chemoleo
Biochemicus Energeticus
Posts: 3005
Registered: 23-7-2003
Location: England Germany
Member Is Offline
Mood: crystalline
|
|
Heat and Cold of dissolution....
Forgive me, I know I should know; but yet it seems that this is a question that baffled even my senior lab members. Disgraceful.
Mix ethanol in water, and it becomes hot. I reasoned, it is because water forms a hydration cage around the CH3 moiety, thus leading to more ordered
water, loss of kinetic energy, decrease of entropy (more ordered system) and release of heat. One lab member got really riled about the term 'kinetic
energy', for some odd reason exclusively preferring the term 'entropy'. Talking about delta H /S and some such.
Anyway, I do think that the hydration model is correct. Irrespective of any reaction enthalpies delta H.
But then, the next question is, why do some solutions become cold?
I.e. dissolve ammonium nitrate, well known from cold packs, in water, and it becomes very very cold.
Why?
Surely there are hydration cages forming around the NH4+ and the NO3-? This should really release energy due to restriction of kinetic movement,
making the solution warm once again. Except that it doesn't.
The term lattice energy was thrown in, by my very grumpy boss. I looked it up, according to wikipedia, http://en.wikipedia.org/wiki/Lattice_energy ,
it refers to the energy to seperate the ions in their *gaseous* states. So nothing to do with solubilisation. Unless its definition is broader than it
seems.
So dissolution of AN in H2O becomes cold... why? Where does it require energy to dissolve it? To break apart ionic bonds, similar to the lattice
energy concept? Why does that not hold for many other salts, such as CaCl2, where the ionic bond is also very strong, and where salt dissolution leads
to *heat release*?
Please clarify, I intend to set some of the boys right on this issue in the lab. Throw-away comments such as lattice energy just don't do, I am
curious about the *physical process* that is behind this phenomenon!
Many thanks 
[Edited on 16-2-2007 by chemoleo]
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
|
|
|
Ozone
International Hazard
Posts: 1269
Registered: 28-7-2005
Location: Good Olde USA
Member Is Offline
Mood: Integrated
|
|
Chemoleo,
The question is what sort of interaction is occuring. I would go into detail, but I had a lab full of students today and I am tired.
As much as I hate it, Wiki has a good starter on the subject here:
http://en.wikipedia.org/wiki/Enthalpy_change_of_solution
Now remember, thermodynamically, (after much derivation) DG (Gibbs free energy) = DH (enthalpic change) - TDS(temperature, K and entropic change).
Thus the overall free energy of your system is effected by both factors.
Ack! And to me, lattice energy involves the energy required to make an ion from a neutral atom. If the molecule is already an ion pair, the energy
whould be put in (or taken out) in order to separate the charges via solvent cage. The difficulty in doing this has a large effect on whether or not
the process is endo or exo thermic.
I'll cough up some more when I've had some rest!
Cheers,
O3
[Edited on 16-2-2007 by Ozone]
[Edited on 16-2-2007 by Ozone]
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
|
|
|
Magpie
lab constructor
Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline
Mood: Chemistry: the subtle science.
|
|
Here is some information from "Chemical Process Principle's", part I, p.318, by Hougen et al, 1962:
"Thus, energy is absorbed in overcoming the attractive forces between the particles of the condensed state."
eg, ammonium nitrate, NaCl?
and:
"... the heat of solution of a substance that forms a solvate or hydrate has generally a large negative value, indicating the evolution of heat when
the unhydrated substance is dissolved."
eg ethanol, CaCl2, and to a much greater extent H2SO4??
I don't know if this is getting down to the exact mechanics that you are looking for but it is a start.
The single most important condition for a successful synthesis is good mixing - Nicodem
|
|
|
Ozone
International Hazard
Posts: 1269
Registered: 28-7-2005
Location: Good Olde USA
Member Is Offline
Mood: Integrated
|
|
Damnit! I should be going to bed, but this is a good (both in interest and pedagogy) thread.
H2SO4 has the additional bonus of dehydrating things which adds to the enthalpy of mixing/solvation the energy of dehydration liberated by the
dehydratee. Entropy, of course, goes down on solvation (multiple molecules become one, more-or-less) which minimizes its negative contribution leading
to generally exothermic processes.
Sorry, I was teaching calorimetry today ,
O3
Multipie, Ha!
Another smack on the noggin is that less CaOH2 dissolves in hot water and more in cold!
[Edited on 16-2-2007 by Ozone]
[Edited on 16-2-2007 by Ozone]
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
|
|
|
Levi
Hazard to Others
Posts: 196
Registered: 24-1-2007
Member Is Offline
Mood: No Mood
|
|
| Quote: | Originally posted by Ozone
Entropy, of course, goes down on solvation
[Edited on 16-2-2007 by Ozone] |
That's nice. Suppose solvation reciprocates or does it just roll over and fall asleep after being "dehydrated"?
Chemcrime does not entail death. Chemcrime is death.
|
|
|
Ozone
International Hazard
Posts: 1269
Registered: 28-7-2005
Location: Good Olde USA
Member Is Offline
Mood: Integrated
|
|
Entropy never sleeps!
Given as much time as it needs, the entropy of a system will always increase.
Nice catch!
And... It does not always do this (I was remarking on that general example). I did forget to mention though, as the temperature goes up, so does the
entropy, especially in systems where there is massive exothermia (such as hydration of H2SO4) which leads to a l-->g phase transition (boiling).
This transition lowers total energy by removing heat (DHvap) whilst increasing entropy (l-g).
I was trying to break it into smaller pieces, viz. single interactions before describing the energy balence of the whole system. There is a reason why
this topic is killer and takes a while to get (I'm an organic chemist so this was a battle).
Oh yes, and in this particular process, there is lattice energy involved as water is being ionized.
Best,
O3
A nice, easily digestible account of free energy and entropy is given here:
http://www.2ndlaw.com/gibbs.html
[Edited on 16-2-2007 by Ozone]
[Edited on 16-2-2007 by Ozone]
[Edited on 16-2-2007 by Ozone]
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
|
|
|
Aurum
Harmless
Posts: 34
Registered: 19-5-2006
Member Is Offline
Mood: No Mood
|
|
Is there any way to work out thermal change when a concentrated solution is diluted?
Edit, found the values for LiCl and LiBr.
[Edited on 18-2-2007 by Aurum]
|
|
|
pantone159
National Hazard
Posts: 586
Registered: 27-6-2006
Location: Austin, TX, USA
Member Is Offline
Mood: desperate for shade
|
|
It takes energy to separate, e.g., the NH4+ and NO3- ions from each other in the lattice in the solid. Some energy comes back from the solvation of
the ions by H2O, but not as much. (Ions w/ smaller charge have generally lower solvation enthalpies.)
The reason NH4NO3 dissolves at all, then, is that the entropy increase is large enough to overcome this, i.e., DG = DH - T*DS is negative, despite DH
being positive, because DS is positive (and large enough, considering the appropriate T).
|
|
|
Ozone
International Hazard
Posts: 1269
Registered: 28-7-2005
Location: Good Olde USA
Member Is Offline
Mood: Integrated
|
|
Excelllent! That defines what would be considered an enthalpic or entropically driven system!
Also note that in one case I was describing lattice energy and that pantone159 is decribing the 3-D space and molecular order in the solid state
(crystal lattice); It turns out that these are really the same, by definition.
Yeaah,
O3
[Edited on 18-2-2007 by Ozone]
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
|
|
|
12AX7
Post Harlot
Posts: 4803
Registered: 8-3-2005
Location: oscillating
Member Is Offline
Mood: informative
|
|
| Quote: | Originally posted by Ozone
Ack! And to me, lattice energy involves the energy required to make an ion from a neutral atom. |
Whoa whoa whoa! That's ionization energy (A = A+ + e-) or electron affinity (A + e- = A-).
With H2SO4, isn't there also something about forming H3O+?
Tim
|
|
|
Ozone
International Hazard
Posts: 1269
Registered: 28-7-2005
Location: Good Olde USA
Member Is Offline
Mood: Integrated
|
|
Quite correct!
I should have worded that differently! Ionization energy is the amount of energy needed to ionize a neutral atom. Lattice energy refers to the amount
of energy required to separate an ionic compound into its substituent ions. I think that the ionization energy is usually rolled into the
DH(formation) when written as 2Na + Cl2--> 2NaCl, but not when written as Na+ + Cl- -->NaCl?
Specifically as defined, the lattice energy is the release of heat as two ions(g) settle down into a crystalline lattice (s), or the reverse. So, I
suppose, that by definition, in liquid phase, we would be looking at the enthalpy related to the solvation (separation) of the solvating ion pair (or
group).
Each individual step involves and individual energy balence. For example, it takes about 2.5 pages of calculus (stat. mech. Yuck) to approximate the
equilibrium constant for the "simple" lattice ionization of NaCl-->Na+ + Cl-. Lattice energy involves the "bond-strength" between non-covalently
bonded atoms, viz. Ionic bonds. This energy is quite large, say -780 kJ/mol (NaCl) and can be *very* large, viz. -3791 kJ/mol for MgO.
Yup, with H2SO4, there are two different Solvation/dissociation energy calculations required to deal with protons 1 & 2 making H30+ (there are two
separations and two different dissociation constants means there are two different sets of partition coefficients).
PChem in a nutshell is sure to lead to the edit button! But, since we started broadly and are slowly tearing it apart, we may eventually have a decent
resource assembled here. I am appreciating the opportunity to refresh my memory in the subject (being more interested in it now then I was when I was
taking the courses).
Onward,
O3
Check out Born-Haber cycles
[Edited on 18-2-2007 by Ozone]
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
