Dear Philippe,
I mixed up the signs and agree with you nice and much more quantitative
explanation.
Best,
Tim
On Thursday 05 January 2017 10:18:53 AM DUMAS Philippe wrote:
> Le Jeudi 5 Janvier 2017 08:56 CET, Tim Gruene <[log in to unmask]> a écrit:
>
> I did not follow all the previous details of this discussion, but I think
> one may say that the statement "The entropy of molecules arranged in a
> crystal must be greatly larger than in solution" is incorrect. Of course,
> there is an increase of entropy (DeltaS = S(solution) - S(crystal) > 0)
> from the ordered crystalline state to the disordered solution state, hence
> S(crystal) < S(solution). Te nice thing is that, without knowing anything
> about the crystallization process, one can also say that DeltaS (per mole)
> = DeltaH/T, where T is the absolute temperature anf DeltaH > 0 is the heat
> absorbed by one mole of molecules going from the crystal state to the
> solution state during the melting of the crystal. Hence, the
> crystallization process, as with ice formation, produces heat (releases
> heat in the solution). Would one measure (in a calorimeter) the heat
> -DeltaH produced during crystallization, and the number of molecules
> having formed the crystal, then one would obtain immediately the DeltaS of
> crystallization. To be accurate: all of that is correct as far as T is the
> equilibrium temperature where the two phases coexist and can exchange
> reversiby (i.e. DeltaG = DeltaH - T DeltaS = 0). For ice melting at
> atmospheric pressure this would be of course 273.15 K. Philippe Dumas
>
> > Dear Nicolas,
> >
> > are you sure this concept is correct?
> >
> > The entropy of molecules arranged in a crystal must be greatly larger than
> > in solution. Hence the driving force for crystallisation is actually a
> > drop in energy, and I am not sure there is actually a barrier.
> >
> > I would take the kids to the play ground and let them run around randomly.
> > When I blow a wistle they should line up in an orderly manner.
> >
> > I am sure it will take some time before some sort of order is achieved,
> > for
> > the kids could face in various directions, or line up in blocks, or other
> > shapes. Once a seed is there, i.e. once e.g. 4-5 kids have created a
> > regular block, it will be much easier for the rest to line up (this is
> > consistent with Patrick's explanation).
> >
> > Cheers,
> > Tim
> >
> > On Wednesday 04 January 2017 05:45:50 PM Nicolas FOOS wrote:
> > > Dear Evette,
> > >
> > > If I was is your situation (explaining nucleation and other concept). I
> > > will discuss in terms of energy.
> > >
> > > I mean obtaining the initial nuclei is the "costly" step in terms of
> > > energy. To represent that, out the classical curve of energy, I will use
> > > a metaphoric representation such as jump over a barrier and run after.
> > >
> > > With this analogy, it's possible to explain that the first step is
> > >
> > > difficult and the second more accessible. If the barrier is to high,
> > > it's impossible to continue and run. If you don't have any barrier it's
> > > easy to run and if you only have a small barrier is not to difficult to
> > > jump over and run. But It also allow you to explain that if you
> > > facilitate the apparition of the first "surface" thanks to appropriate
> > > method (seeding, dust...) you can help the first step (to continue with
> > > the barrier story, it like you have ladder to help, or the ability to>
> > > decrease the size of the barrier.
> > >
> > > For why the crystal and how, I will maybe use the example of orange
> > > pyramid in the food store. Orange are stable together because they have
> > > enough contact, because they have relatively homogeneous shape. If you
> > > mixed orange with water melon it's difficult to obtain nice pyramid.
> > >
> > > For crystallization experiment which work, I have no Idea out of the one
> > > you already mentioned.
> > >
> > >
> > > Hope this help.
> > >
> > > Nicolas
> > >
> > > Nicolas Foos
> > > PhD
> > > Structural Biology Group
> > > European Synchrotron Radiation Facility (E.S.R.F)
> > > 71, avenue des Martyrs
> > > CS 40220
> > > 38043 GRENOBLE Cedex 9
> > > +33 (0)6 76 88 14 87
> > > +33 (0)4 76 88 45 19
> > >
> > > On 30/12/2016 11:06, Radisky, Evette S., Ph.D. wrote:
> > > > Can anyone point to some especially useful resources to help explain
> > > > to kids (pre-chemistry, ~age 10-12) how and why molecules crystallize?
> > > > Maybe a good online movie or animation? I am especially needing help
> > > > with the concept of nucleation, and why nucleation is slower and then
> > > > crystal growth faster once nuclei have formed. I have been
> > > > supervising some experiments growing sucrose crystals from
> > > > supersaturated solutions, which have worked really well, but I am
> > > > having more difficulty in explaining the underlying fundamental
> > > > concepts in a way that is understandable to the kids.
> > > >
> > > > Thanks!
> > > > Evette
> > > >
> > > > Evette Radisky, PhD
> > > >
> > > > Associate Professor of Cancer Biology
> > > >
> > > > Mayo Clinic Cancer Center
> > > >
> > > > Griffin Cancer Research Building
> > > >
> > > > 4500 San Pablo Road <x-apple-data-detectors://3/0>
> > > >
> > > > Jacksonville, FL 32224 <x-apple-data-detectors://3/0>
> > > >
> > > > tel: 904-953-6372 <tel:904-953-6372>
> > > >
> > > > fax: 904-953-0277 <tel:904-953-0277>
> >
> > --
> > --
> > Paul Scherrer Institut
> > Dr. Tim Gruene
> > - persoenlich -
> > Principal Investigator
> > Biology and Chemistry
> > OFLC/102
> > CH-5232 Villigen PSI
> >
> > Phone: +41 (0)56 310 5297
> >
> > GPG Key ID = A46BEE1A
--
--
Paul Scherrer Institut
Dr. Tim Gruene
- persoenlich -
Principal Investigator
Biology and Chemistry
OFLC/102
CH-5232 Villigen PSI
Phone: +41 (0)56 310 5297
GPG Key ID = A46BEE1A
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