If i boil down a Zinc Sulfate solution or a Zinc Chloride solution will it just be a zinc sulfate powder or will it decompose to Zinc Oxide?
-AMchemistryDoctorOfPhilosophy - 15-7-2012 at 20:07
if you heat zinc sulfide, any bonded water will evaporate. Once it's anhydrous, is can be heated to 680 °C before it decomposes to the oxide.
Zinc chloride will melt and then boil on heating.woelen - 15-7-2012 at 22:30
TheAMchemistry87 talks about sulfate, not sulfide. The sulfate indeed is stable and can be evaporated to dryness and loose its bonded water.
The chloride is less stable. From aqueous solution you can prepare the hydrate, but the pure anhydrous salt cannot be formed by simply heating. The
chloride will loose HCl, besides water, and an insoluble mix of zinc hydroxide/oxide and zinc chloride will remain behind. Maybe even all chloride is
lost as HCl and zinc oxide remains behind, but I'm not sure about that.
[Edited on 17-7-12 by woelen]DoctorOfPhilosophy - 15-7-2012 at 22:54
Oops, that's a typo. But here is the MSDS saying that zinc sulfate does decompose
Zinc sulfate is stable at ordinary temperature, but at sufficiently high temperature, it indeed can be decomposed, like most sulfates. Some sulfates
loose SO3, others loose SO2 or a mix of both.weiming1998 - 16-7-2012 at 03:57
If i boil down a Zinc Sulfate solution or a Zinc Chloride solution will it just be a zinc sulfate powder or will it decompose to Zinc Oxide?
-AMchemistry
To answer your question, first we need to know what zinc sulfate and zinc chloride solutions are made of.
Both zinc sulfate and zinc chloride are in the form of Zn2+ and SO4(2-) ions. The Zn2+ ions are Lewis acidic, and forms an adduct with water,
[Zn(H2O)6]2+, which hydrolyzes further to produce [ZnOH(H2O)5]+ and H+ ions, making the solution mildly acidic. This causes very small amounts of HCl
to exist in equilibrium with the other species in a zinc chloride solution and very small amounts of H2SO4 and HSO4- to exist in a solution of zinc
sulfate.
The two acids are different, HCl being far more volatile, so when zinc chloride solution is boiled dry, the small amounts of HCl escape, causing a
shift in the equilibrium, with all the HCl finally leaving only dry ZnO left. Approximate equation: [Zn(H2O)6]2+(aq) +
2Cl-(aq)=====>ZnO(s)+2HCl(g)+5H2O(g).
Sulfuric acid, on the other hand, is less volatile than water, and will not boil off even when the temperature is enough to dehydrate the ZnSO4
hydrate to ZnSO4 anhydrous. So nothing happens when you dehydrate it. You just get anhydrous ZnSO4. At much higher temperatures though, it can
decompose to SO3, which all sulfates except reducing sulfates (like iron (II) sulfate and ammonium sulfate) decompose to. bbartlog - 16-7-2012 at 08:02
Quote:
with all the HCl finally leaving only dry ZnO left. Approximate equation: [Zn(H2O)6]2+(aq) + 2Cl-(aq)=====>ZnO(s)+2HCl(g)+5H2O(g).
Not so simply, though. The first decomposition (after the monohydrate ZnCl2.H2O is obtained) is roughly to Zn(OH)Cl,
ZnCl2.H2O -> Zn(OH)Cl + HCl,
then to ZnO plus ZnCl2,
2Zn(OH)Cl -> ZnO + ZnCl2 + H2O (somewhere around 300C)
...and then finally if enough heat is applied the ZnCl2 is also vaporized, presumably somewhere around the boiling point of 756C. At which point you
would have ZnO, but only because you'd vaporized the remaining ZnCl2.
