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aga
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[*] posted on 25-6-2014 at 12:38
Catalysts


Does a good quantum-based explanation exist describing how catalysts work ?

Google is having a day off.




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[*] posted on 25-6-2014 at 13:39


There is unstable product that decomposes again to reactant and usually two or more new molecules.

For example MnO2 gives unstable product that will decompose again to MnO2 and O2 with hydrogen peroxide.
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[*] posted on 25-6-2014 at 13:53


hmm. Manganese dioxide and hydrogen peroxide making steam and oxygen.

Is that a catalytic decomposition of hydrogen peroxide ?

If so, how does the rapid disassociation work at the ionic, electron, or quantum level ?




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[*] posted on 25-6-2014 at 15:05


It's not quantum physics that MnO2 gives unstable product but it's a matter of chemical property of that compound.

[Edited on 25-6-2014 by Random]
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[*] posted on 25-6-2014 at 20:37


The CNO cycle, where carbon acts as a catalyst, should be amenable to quantum analysis.

[Edited on 26-6-2014 by Paddywhacker]
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[*] posted on 26-6-2014 at 01:09


CNO cycle inside a star ?
Wow ! thanks

[Edited on 26-6-2014 by aga]




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[*] posted on 26-6-2014 at 04:49


aga:

In most cases of catalysis quantum mechanics needs to be invoked no more or no less than in catalyst-free reactions. It's all just chemistry. Start understanding the basics of catalysis and much will be revealed.

The CNO cycle isn't chemistry of course. But carbon does act as a catalyst there. Not the greatest starting point for understanding chemical catalysis though...

[Edited on 26-6-2014 by blogfast25]




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[*] posted on 26-6-2014 at 08:45


oh Lord, you bass's got me GOOGLELOST* reading about nucleosynthesis

*googlelost=following interesting link after link after link




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[*] posted on 26-6-2014 at 12:48


sure it isn't Googlinkistificated ?



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[*] posted on 30-6-2014 at 12:09


Seems that i totally misunderstood catalysis, in that the catalyst Does react, however the process ends up with the catalyst back in it's original form.

Detritus here we come.




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[*] posted on 30-6-2014 at 12:38


Hmm. MnO2 remains UNCHANGED when mixed with H2O2 I'm pretty sure. Now, the in "old school" Hydriodic acid/Red phosporous reduction of ephedrine there is a "triangular" cycle. Heat breaks up HI, the free I combines with P, then gets hydrolysed back into HI and P. When the reaction is over, the P is all still there but it DID take place in the reaction hence not a catalyst as I understand the term.

I think I got that right.




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[*] posted on 1-7-2014 at 11:55


That is how i understood the term too : Catalyst does NOT take part in the reaction, yet accellerates the reaction.

Seems that it DOES react, yet ends up back in it's original form.

Begs the question tho, in an exhaust pipe, there is pressure pushing stuff out the tail-pipe, so anything that 'gets involved' in the reaction stands a reasonable chance of being blown out the back, be it a gas, liquid or solid.

[Edited on 1-7-2014 by aga]




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[*] posted on 1-7-2014 at 12:02


arkoma, Are you sure that MnO2 remains unchanged in the decomposition of peroxides?
I have done the reaction quite a few times and it [MnO2] seems to be unsuitable in decomposing more peroxide afterwards.




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[*] posted on 1-7-2014 at 12:04


Is there a way to definitively tell if it's MnO2 afterwards ?
(a way available in a home lab)




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[*] posted on 1-7-2014 at 12:37


Quote: Originally posted by aga  
Is there a way to definitively tell if it's MnO2 afterwards ?
(a way available in a home lab)


Plenty of ways.

For instance, MnO2 is such a powerful oxidiser that it can oxidise chloride ions quantitatively:

MnO2(s) + 4 HCl(aq) === > MnCl2(aq) + Cl2(g) + 2 H2O(l)

This is in itself the basis for methods to determine the composition of MnO2 (and the basis of some chlorine generators).

It's well documented that MnO2 is often slightly deficient on oxygen, with respect to its theoretical formula.

[Edited on 1-7-2014 by blogfast25]




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[*] posted on 1-7-2014 at 13:13


Do what ?

MnO2 has less oxgen than the derived formula for MnO2 says it should ?

So it's MnO and MnO2 as a mixture or is that not possible ?




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[*] posted on 1-7-2014 at 13:21


You guys are making this more complicated than it needs to be.
Just weigh the oxide before and after.
No need to generate chlorine. BTW, that react takes a long time doesn't it? It starts off fast, giving off lots of chlorine, but then it slows way down. You'll need to boil it in hydrochloric acid for a while to get the reaction to go to completion. Then you'll need to analysis how much chlorine was produced, the whole process would be a pain in the chode.




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[*] posted on 1-7-2014 at 13:33


Quote: Originally posted by Zyklon-A  
done the reaction quite a few times and it [MnO2] seems to be unsuitable in decomposing more peroxide afterwards.

Er, you said that, and grabbed my attention.
Are you now saying that it's a Weighing error ?
I dearly hope not.


[Edited on 1-7-2014 by aga]




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[*] posted on 1-7-2014 at 15:13


A weighing error? Who said that?



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[*] posted on 1-7-2014 at 15:29


Quote: Originally posted by Zyklon-A  
arkoma, Are you sure that MnO2 remains unchanged in the decomposition of peroxides?


Please READ my posts; I said PRETTY SURE

Same S.N.A.F.U. in "chemophobia". The label did NOT claim the oxygen was carcinogenic, but the combustion products of the oxygen. Sheesh. Slow down and read.

edit

Quote:
The manganese dioxide is not consumed in the reaction, and thus may be recovered unchanged, and re-used indefinitely. Accordingly, manganese dioxide catalyses this reaction.[4]


[4] Richard I. Masel “Chemical Kinetics and Catalysis” Wiley-Interscience, New York, 2001. ISBN 0-471-24197-0.


So yeah, I am PRETTY SURE

[Edited on 7-1-2014 by arkoma]




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[*] posted on 2-7-2014 at 02:25


Quote: Originally posted by Random  
It's not quantum physics that MnO2 gives unstable product but it's a matter of chemical property of that compound.


First of all, quantum mechanics define ALL reactions. Chemical properties are inherently quantum mechanical in nature. Catalysts work by allowing a reaction to proceed through a pathway that has a small activation energy. This means that a catalyst binds to starting materials

A + Cat ⇌ A-Cat

In A-Cat system A is activated to further react with other compounds. This activation is a result of higher nucleo/electrophilicity induced by the binding to the catalyst. Basically in A-Cat system the energies (and thus shapes) of HOMO and LUMO orbitals are different from pure A. If the catalyst is chosen carefully this shape and energy deformation will allow further reaction with compound B (for example LUMO activation makes Nu attacks easier). This leads to formation of compound A-B and, depending on the system, the catalyst is released at this stage or at a later stage.

A-Cat + B ⇌ A-B + Cat

or

A-Cat + B ⇌ A-B-Cat ⇌ A-B + Cat

Catalysts aren't perfect, they degrade over time (as they are involved in chemical reactions). Usually this is accounted by measuring turnover numbers, roughly speaking how many cycles a catalyst can stay active.

[Edited on 2-7-2014 by kavu]
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[*] posted on 2-7-2014 at 04:05


Quote: Originally posted by Zyklon-A  
You guys are making this more complicated than it needs to be.
Just weigh the oxide before and after.
No need to generate chlorine. BTW, that react takes a long time doesn't it? It starts off fast, giving off lots of chlorine, but then it slows way down. You'll need to boil it in hydrochloric acid for a while to get the reaction to go to completion. Then you'll need to analysis how much chlorine was produced, the whole process would be a pain in the chode.


No, it's a simple way to demonstrate your catalyst remains unchanged. That after you've used it as a catalyst for H2O2 decomposition it still oxidises chloride ions in acid medium.

The reaction is no different from any other in terms of kinetics. Use an excess HCl to keep reaction rate high. Not complicated. Nor do you have to generate vast amounts of chlorine: use only a few mg of MnO2.

As regards 'simple' weighing that would work but in practice isn't as easy as it sounds. The material would have to be quantitatively collected, dried and accurately weighed.

Quote: Originally posted by aga  

MnO2 has less oxgen than the derived formula for MnO2 says it should ?



Yes, but it's close to MnO2 (like MnOx with x = 1.995 or thereabouts, it can vary from one product to another). Presumably a small amount of the Mn is not in oxidation state IV. Considering it's not the most stable of compounds and very eager to oxidise stuff, perhaps that should come as no great surprise.

[Edited on 2-7-2014 by blogfast25]




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[*] posted on 4-7-2014 at 14:18


Groovy.
Thanks to you all for getting stuck into this.

So: a Catalyst IS involved with the reaction it catalyses, however it ends up back in it's original state at the end.

I was taught otherwise, and could not see any way that was possible.

So no special Magic happens with catalysts, just that the Route is altered.

Thanks to You, i have learned something.

@Zyklon: claiming that the catalyst was unsuitable to do the same thing again suggested some error somewhere, and i guessed at weighing.




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[*] posted on 5-7-2014 at 05:36


Aga:

To see the most amazing bag of catalysts, have a look in the mirror. All living systems use enzymes as biocatalysts and these catalyse the most incredible metabolic reactions (and at low temperature too!) imaginable.

For instance Glycosis, the conversion of glucose to pyruvate and the first step in the fermentation of sugar to ethanol, uses some 10 different enzymes to achieve that complex chemical reaction(s):

http://en.wikipedia.org/wiki/Glycolysis#mediaviewer/File:Gly...

Enzymes make our man made catalysts look like dumb, blunt, boring instruments.






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[*] posted on 5-7-2014 at 05:42


I checked the mirror, and there was just an ugly bag of mostly-water.




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