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 <[email protected]> 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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