Sciencemadness Discussion Board

Catalysts

aga - 25-6-2014 at 12:38

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

Google is having a day off.

Random - 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.

aga - 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 ?

Random - 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]

Paddywhacker - 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]

aga - 26-6-2014 at 01:09

CNO cycle inside a star ?
Wow ! thanks

[Edited on 26-6-2014 by aga]

blogfast25 - 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]

arkoma - 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

aga - 26-6-2014 at 12:48

sure it isn't Googlinkistificated ?

aga - 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.

arkoma - 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.

aga - 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]

Zyklon-A - 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.

aga - 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)

blogfast25 - 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]

aga - 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 ?

Zyklon-A - 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.

aga - 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]

Zyklon-A - 1-7-2014 at 15:13

A weighing error? Who said that?

arkoma - 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]

kavu - 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]

blogfast25 - 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]

aga - 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.

blogfast25 - 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.


aga - 5-7-2014 at 05:42

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

AJKOER - 5-7-2014 at 09:19

Aga:
I agree.
---------------------
Here is the technical definition of a catalyst (source: http://chemistry.about.com/od/chemistryglossary/a/catalystde... ):

"A catalyst is a subtance that increases the rate of a chemical reaction by reducing the activation energy, but which is left unchanged by the reaction."

Here is a different source with discussion ( link: http://www.chemicool.com/definition/catalyst.html ), to quote:

"Definition of Catalyst
A catalyst is a substance that speeds up a chemical reaction, but is not consumed by the reaction; hence a catalyst can be recovered chemically unchanged at the end of the reaction it has been used to speed up, or catalyze.

Discussion
In order for chemicals to react, the species involved in any reaction must undergo a rearrangement of chemical bonds.
The slowest step in the bond rearrangement produces what is termed a transition state - a chemical species that is neither a reactant nor a product, but is an intermediate between the two. Energy is required to form the transition state. This energy is called the Energy of Activation, or Ea. Reactants with energy lower than Ea cannot pass through the transition state to react and become products.
A catalyst works by providing a different route, with lower Ea, for the reaction. In any given time interval, the presence of a catalyst allows a greater proportion of the reactant species to acquire sufficient energy to pass through the transition state and become products.
Catalysts cannot shift the position of a chemical equilibrium - the forward and backward reactions are both accelerated so that the equilibrium constant Keq is unchanged. However, by removing products from the reaction mixture as they form, the overall rate of product formation can in practice be increased."
----------------------------------------------------

Now, here is a real world test. You have Ethyl alcohol which is not completely devoid of water. You add Aluminum, but no reaction. You add a small amount of Iodine. The reaction starts. Assume all of the iodine can be recovered as an iodide. Is the iodine a catalyst? Is the iodide a catalyst here? If heating the iodide liberates I2, does that constitute "recovered chemically unchanged"? How do you define here "the end of the reaction" when there are reactions (plural)?

[Edited on 5-7-2014 by AJKOER]

blogfast25 - 5-7-2014 at 10:15

Quote: Originally posted by AJKOER  

Now, here is a real world test. You have Ethyl alcohol which is not completely devoid of water. You add Aluminum, but no reaction. You add a small amount of Iodine. The reaction starts. Assume all of the iodine can be recovered as an iodide. Is the iodine a catalyst? Is the iodide a catalyst here? If heating the iodide liberates I2, does that constitute "recovered chemically unchanged"? How do you define here "the end of the reaction" when there are reactions (plural)?



If the initial I2 is recovered as iodide then I2 is not a catalyst in the strict sense of the word.

Nor is the iodide a catalyst here: it forms by reacting I2 with the aluminium, removing its oxide layer and making it more reactive towards the absolute alcohol. Had you added iodide instead of iodine no such effect would have been observed.

How can heating iodide yield iodine without oxidation? Only at temperatures where an iodide begins dissociating is this possible:

MI(g) + high T < === > M(g) +1/2 I2(g), shifts to the right at high T, for an exothermic iodide.

For a catalyst to qualify as a catalyst it needs to remain chemically unchanged while on the job. Iodine does change (and quite dramatically too!) when it gets this ethanol Al lark going: it becomes iodide, permanently. Not a catalyst. Subsequent, SEPARATE, recovery of the iodide as iodine isn't part of the (non-catalysed) reaction.


[Edited on 5-7-2014 by blogfast25]

AJKOER - 5-7-2014 at 14:10

Blogfast obviously known his stuff, but he did miss a small point. The catalyst is defined in reference to a reaction. Did anyone notice that I did not precisely define the reaction of interest?

For example one could argue that I was looking at constructing a galvanic cell to remove the remaining water in the alcohol. Aluminum is the anode, oxygen surface contact is the cathode and Aluminum iodide is the electrolyte! As the latter remains unchanged and promotes the reaction, it may meet our technical definition of a catalyst for the galvanic cell, where the formation of Al(OH)3 also removes water. Now, it is most likely true that this would be an inconsequential reaction in the overall reactions, but that not in the definition either!

blogfast25 - 6-7-2014 at 04:59

Quote: Originally posted by AJKOER  
For example one could argue that I was looking at constructing a galvanic cell to remove the remaining water in the alcohol. Aluminum is the anode, oxygen surface contact is the cathode and Aluminum iodide is the electrolyte! As the latter remains unchanged and promotes the reaction, it may meet our technical definition of a catalyst for the galvanic cell, where the formation of Al(OH)3 also removes water. Now, it is most likely true that this would be an inconsequential reaction in the overall reactions, but that not in the definition either!


You're playing with words. The electrolyte does not take part in the redox reactions of the cell, it can therefore not be considered a catalyst. Catalysts always take part in the reaction they catalyse, no matter how subtly. The iodide ions do not, they are merely charge carriers.