sign up log in tour help

Take the 2-minute tour Chemistry Stack Exchange is a question and answer site for scientists, academics, teachers and students. It's 100%free, no registration required.

How do copper sulphate and the like change crystal structure on adding water of hydration?

, in its hydrated form, is crystalline, whereas the anhydrous form is amorphous.Copper sulphate

has a similar story-- on heating the crystalline dihydrate we get an amorphous hemihydrate. (Gypsum in fact has two differentcrystalline forms of the dihydrate, and has an anhydrous form as well).Gypsum

I've never really grasped how the water makes it crystalline. I suspect it has to do with the relative sizes of the compounds being re-compensated by ligands, but I'm not sure. How does the water affect/create the crystalline properties?

inorganic-chemistry coordination-compounds crystal-structure

asked May 29 '12 at 5:23

ManishEarth 5,787 3 21 64

2 Answers

I learned something in trying to answer this question since I've never quite understood what thewater was doing in hydrated crystals either. The waters of hydration help to stabilize the crystallineform by holding the crystal together with hydrogen bonding. Water incorporated into the crystalallows the negative dipole of the water to partially diffuse the positive charge and the positive dipoleof the water to partially diffuse the negative charge, and would help to minimize repulsive forcesbetween ions of like charge "close" in the crystal. Speaking of ...$V 0 40

)

Copper is surrounded by six oxygen atoms, provided by two different sulfate groups and fourmolecules of water. A fifth water resides elsewhere in the framework but does not bind directlyto copper.....Water of crystallization is stabilized by electrostatic attractions, consequentlyhydrates are common for salts that contain +2 and +3 cations as well as -2 anions. ( )1 Ref

A more that takes some looking at to get the sense of is given below. , with iron with a 3+ charge, forms various hydrates with x = 6, 7, 9, 10.

(Disclaimer....actually this is the crystal structure for coquimbite with has an occasional Al for Fesubstitution thrown in, but the principle is the same)

interesting reference 0'F

40

Y)

In this picture, the sulfate ions are shown as tetrahedra and the ions are shown as octahedra(presumably because ions have 6-coordination sites). The crystal is formed from "clusters of6 sulfate tetrahedra and 3 iron octahedra which share only corners" and there are two clusters perunit cell. Each of the clusters is linked to the others through hydrogen bond provided by theincorporated water, and parallel chains are held together only through hydrogen bonds.

'F

'F

The individual chain segments are linked through hydrogen bonds only. The geometricalarrangement of the chains gives rise to "channels," ... which are occupied by water moleculeslinked to the chains by hydrogen bonds.

So, you can see how removing the waters of hydration would cause the crystalline structure to

break down and result in an amorphous substance.

Further information from the paper: Coquimbite has . The crystal structure shows that, ofthe 3 Fe found in a cluster, one Fe is associated with 4 oxygens from sulfate groups, the second Feis surrounded by 3 oxygens from sulfate groups and 3 water molecules, and the third Fe (or Al) issurrounded by 6 water molecules, so the octahedral representation does make sense.

0)

edited May 31 '12 at 5:17 answered May 30 '12 at 0:12Janice DelMar2,802 12 23

Hmm, interesting. I'll wait a bit and then accept. Any reason why ions are shown as octahedra? Is it becausethere's some coordination compound being formed? (Probably it, the "incorporated water" may be acting as a ligandon the Iron side)

'F

ManishEarth May 30 '12 at 0:53

Could you incorporate that into your post? Thanks :) ManishEarth May 30 '12 at 9:11

[Manishearth, I'm not sure I'm even answering the right question below. After reading your questionmore carefully, I think you were more interested in how water molecules help crystals form, ratherthan whether or not the anhydrous forms can form crystals also (versus being strictly amorphous).So, oops. Maybe some one else has a good answer on the water molecules part?]

I suspect the answer is an issue of process rather than of fundamentals.

That is, if you could find a non-aqueous medium that readily dissolves the anhydrous form of somesalt, my very strong suspicion is that you could grow excellent clear crystals of it that would lookalmost nothing like the hydrous crystalline forms.

The problem for metallic-anion salts in particular is that finding a liquid that dissolves the saltwithout simultaneously forming strong coordination complexes with it is going to be tricky, since I'mguessing it's usually the coordination effect that help ionize the units of the material to make itsoluble!

Here's a wild guess: Someone, somewhere, has likely done research on how to grow crystals ofsubstances such as anhydrous (hmm, why does this phrase "dead burnt plaster" come tomind? :) or anhydrous by using low-temperature molten solutions of... something or other?Not sure what!

$B40

$V40

And what a delightful crystal that would be to have! It would be way out of the ordinary despitebeing so very... ordinary?... in one sense in its composition.

Apparently, do indeed exist, and are easier to createthan I thought. You just add concentrated sulfuric acid [a] to ordinary copper sulphate pentahydratesolution and let it evaporate slowly. Since is one of the most aggressive drying agents inexistence, my guess is that the just can't compete for the remaining water molecules andis forced to crystallize without hydration.

Update: clear crystals of anhydrous $V40

)

40

$V40

[a] Please, never ever attempt to use concentrated anywhere except in a real lab withprotections and procedures fully in place. This is a compound that wants water so badly that it willysynthesize water right out of the hydrogen and oxygen bonded into proteins and carbohydrates,leaving only charred carbon. Ironically, the actual acidity of concentrated (vs hydrated) isquite moderate, comparable to that of vinegar.

)

40

)

40

Addenda:

-- Another common-exotic crystal may be lithium fluoride, , although the accuracy of thatseems to depend on who you read. Unique how, you say? Even though lithium was an early resultof the Big Bang, lithium and fluorine are both consumed rapidly by large hot stars, especiallyfluorine. Their existence on earth thus appears to depend on processes such as proton irradiationand possibly even intensive neutrino irradiation tht only occur the explosion of a supernovastar, rather than being inherited from before the cataclysmic event. Pretty much all of the elements(except ) here on Earth are star-forged of course, but the elements in crystals thus may beremnants of the supernova event itself. That would... cool!)

-J'

during

) -J'

-- And speaking of unique, wearable crystals: I've always wondered if there's a market out there fordiamond crystals synthesized using carbon that was captured as carbon dioxide during thecremation of a loved one. If there is, feel free -- that is definitely not my bailiwick!

edited May 29 '12 at 22:47 answered May 29 '12 at 16:53

Terry Bollinger1,396 4 13

1

As a possibly useful addition: structures for , and . Also, (Though, made from ash rather than carbon dioxide.)

chalcanthite, the pentahydrate chalcocyanite, the anhydrouscopper sulphate cremation diamonds. Aesin May 29'12 at 18:00

Aesin, that cremation diamonds link is... remarkable! I had that idea longer ago than I care to admit. Clearly amissed opportunity. And thanks for all the structures, wow. Terry Bollinger May 29 '12 at 22:37

Yeah, I was looking for how it forms a crystal. It's OK, I know about conc (though others may not). Ofcourse, in concentrated form it only goes till )

40

, making it not that great, but the water thingy is always there.And the anhydrous crystals concept is interesting--most probably they just retain a brittle structure afterdehydration.

)40

ManishEarth May 30 '12 at 0:43

Hmm. You have a solid correct answer now -- I'm inclined just to delete this one as overly irrelevant?...Terry Bollinger May 30 '12 at 11:22

@TerryBollinger I wouldn't delete it...the suggestion to dehydrate the copper (II) sulfate with sulfuric acid wasinteresting I thought. I have no idea what would happen, and I would try it if I still had a lab. Multiple answersusually give interesting ways to think about things. Janice DelMar May 31 '12 at 1:16