HgDinis25
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Zinc reduction power
Apart from the general displacement reactions, wich can be consulted in a reactivity series, are there any more interesting zinc reductions?
For instance, are there any conditions that allow zinc to reduce Silicon Dioxide, to produce elemental Silicon?
What about other exotic reductions?
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blogfast25
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Quote: Originally posted by HgDinis25  | Apart from the general displacement reactions, wich can be consulted in a reactivity series, are there any more interesting zinc reductions?
For instance, are there any conditions that allow zinc to reduce Silicon Dioxide, to produce elemental Silicon?
What about other exotic reductions? |
SiO2 + 2 Zn === > Si + 2 ZnO is not thermodynamically favourable by a long shot, ΔH (standard) = + 200 kJ/mol.
Check the 'chromium compounds' thread for the reduction of Cr(III) to Cr(II) (even though it didn't really work for me).
Possibly also reduction of various V oxidation states to lower ones.
[Edited on 20-3-2014 by blogfast25]
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HgDinis25
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Yes I've been reading the chromium topic for the last few days, nothing wokable for zinc I guess. I am also aware of the Vanadium recudtion generating
different colours (according to the different states of oxidation of V).
I was really hopping that the silicon isolation could work. Did you calculate the enthalpy using the Si - O bond, as a double covalent bound?
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DraconicAcid
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| Quote: Originally posted by HgDinis25 | Yes I've been reading the chromium topic for the last few days, nothing wokable for zinc I guess. I am also aware of the Vanadium recudtion generating
different colours (according to the different states of oxidation of V).
I was really hopping that the silicon isolation could work. Did you calculate the enthalpy using the Si - O bond, as a double covalent bound?
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Silicon dioxide does not contain double bonds; only single ones.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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blogfast25
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| Quote: Originally posted by HgDinis25 | Yes I've been reading the chromium topic for the last few days, nothing wokable for zinc I guess. I am also aware of the Vanadium recudtion generating
different colours (according to the different states of oxidation of V).
I was really hopping that the silicon isolation could work. Did you calculate the enthalpy using the Si - O bond, as a double covalent bound?
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No, no, the reduction of Cr III to Cr II is possible with zinc (see the link in that thread) but it isn't easy.
Calculate the enthalpies (or better free energies) from tabulated values for the reagents and reaction products using Hess' Law.
NIST webbook is a great online resource for tabulated values of ΔH<sub>formation</sub>. Also, CRC Handbook, for ΔH, ΔG and
entropies of a large number of compounds.
Starting from bond energies for reaction enthalpies is tedious and only approximate.
SiO2 is a network type polymer, it contains no double bonds.
[Edited on 20-3-2014 by blogfast25]
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HgDinis25
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Yes, you're right I was completly wrong about the double bonds, it makes a network with only single bonds (I actually went check the structure of the
polymer xD).
Anyway, any more interesting reductions with zinc? I am mainly aiming at isolating some elements, and I'm out of interesting ideas at the moment.
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blogfast25
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| Quote: Originally posted by HgDinis25 | | Anyway, any more interesting reductions with zinc? I am mainly aiming at isolating some elements, and I'm out of interesting ideas at the moment.
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Your best bet remains aluminothermic reductions. Ni, Co, V, Nb, Si, Ti for instance are all feasible. Zinc has far too low heat of formation to be of
much use in pyrometallurgy.
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HgDinis25
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What about using Zinc powder to displace least active metals from their solutions? It's a good way to isolate a few elements.
Anyway, can I assume that all rare earth metals can be displaced by zinc powder?
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blogfast25
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You are assuming very wrongly. For instance for:
La<sup>3+</sup> + 3 e == > La, Ered = - 2.379 V
And for: Zn == > Zn<sup>2+</sup> + 2 e, Eox = + 0.7618 V
Ered + Eox << 0, so the reaction cannot proceed (ΔG > 0)
http://en.wikipedia.org/wiki/Standard_electrode_potential_(data_page)
The REs are very electropositive elements. Mainly electrolysis (of molten chlorides) is used for their preparation. Aluminothermy e.g. doesn't work at
all for these elements.
Zn can reduce a few metals in solution, those for which Ered is no smaller than - 0.7618 V. That's a few but not that many.
[Edited on 21-3-2014 by blogfast25]
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