water requires understanding either the final
thermochemical state (delta G) of carbon dioxide in air and liquid water,
or, the difference of thermochemical states between all the intermediate
steps in going from gas to liquid, or liquid to gas.  The intermediate
steps may include enthalpy of solution, enthalpy of vaporization, ...  You
also have to know the change in entropy.  But even then, you're not done
because carbon dioxide, once it's in a liquid form, has to go into
equilibrium with other states such as carbonic acid and the various
carbonates.  And even after you know all the intricate chemical
permutations and incorporate it all into an overall equilibrium constant,
you have to consider things like how deep is the water?  For example, a
really thin layer of water has a high surface area to volume ratio.  The
thermochemical equilibrium will change near the surface because different
things are affecting the solution (e.g. surface tension).  So since it's
all quite complicated, it's easier just to measure the value empirically. 
The empirical nature of the Henry's Law constant is a convenient way of
glossing over all this complexity into a convenient and useful number.

This is a really tough question if you want it answered properly.  I only
hope to have given you a peep into the kind of issues involved.  As for the
bottle of beer, the Henry's law constant is different for two compounds. 
I'm sure what was meant was that the partial pressure of carbon dioxide in
the air inside the closed bottle was in thermochemical equilibrium with the
concentration (mol/l) of carbon dioxide in the liquid.    You can't of
course, know this for sure, because maybe somebody picked up the bottle and
shook it, or maybe it was taken out of the refrigerator and the temperature
of the gas above the liquid is warming faster than the liquid.  He probably
was talking about some stale beer inside a dusty factory that's been
untouched and untampered with for ages.  Yuck.  I like my beer cold and
with a good head of foam.

I'm sure you have other questions... pick up a book on thermochemical
equilibrium or statistical thermodynamics.  Paul Atkins has a good book on
General Chemistry as well.

Current Queue | Current Queue for Chemistry | Chemistry archives

Try the links in the MadSci Library for more information on Chemistry.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-2002. All rights reserved.