Author Topic: hypo, hypo.... (shakes head in disbelief)  (Read 1888 times)

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indole_amine

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hypo, hypo.... (shakes head in disbelief)
« on: September 30, 2004, 01:52:00 AM »
"so G=H-TS becomes negative. this formula also makes clear why solids melt/sublime on heating:
suddenly the entropy term doesn't make the free energy G negative anymore!"


You can't say something like this as an answer to the question "What is the deciding factor for whether an amine will form crystals or an oil in its salt and freebase form? Is it the the way the molecule is structured or something like that or must we just go on from what we know from previous experiences?"
 - if you really believe you are of any help in this case, then I'm afraid to tell you, but you ain't!  :(

salts: high energy (ionic) bonds, outweighing entropy loss
non salts: low energy bonds (van der waals, induced dipole,...), only outweigh entropy
loss at lower temperatures.
(ignoring other cases like metals and polymers)


Well, according to this your explanation, all compounds being NO salt would automatically have low melting points and would be hard to crystallize properly???

And tars are polymers, which of course can become solid - even in your part of the woods (I sincerely invite you to a cleaning session with various solvents, several brushes ans lots of polymerized crap residing in several RBFs, if you don't believe me)...

Further, salts also can have other than ionic bonds (re: complex salts)...


And all that entropy, melting point and sublimation stuff has nothing to do with the physical principle behind the formation of ionic NaCl salt lattices in supersaturated (aequous?) solutions, with all due respect...


indole_amine

indole_amine

  • Guest
tss....
« Reply #1 on: September 30, 2004, 02:03:00 AM »
To all others: I translated the whole thing from a page dedicated to chilren! Highschool chemistry...  ;D

http://www.u-helmich.de/che/09/04-ionen/ionen07.html


(even if you don't understand a single word: they have the same pictures in the same context - is this proof/"reference" enough?)


indole_amine

indole_amine

  • Guest
in easier terms
« Reply #2 on: September 30, 2004, 02:27:00 AM »
Well actually you could've helped someone in explaining, hypo: but apparently you decided for destructive criticism and against detailed explanations, so the important part stayed hidden inbetween the lines...

For solid substances, the degree of "ease of crystal formation" can be approximated by determining the type of bonding between the molecules when crystal formation occurs. The weaker the forces holding the molecules together are, the more difficulties are to be expected when crystallization is attempted (or at least the crystals are less stable when being held together by weaker forces (ionic, dipol-dipol, van-der-waals etc. - like hypo said).

Why don't you just post detailed, helpful information like this (apparently you KNOW all those things), but start criticising others instead? You gain nothing but bad reputation if doing so...

indole_amine

hypo

  • Guest
cool down...
« Reply #3 on: September 30, 2004, 02:44:00 AM »
well, imho the page you translated didn't answer the question of _why_
some things form salts and others don't, just _how_.

> all compounds being NO salt would automatically have low melting points
> and would be hard to crystallize properly???

not taking into account other cases like polymers and metals: generally yes!
methamphetamine freebase: <0°
methamphetamine.HCl: 134°C

> And all that entropy, melting point and sublimation stuff has nothing to do with the
> physical principle behind the formation of ionic NaCl salt lattices in supersaturated
> (aequous?) solutions, with all due respect...

sure it does. the salt crystallises in the lowest energy configuration. which is a huge
crystall, because it tries to minimise those not favorable surface positions of atoms.

> but start criticising others instead?

i didn't criticise you. i only pointed out things that were wrong and then i completed
the picture.

> You gain nothing but bad reputation if doing so...

i guess my reputation is as bad as it can get - i can do what i want.  ;)


Xaja

  • Guest
This is funny heh...
« Reply #4 on: September 30, 2004, 09:29:00 PM »
But I'm with hypo. To understand most of this stuff you need to know about Gibbs Free Energy, equations of state, gas laws, entropy, enthalpy etc etc. If you don't know the background, you can't grasp the principles.

I guess you can explain some stuff, like the ion arrangements of salts as above, but in a way it is like getting a 'Daddy, what makes the car move?' question. No offence Ascension, learning half the reason the Hive is here, am just making an analogy between questions.


indole_amine

  • Guest
suggestion
« Reply #5 on: October 02, 2004, 11:19:00 AM »
well, imho the page you translated didn't answer the question of _why_
some things form salts and others don't, just _how_.


Yes? I would say its obvious: things capable of dissociating in cations and anions are capable of formic ionic salts. Other "things" can't accomplish that.  ;)

> all compounds being NO salt would automatically have low melting points
> and would be hard to crystallize properly???

not taking into account other cases like polymers and metals: generally yes!
methamphetamine freebase: <0°
methamphetamine.HCl: 134°C


...that is one example. Sulfur is another (different crystal types; all with mp = roughly 100°C)... :)

(And please tell me: what was wrong with the above explanation on how ionic salts form? Incomplete maybe, not the only thing one should know maybe - but essentially wrong?)

Hypo, maybe you could explain at this point why elementary sulfur (for example) does form crystals? It would fit here very well. And giving practical examples is always easier to understand than just saying "G = H - TS < 0", don't you think?


indole_amine

Xaja

  • Guest
Allow me...
« Reply #6 on: October 02, 2004, 06:00:00 PM »

maybe you could explain at this point why elementary sulfur (for example) does form crystals?




Sulfur forms extremely strong bonds to itself. Its called catenation. It forms rings of 8 sulfur atoms. It does this because it has 4 sp3 hybridised orbitals, two have a lone pair of electrons and two have one electron which form bonds with the single electron of another S atoms sp3 orbital. Hence the formation of two bonds, leading to rings. 8 most stable arrangement - bond angles must be at lowest energy conformation with 8 S atoms. Because things tend to move towards lowest energy state, other allotropes of S rings tend to become S8 rings. Because atoms tend towards lowest energy conformation, molecules stack as orderly as possible: = crystal formation. There is a pretty simple explaination without really going into thermodynamics of it all...  8)




hypo

  • Guest
Why freebases tend to be oils and salts solids
« Reply #7 on: October 03, 2004, 02:49:00 AM »
> Sulfur is another (different crystal types; all with mp = roughly 100°C)

What's your friggen' point? sulfur doesn't resemble methamphetamine at all. so let's compare to a corresponding salt:

S: ~100°C
Na2S: >900°C


See? your example impressively supports my statement saying that salts generally have higher melting points than non-salts.

> Hypo, maybe you could explain at this point why elementary sulfur (for example) does form crystals?

Erm, please reread my posts above: there are different types of bonds: some bring more energy, some less.
Many weak bonds can bring as much energy as one strong bond and note how in the case of amphetamine there's a rather big non-polar molecule around the ionic bond. lets remove this non-polar thingy and we get:

NH4Cl: 340-341°C

Surprise, surprise, the melting point increases!


Let me recapitulate:

Q: why do many of our beloved amine freebases exist as oil at STP conditions whereas the corresponding salts are practically always solids.

A: Whether at given conditions the thermodynamically favored form of a substance is a liquid or a solid depends on two factors:
1) Formation of solids means a reduction of entropy (molecules are restricted in their movement and there is formation of an ordered structure).
2) Formation of solids liberates bonding energy.
If the energy gained by the new bonds outweighs the energy corresponding to the loss of entropy, then the thermodynamically favored form will be a solid. since on one side there are only weak bonds between the freebase molecules and on the other side the salts form very strong ionic bonds, the former will often not solidify at STP, whereas the latter almost always will.

(Of course if one is bored one could imagine an exception to this rule, where for example the amine functionality only makes up for a small part of the molecule and for whatever reason the formation of the corresponding salt breaks the crystal structure in such a way that the salt melts at a lower temperature than the freebase, but this would be a very esoteric case, irrelevant for our purposes.)

Alright, someone who doesn't get it now will probably never get it, so i rest my case.


indole_amine

  • Guest
good one
« Reply #8 on: October 03, 2004, 04:07:00 AM »
Wow!

Now that's a lot more understandable than "G=H-TS becomes negative" dare I say!

someone who doesn't get it now will probably never get it - this time I completely agree...  ;)

(BTW sulfur was just the first element I could remember being crystalline - besides metals of course...)


indole_amine