Sciencemadness Discussion Board - Sodium Zincate decomposition products!

Sodium Zincate decomposition products!

Sickman - 4-11-2005 at 02:11

I have a question about a reaction that is often used as a demostration in many highschool classrooms it goes like this;

NaOH + Zn---> Na2ZnO2 + H2 right?

Sodium hydroxide is reacted with metallic zinc releasing hydrogen gas and forming Sodium Zincate.

Now comes my problem: in the demonstration copper pennies are dipped in sodium zincate solution which decomposed, depositing elemental zinc on the surface of the copper penny which is latter heated in a flame to make brass. Hence copper to silver to gold color reaction. Pretty neat! What I am wondering is if:

Na2ZnO2 + Cu ---> ZnCu

where does the Na2 sodium and O2 oxygen go? Does it produce sodium oxide NaO, sodium peroxide NaO2?

12AX7 - 4-11-2005 at 09:39

Gnaw, no sodium oxides.

I don't know how the reaction can possibly proceed as you describe. Zinc is very reactive and copper is practically inert, it can not possibly displace Zn to metal.

Tim

stygian - 4-11-2005 at 17:42

If I knew more about redox (i'll admit, the concept i understand but can't balance a simple reaction) I'd say that the zincate ion, in which i assume the oxygens abstract electrons from the zinc, oxidizes the copper. This reaction can be found on google with little effort.

ANy sodium oxide, peroxide, or superoxide, forms the hydroxide, and liberates oxygen, i think. (except Na2O. no oxygen produced there)

chemoleo - 4-11-2005 at 20:05

Sickman, do you *know* this to be true? Or are you guessing?
Cu cannot reduce Zn2+ to the metal due to the redox potential. Zn could, however, reduce a Cu2+ solution (i.e. forming Cu on a zinc penny, forming your brass).

You sure you havent gotten things mixed up?

stygian - 5-11-2005 at 10:58

I have a question. Would ZnO2(2-) be in +2 state? Or would zincs complex-forming and lewis acid (it is right?) have something to do with this?

Sickman - 5-11-2005 at 12:49

Sorry fella's for the confusion. I guess what has got me confused is Sodium Zincate itself. I can only find "sodium zincate" in searches on the internet and I can't find it in any chemistry books that I own and I own several including the Merck Manual 13th Edition which completly ignores it's existance.

Stygian, believe me! Iv'e searched google and the problem I promise you will find is the that every other post will give you a different formula for Sodium Zincate.

Some say

Zn + 2NaOH + 2H2O--->Na2Zn(OH)4 + H2
Na2Zn(OH)4 + H2

Others say

Zn + NaOH + H2O--->Na2ZnO2 + H2
Na2ZnO2 + H2

All agree that the reaction between a strong sodium hydroxide solution and zinc will liberate Hydrogen gas Ok. So any way, If Sodium hydroxide losses it's hydrogen, would not the Sodium zincate just be:

Na2ZnO2

Which according to many websites reacts with Cu copper in our case the copper coating on a penny. None of the websites seem to explain at this point what happens to our Na sodium and Oxygen, but they all seem to agree that copper reduces sodium zincate to metallic zinc. Which of course is why I 'm wondering about my Sodium and oxygen. Would it form Na2O sodium oxide which would react with the water again to make sodium hydroxide or would it form Na2O2 sodium peroxide which would also form sodium hydroxide along with hydrogen peroxide which would quickly decompose into oxygen and water. Again most of the websites say that the zincate reduces to a metallic zinc coating on the surface of the copper penny, which is heated causing the pure copper and pure zinc to fuse forming the metallic alloy brass. Again they don't seem to bother to let you know about the sodium and oxygen.

12AX7 - 5-11-2005 at 12:51

Quote:

Originally posted by stygian
I have a question. Would ZnO2(2-) be in +2 state? Or would zincs complex-forming and lewis acid (it is right?) have something to do with this?

Something like that, I guess. It's just a rearrangement, no oxidation state has been finagled (O is always 2-, ZnO2 is 2-, so the positive charge must be 2+ in the Zn ion) so there is very little energy (fraction of an eV?) difference. I suppose its complexation behavior could explain this ability, but on the other hand, nearby aluminum is more strongly amphoteric but does not complex as strongly (except H2O of course, which then again could explain its acid/base behavior?).

About Na2ZnO2: note that this formula can be witten with ANY NUMBER of water molecules hydrating it, without changing the <B>overall</B> reaction. That is, only the zinc is affected.

Observe how I can write the supposed compound and reaction:

Sodium zincate:
Na2ZnO2
+2H2O:
Na2Zn(OH)4
+2H2O:
Na2H2Zn(OH)6
Etc. Note the hydrogen added, as if zincate were polybasic (it isn't), with corresponding hydroxyls (totalling n(H + OH)) at the end.

The reaction, overall:
Na2Zn(OH)4 + R = 2NaOH + R(OH)2 + Zn
(I'm writing R instead of something specific such as Cu, because Cu cannot displace Zn. Mg could, but the resulting alkaline solution would dissolve the zinc; Al would work better, but mind that it is trivalent so the equation above needs to be adjusted to stoichiometry.)
Half reaction:
ZnO2(2-) + R = Zn + RO2(2-) (assumes R is also amphoteric)
or, showing all charges, Zn(2+) + 4OH(-) + R(0) = R(2+) + 4OH(-) + Zn

The whole reaction, as ions, can be written:
(note the absurd amount of hydration: this is IN SOLUTION and most of the water does not react!)
2Na(+) + [Zn(2+) + 4OH(-)] + R(0) + 99H2O ==> 2Na(+) + R(2+) + 4OH(-) + Zn(0) + 99H2O.

Note I bracketed the zincate ion, which is written in fully expanded form. The assumption is that four hydroxyls associate with the doubly-charged zinc cation, forming a complex anion. To conserve charge of the solution, strong sodium ions balance it out.

You will also note, easier to see with the bracketing, that the Zn ion can be alone as a cation or combined in an anion, and R can still displace it if it has the energy. This makes sense because the potential series of metals works at pretty much any pH (and such anions usually only assemble at high pH).

Edit: simulpost.

Tim

[Edited on 11-5-2005 by 12AX7]

darkflame89 - 6-11-2005 at 02:29

The above reaction where zincate reacts with copper does exist. It appears that the redox potential of the zincate is below that of the copper(strange), allowing zinc to be formed on the surface of copper metal.

But if anyone is hoping that the same would occur for aluminate, then it is rather disappointing that the redox potential of aluminate is above that of the copper, and no such displacement reaction occur...

guy - 6-11-2005 at 15:11

The reaction seems impossible theoreticaly because
[Zn(OH)4]2- + 2e- ---> Zn + 4OH- is E= -1.22 V

and
Cu --> Cu2+ + 2e is E= -0.337

added up its negative and unfavorable

AND also this site says that no copper dissolves in the solution, so could it be a self oxidizng reaction with copper as a catalyst?

[Edited on 11/6/2005 by guy]

darkflame89 - 7-11-2005 at 01:20

I was wrong then..

...it appears that from the above article, nascent hydrogen formed on the surface, reduced zincate to zinc? Might be possible..read somewhere that hot water is decomposed by zinc-copper couple..

woelen - 7-11-2005 at 05:09

This experiment with copper coins, being covered with zinc metal only happens, when the copper coin actually touches a piece of zinc. If you take a zinc-plated copper coin and put it in the NaOH-solution with zincate ion, then the zinc quickly dissappears and the coin becomes red again. As soon as some pieces of zinc are touching the copper coin, it becomes zinc-plated in seconds again.

An explanation of this phenomenon with some pictures I've put on my website. This actually is a very neat experiment.

http://woelen.scheikunde.net/science/chem/exps/copper+zinc/i...

chloric1 - 7-11-2005 at 09:55

Darkflame_ recently been reading about the hydrogen overvoltage of zinc metal. This is in regards to the metal's electrolytic isolation from zonc sulfate. Actually the zinc ion needs higher voltage than that to decompose water! But the high hydrogen overvoltage permits the isolation of zinc metal from acidic media none lthe less

I am sure the zincate reduction is some other hydrogen overvoltage "trick" also. I vividly remember this experiment 15 years ago in my high school. I was puzzled because I could not rationalize how copper got the zinc to deposit. But it sure was fun to stick it in the flame to make it brassy!

[Edited on 11/7/2005 by chloric1]

[Edited on 11/7/2005 by chloric1]