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Brain&Force
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Probably because the hydrogen peroxide is protonated. I guess. That seems probable, but I'm not sure. It also could be because the solution is
resistant to oxidation.
At the end of the day, simulating atoms doesn't beat working with the real things...
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vmelkon
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Quote: Originally posted by Brain&Force  | | Probably because the hydrogen peroxide is protonated. I guess. That seems probable, but I'm not sure. It also could be because the solution is
resistant to oxidation. |
You mean when H2O2 gets a H+, it is more stable?
Signature ==== Is this my youtube page? https://www.youtube.com/watch?v=tA5PYtul5aU
We must attach the electrodes of knowledge to the nipples of ignorance and give a few good jolts.
Yes my evolutionary friends. We are all homos here. |
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blogfast25
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I think H2O2 is much, much less prone to protonation than H2O. H2O2 is a very weak acid: pK = 11.75 (Wiki).
In H2O the oxygen atoms have 2 lone electron pairs available to share with a proton, in H2O2 only 1. [Edit: this is wrong and has been corrected in my
post below this one]
For that reason, in watery solution any protons get lapped up by water, leaving little for H2O2, I think.
I could be wrong on that.
In alkaline conditions H2O2 would deprotonate slightly but going by the pKa not much either.
[Edited on 12-7-2014 by blogfast25]
[Edited on 12-7-2014 by blogfast25]
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AJKOER
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In my opinion, part of the reaction may be electrochemical based essential on a half reaction involving oxygen, of which, conc H2O2 is a good source.
The NaCl serves both as a good electrolyte and as a supplier of chloride ions for ligand formation side reactions.
The reaction using dilute H2O2 and heat should also progress, but less vigorously.
To test my electrolyte hypothesis, replace NaCl with a small amount of ZnCl2 or NH4Cl being mindful of pH effects (that is, in comparison to NaCl and
dilute HCl to form an equivalent pH), and see how the reaction proceeds.
[Edited on 12-7-2014 by AJKOER]
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papaya
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| Quote: Originally posted by blogfast25 |
In H2O the oxygen atoms have 2 lone electron pairs available to share with a proton, in H2O2 only 1.
[Edited on 12-7-2014 by blogfast25] |
How is this true?
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blogfast25
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@papaya:
Look at the electronic structure of water: each hydrogen atom shares a pair of electrons with the oxygen atom in a so-called sigma molecular orbital
(bonding orbital). But even after this bonding each oxygen atom still has two electron pairs left that are non-bonding (oxygen has 6 valence
electrons).
When a water molecule protonates (to the hydronium ion - H3O+) one of these non-bonding pairs bonds to the extra proton.
However I was wrong above: the oxygen atoms in H2O2 also have two non-bonding electron pairs. My bad.
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blogfast25
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| Quote: Originally posted by AJKOER | In my opinion, part of the reaction may be electrochemical based essential on a half reaction involving oxygen, of which, conc H2O2 is a good source.
The NaCl serves both as a good electrolyte and as a supplier of chloride ions for ligand formation side reactions.
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I think we can all more or less agree that the tetrachloro cuprate ion is involved here and it would be useful to replace the sodium with ammonium or
potassium.
That NaCl is a good electrolyte is irrelevant here: the moment you've got a copper salt in there the solution becomes an electrolyte. But if some
electrolytic effect is taking place it is not two half reactions taking place on separate electrodes, requiring an electrolyte to close the circuit.
The effect of pH is truly striking and hard to explain.
[Edited on 12-7-2014 by blogfast25]
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papaya
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The effect of pH may be is due to
H2O2 <=> H+ + HOO-
equilibrium - it's known that even pure peroxide solutions are more stable in acidic solutions than in basic, I also already provided 2 links to
relevant papers in this thread, from which it seems there's possibility for many mechanisms working in parallel (including free radical ones).
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blogfast25
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| Quote: Originally posted by papaya | The effect of pH may be is due to
H2O2 <=> H+ + HOO-
equilibrium - it's known that even pure peroxide solutions are more stable in acidic solutions than in basic, I also already provided 2 links to
relevant papers in this thread, from which it seems there's possibility for many mechanisms working in parallel (including free radical ones).
|
It's possible but the Ka of that equilibrium is very, very small: Ka = 10<sup>-11.75</sup> (compare to acetic acid e.g.: pKa = 4.76). Even
at pH, say 6, the vast majority of the H2O2 will be as H2O2, not as HO2(-). You can calculate the % of H2O2 present as HO2(-) as a function of pH very
easily: it's very small peanuts.
The first deprotonation of H2O2 is actually weaker than the first deprotonation of boric acid!
[Edited on 12-7-2014 by blogfast25]
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blogfast25
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While Papaya’s find is interesting, it was always unlikely that this would have gone unnoticed by other chemists so far. In fact a superficial
google provides plenty references:
http://www.chemicalforums.com/index.php?topic=7170.0
Video showing the decomposition of H2O2 by CuCl2 solution (no NaCl or other chloride):
https://www.youtube.com/watch?v=-Tm2x8iZQCE
An article that presents a partial explanation for the phenomenon and provides activation energies:
http://link.springer.com/article/10.1007%2Fs007060170004#pag...
Atomistry.com article that in fact states that NaCl on its own has a stabilising effect on H2O2, as does very dilute H2SO4 (same source):
http://oxygen.atomistry.com/catalytic_decomposition_hydrogen...
So, not ‘new’ but possibly not yet fully explained either. And no references to pH, other than the H2SO4 claim by atomistry.com...
--------
On a side note, here an interesting link on the oxidation of Rochelle's salt to formate and CO2, using hydrogen peroxide as oxidiser and CoCl2
solution as catalyst:
http://www.nuffieldfoundation.org/practical-chemistry/involv...
It appears to be rather colourful too.
[Edited on 13-7-2014 by blogfast25]
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AJKOER
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Actually, in my opinion, it is not so much CoCl2 that is responsible for the heightened power of H2O2 in this reaction, but I suspect, it is the
formation of the more reactive singlet oxygen. The latter is formed by the action of H2O2 on HOCl or a hypochlorite in a basic ionic medium. For
example, passing Cl2 gas into a cold (4 C) conc solution of NaOH in 30% H2O2.
The hydrolysis of CoCl2 supplies HCl that is turned into Cl2 and water (and HOCl) by the H2O2.
The same mechanism may also occur with this thread's topic of interest. There is a possible manner to confirm the presence of Singlet oxygen, run the
reaction in the dark and with small additions of H2O2, is there luminescence?
[Edited on 13-7-2014 by AJKOER]
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blogfast25
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| Quote: Originally posted by AJKOER | | Actually, in my opinion, it is not so much CoCl2 that is responsible for the heightened power of H2O2 in this reaction, but I suspect, it is the
formation of the more reactive singlet oxygen. |
Opinions we're not short of. Evidence however is fairly scarce (and all that really matters in science).
What makes you think singlet oxygen plays a part here?
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AJKOER
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Blogfast:
Here is a reference on singlet oxygen formation http://www.researchgate.net/publication/15390248_Singlet_mol... [Edit] and a quote from the abstract citing a pH link to the formation of singlet
oxygen in the presence of chloride ions:
"ABSTRACT A study of the pH profile of the decomposition of aqueous hypochlorite has revealed the evolution (onset at pH 8) of single (1 delta g)
molecular oxygen (singlet spin state dioxygen) detected spectroscopically (1268 nm), prior to the appearance of chlorine (onset at pH 5.5). The
possible mechanism of the singlet state dioxygen evolution is presented, and the origin of its chloride ion dependence is discussed, especially in
reference to chloride ion dependence of singlet molecular oxygen evolution in biological systems. Recent epidemiological analyses of the correlation
of human cancer with chlorinated water supplies focus attention on the singlet oxygen mechanisms of DNA lesion formation."
Here is a text quote relating to preparation:
"The generation of singlet molecular oxygen by the chemiluminescent reaction of hydrogen peroxide with aqueous hypochlorite is well known and is now a
standard method (5-9)"
The full article may be available at https://www.google.com/url?sa=t&source=web&rct=j&...
Interestingly, the cobalt chloride catalyzed reaction proceeds slowly with just H2O2, but with CoCl2, somehow the oxidation (via oxygen) proceeds
more rapidly.
[Edited on 14-7-2014 by AJKOER]
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blogfast25
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Interesting as that may be, it says sweet FA about any role CoCl2 could play in there. Or CuCl4(2-) for that matter.
I have no idea how that Rochelle's salt oxidation with H2O2 is catalysed by CoCl2 but I could propose [hypothesise] that CoCl2 forms some activated
complex with the tartrate anion. That would be as good or bad a guess as invoking singlet oxygen.
We really don't know (yet).
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AJKOER
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Actually, there is some depth on the employment of singlet oxygen in organic synthesis. See, for example, "Singlet Oxygen in Organic Synthesis",
(McKerrall, 2011), link: https://www.google.com/url?sa=t&source=web&rct=j&...
Also, "Singlet Oxygen as a Reagent in Organic Synthesis - Handbook of Synthetic Photochemistry", link: https://www.google.com/url?sa=t&source=web&rct=j&...
And, "SINGLET OXYGEN: A REAGENT IN ORGANIC - IUPAC", link: https://www.google.com/url?sa=t&source=web&rct=j&...
But a search on CoCl2 is far less expansive, see, for example, "Handbook of Reagents for Organic Synthesis, Reagents for ...,
http://books.google.com/books?id=3jLhNq1B2xsC&pg=PA671&a...
Philip L. Fuchs - 2011 - Science
Table 1 Cobalt-catalyzed trimethylsilylmethylmagnesium-promoted styrylation of alkyl halides cat [CoCl2(dpph)] + Alkyl–X Ar Me3SiCH2MgCl ArAlkyl
ether, ..."
which is not convincing evidence, but possibly more supportive for a singlet oxygen argument.
A well discussed and important reaction involving singlet oxygen in inorganic chemistry occurs in Atmospheric Chemistry. To quote, for example,
"Nitrous oxide diffuses from the earth to the troposphere, where it is gradually introduced into the stratosphere by turbulent diffusion. There it is
exposed to ultraviolet radiation that breaks, it into its components, atmospheric nitrogen (N2) and singlet oxygen (o). These single oxygen atoms
react further with nitrous oxide to produce significant amounts of nitric oxide (NO). During the chain of chemical alterations that follows, nitric
oxide steals an atom from the ozone, combining with it to become nitrogen dioxide (N02) and free oxygen (o2). "
Source: Proc.Nat.Acad.Sci.USA Vol.69,No.9,pp.2369-2372,September1972 , title "Newly Recognized Vital Nitrogen Cycle", by Harold Johnston, link: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC426942/
So little known singlet oxygen, with a short (under 75 minutes) but apparently active life span, is contributing to the depletion of the protective
ozone layer that keeping us all alive.
[Edited on 14-7-2014 by AJKOER]
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blogfast25
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The Nuffield link on tartrate oxidation with H2O2 and CoCl2 provides a tentative explanation for that reaction:
"Cobalt(ll) ions are pink. The hydrogen peroxide initially oxidises the cobalt(II), Co2+, to cobalt(lll), Co3+, which is green. The cobalt(III)
bonds to the tartrate ion, allowing the oxidation to take place. The CO3+ is then reduced back to CO2+ and the pink colour returns."
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