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AJKOER
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Quote: Originally posted by AJKOER  | ......
Now, the reaction:
MgCl2 + 3 O2 --> Mg(ClO3)2
if valid for the solid salt exposed to air appears to be largely unexplained or attributed to other compounds/impurities or even special physical
surface conditions.
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I came across the following, that presents a possible answer (link: http://quizlet.com/5215096/chapter-15-flash-cards/ ). To quote:
"Calculate Kc for the reaction
MgO(s) + Cl2(g) ⇌ MgCl2(s) + O2(g)
and indicate whether the value will be larger or smaller at a lower temperature."
The supplied answer is "4.47, larger". So at a higher temperature, passing air through a hot concentrated MgCl2 solution could be similar to
performing a chlorination reaction.
Further, passing air through a hot concentrated Mg(ClO)2 solution, that is undergoing disproportionation forming MgCl2 and Mg(ClO3)2:
3 Mg(ClO)2 ----) 2 MgCl2 + Mg(ClO3)2
may further increase yield via the asserted formation of HOCl by the action of chlorine on water, and the resulting action of the hypochlorous acid on
the hypochlorite:
4 MgCl2 + 2 O2(g) = 4 MgO(s) + 4 Cl2
4 Cl2 + 4 H2O = 4 HCl + 4 HOCl
Mg(ClO)2 + 2 HOCl ----) 2 HCl + Mg(ClO2)2
Mg(ClO2)2 + 2 HOCl ----) 2 HCl + Mg(ClO3)2
8 HCl + 4 MgO ---) 4 H2O + 4 MgCl2
-----------------------------------------------
Net: 2 O2(g) + Mg(ClO)2 ---warm 4MgCl2 & some H2O--) Mg(ClO3)2
So an excess of hot MgCl2 in the presence of some water acts as a catalyst for the formation of Mg(ClO3)2, which remains stable as the MgO formed from
MgCl2 serves as a pH buffer.
The actual action of air was noted in the opening thread, to quote Lunge again:
"In both solutions the hypochlorite is easily converted into chlorate, not merely by heating to 50°, but even by prolonged agitation by a current of
air at ordinary temperatures. At 70° C, from the first mostly chlorate was formed, with a little chloride, produced by loss of oxygen. Hence
magnesium hypochlorite in statu noscendi does not possess much stability and is easily transformed into chlorate."
I would amend the last sentence above to include magnesium chloride as well. This was recently, on another topic, brought to my attention on a patent
discussing the preparation of an adduct of MgCl2 in an atmosphere of nitrogen, and not air.
[EDIT] Per this comment:
| Quote: Originally posted by woelen | | In my second life in a parallel universe I make Mg(ClO3)2 routinely from MgCl2 and O2. I just open my container with MgCl2 and leave it standing
overnight. The next morning I have Mg(ClO3)2 and I can make some quick and loud bangs in the streets again. |
As:
2 HOCl = Cl2O + H2O
and that my cited reaction is for solid MgCl2:
MgO(s) + Cl2(g) ⇌ MgCl2(s) + 1/2 O2(g)
My analysis above also supports the formation of Mg(ClO3)2 in the presence of anhydrous MgCl2, O2 and water vapor via heat and HOCl/Cl2O as well.
Note, heat can be self generated by the action of water vapor on the anhydrous MgCl2 salt.
[Edited on 28-7-2014 by AJKOER]
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blogfast25
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Wrong right from the start. Read the quizlet again:
”At a certain temperature, Kc = 0.0500 and △H = +39.6 kJ for the reaction below.
2MgCl2(s) + O2(g) ⇌ 2MgO(s) + 2Cl2(g)”
Kc is very small, indicating that the equilibrium lies strongly to the LEFT, as suggested also by △H > 0.
This equilibrium would shift slightly to the right at higher temperatures because the reaction left to right is endothermic.
Your “analysis” doesn’t support a g-d damn thing because it starts from a false premise.
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aga
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AJ. Please.
Blowing AJ smoke is getting painful, even to me, a total noob.
Clearly you are adept at Online Research, and gleaning info from published work.
That is quite a skill, and all credit to you for it.
Many of the things you have found really are of great value.
Your avid interest and enthusiasm are remarkable.
I am in two minds as to whether you are blogfast25's insane alter-ego, or a separate personality.
U2U me please.
It is time to move on, Up and forwards.
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The Volatile Chemist
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| Quote: | | I am in two minds as to whether you are blogfast25's insane alter-ego |
Sorry to encourage it, but LOL. Im
sure personal experimentation on everyone's own part should solve the questions given.
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AJKOER
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Yes, I agree based on the quiz constructed numbers (note that the precise temperature in the quiz question is not specified, perhaps for a reason?),
that it appears less than likely.
But, that being said, the preparation of MgCl2.nEtOH in a patented commercial process goes to the added expense of reacting anhydrous MgCl2 with dry
ethanol in an atmosphere of pure N2. This, together with a reported formations of the chlorate both from the dry salt in the presence of water vapor,
and in aqueous magnesium hypochlorite solutions with air at a mild temperature, suggests to me that the possible action of oxygen on MgCl2 in the
presence of some water/excess chloride proceeds more likely than not in a matter of hours. In particular, in a hot concentrated solution of chloride
in an open vessel (no pressure considerations), I would expect the reaction below to moved more to the right:
O2 (g) + 2 MgCl2 (aq) ---heat,excess chloride--) 2 MgO (s) + 2 Cl2 (g)
Note also, the quiz reaction is in the complete absence of water vapor, technically a different situation.
I agree that one suggested resolution is further testing.
[EDIT] The basic solid reaction as cited in the quiz appears to be published. See http://www.sciencedirect.com/science/article/pii/00406031778...
Details:
Thermochimica Acta
December 1977, Vol.21(3):349–354, doi:10.1016/0040-6031(77)85003-X
"The reaction of oxygen with magnesium chloride" by Matthew C. Ball
"Abstract
The kinetics and mechanisms of the reaction between magnesium chloride and oxygen have been studied over a range of oxygen partial pressures. At low
temperatures the reaction is controlled by a phase-boundary process having an activation energy of 74.0 kJ, which changes to one of diffusion of
chlorine at higher temperatures. The activation energy for the diffusion process is 41.2 kJ and there is a considerable loss of entropy in the
formation of the transition state."
---------------------------------------------
Found the answer in the presence of O2 and water vapor! The reaction apparently per Patent 2,039,653 (see page 2 at https://docs.google.com/viewer?url=patentimages.storage.goog... ) produces a mixture of Cl2 and HCl, in proportion to the amounts of O2 and water
vapor employed at a temperature between 400 to 650 C. This is a bit higher than I was hoping for and may be reflective of the requirements for a rapid
reaction for the particular goals of that patent. Interestingly, this article http://www.researchgate.net/publication/229299014_The_effect... notes that the hydrolysis of magnesium chloride hydrate (or, MgCl2 and water
vapor) can commence below 250 C in the presence of solid NaCl.
[Edited on 29-7-2014 by AJKOER]
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blogfast25
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Try it and get back to us. Now please stop boring us with your mythological schemes.
Put up or shut up.
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AJKOER
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Today I pass air using an air pump (designed for a fish tank) into a solution of RT and also hot concentrated solution of MgCl2 with some CaCl2 as
well (Magnesium chloride spray is a topical skin treatment remedy).
There was no apparent reaction (no visible formation of MgO) in the RT solution after a 30 minutes of treatment with air.
Interestingly, there was after only 5 minutes in the hot concentrated solution some small white specks visible.Then, the entire solution turned to
gel. Apparently, the bubbling of air through a solution is also dehydrating causing what I observed, as upon rehydrating, the salts all dissolved.
If the asserted reaction with O2 does occurs, it is my assessment that both elevated temperature and highly ionic conditions are probably required.
Prolonged reaction time at somewhat lower temperatures under the proper conditions are hard to test (dehydration effect for one and continual
reheating).
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blogfast25
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The product that would form is Mg(OH)2, not MgO, because MgO + H2O === > Mg(OH)2 is thermodynamically favourable. The poor solubility of the latter
might help explain what you describe as 'gel'. Could chlorine be smelled?
But if reacting air oxygen with an aqueous chloride would proceed at an appreciable, practical rate, don't you think we would be massively doing it to
hot brine, instead of electrolysing it?
Air oxygen is a very sluggish oxidiser at modest temperatures, largely due to that strong double bond: O<sub>2</sub> is O=O. Without it O2
would be about as reactive as F2 and thus wholly unsuitable to sustain life.
If your MgCl2 solution is concentrated (how long is a piece of string?) you're in highly ionic conditions (although I'm not sure why that should help,
apart from the concentration effect itself).
[Edited on 2-8-2014 by blogfast25]
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AJKOER
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My current opinion is, in effect, that the reaction as written:
MgCl2 (aq) + 3 O2 --> Mg(ClO3)2 (aq)
which implies a direct reaction between, say, anhydrous magnesium chloride and moist air/O2 resulting in chlorate, is missing key catalytic elements.
My take on possible paths producing chlorate in a system with O2/air and say anhydrous MgCl2 with exposure to water vapor (producing heat along with a
possible beneficial influence of solar light) in the presence of, say, transition metal impurities is based on a Fenton (or Fenton-type) source of
hydroxyl radicals. The latter radical is normally involved in the construction of intermediates leading to significant chlorate formation, as will be
detailed.
First, note that MgCl2 on absorbing water vapor also forms a slightly acidic solution. To quote a source (link: http://www.chemguide.co.uk/inorganic/period3/chlorides.html ) as this appears to be a point in the educational media with apparent diverging
reports, to quote:
"Magnesium chloride dissolves in water to give a faintly acidic solution (pH = approximately 6).
Note: This is one point when you need to know exactly what your examiners want you to say about this by looking at your syllabus, past papers and
mark schemes.
Some examiners simply say that magnesium chloride just dissolves in water. However, that wouldn't account for the slightly lowered pH. On the other
hand, there is no point in learning a complicated bit of chemistry if all you need is a simplification. Be aware that it is a simplification, though.
When magnesium ions are broken off the solid lattice and go into solution, there is enough attraction between the 2+ ions and the water molecules to
get co-ordinate (dative covalent) bonds formed between the magnesium ions and lone pairs on surrounding water molecules.
Hexaaquamagnesium ions are formed, [Mg(H2O)6]2+ :
MgCl2(s) + 6 H2O(l) → [Mg(H2O)6]2+ + 2 Cl(aq)
ions of this sort are acidic - the degree of acidity depending on how much the electrons in the water molecules are pulled towards the metal at the
centre of the ion. "
A proposed mechanism in this acidic environment to chlorate starts with a process referred to as metal autoxidation requiring the presence of
transition metal salts:
Fe(2+)/Cu+ + O2 ↔ Fe(3+)/Cu(2+) + ·O2-
where copper is a more likely active in mildly acidic environments than iron. Source: https://www.researchgate.net/publication/11374766_Generation...
Then, one possible path:
·O2- + H+ = ·HO2
·HO2 + ·HO2 → H2O2 + O2
Fe(2+)/Cu+ + H2O2 → Fe(3+)/Cu(2+) + ·OH + OH-
Fe2+ + Cu2+ ↔ Fe3+ + Cu+
where the above coupled redox reaction between iron and copper salts (and or other transition metals including, for example, manganese and even trace
amounts of cobalt) could foster the Fenton (or Fenton-type) reaction (see discussion and references at https://www.sciencemadness.org/whisper/viewthread.php?tid=53... ) along with the presence of any solar light which would likely enhance the
formation of the hydroxyl radicals via a photo-Fenton mechanism.
Next, recent research suggests a reaction between the hydrochloric acid and a hydroxyl radical (produced in the Fenton reactions cited above) that is
joined with a single water molecule resulting in the monoatomic chlorine radical:
HCl + ·OH.(H2O) → ·Cl + (H2O)2 (Source: see "The exothermic HCl + OH·(H2O) reaction: Removal of the HCl + OH barrier by a single water molecule"
by Guoliang Li, et al, J. Chem. Phys. 140, 124316 (2014); link: http://scitation.aip.org/content/aip/journal/jcp/140/12/10.1... )
Then: ·Cl + ·Cl → Cl2
And also, creating a longer lasting (but last active) intermediate than the monoatomic chlorine radical, which could act in place of ·Cl in cited
reactions below:
·Cl + Cl- → ·Cl2- (See http://pubs.acs.org/doi/abs/10.1021/jp9929768 )
Next, in the presence of heat or solar light, could overcome any energy barrier for the reactions to proceed to chlorate as follows:
·OH + Cl2 → HOCl + ·Cl
HOCl + ·Cl → HCl + ·ClO
Source: See "Pathways for the OH + Cl2 → HOCl + Cl and HOCl + Cl → HCl + ClO Reactions", by Hongyan Wang, at http://pubs.acs.org/doi/abs/10.1021/acs.jpca.5b01273
Then:
·OH + HOCl = H2O + ·ClO (See Table 3.5, p. 53 at https://www.google.com/url?sa=t&source=web&rct=j&... )
·ClO + ·ClO = Cl2O2 (See http://pubs.acs.org/doi/abs/10.1021/j100286a035?journalCode=... )
Cl2O2 + HOCl = HClO3 + Cl2
Reference for the above reaction, please see Eq 8 in "Effect of Chloride Ion on the Kinetics and Mechanism of the Reaction between Chlorite Ion and
Hypochlorous Acid" by Balazs Kormanyos, et al., 2008, at https://www.researchgate.net/publication/23141635_Effect_of_... ).
Also, this source http://isiasistemi.it/applications/desalination-process/isia... based upon field observations in a desalination process, to quote:
"One of the most undesirable byproducts in generators is the chlorate ion (ClO3-). In some generators that operate with relatively low initial
reactant concentrations, a significant amount of chlorate is formed by reactions with {Cl2O2}, as shown in reaction:
{Cl2O2} + H2O = ClO3- + Cl- + 2H+
{Cl2O2} + HOCl = ClO3- + Cl- + H+
{Cl2O2} + 3HOCl + H2O = 2ClO3- + 5H+ + 3Cl- "
I would also note that bicarbonate (as sourced here from CO2 in air) has been cited in patents as a promoter of chlorate formation likely due to pH
control and the reaction sequence:
·OH + HCO3- → H2O + ·CO3-
·CO3- + OCl- → CO3(2-) + ·ClO
forming the ClO intermediate chlorate precursor where the carbonate radical anion is more selective and has a greater extended life span in solution
than the hydroxyl radical (source: see p. 28 at https://www.google.com/url?sa=t&source=web&rct=j&... ) that created it.
----------------------
Another possible path to chlorate starting with the possible creation of chlorine from MgCl2 in the presence of again some transition metals:
Cl2 + H2O = HCl + HOCl
Fe2+/Cu+ + HOCl → Fe3+/Cu2+ + ·OH + Cl-
where both iron and copper are likely active in mildly acidic conditions, as occurs in the human body, with HOCl. Source: See "Fenton chemistry in
biology and medicine*" by Josef Prousek, to quote reaction (15) on page 2330:
"For Fe(II) and Cu(I), this situation can be generally depicted as follows [20,39],
Fe2+/Cu+ + HOX → Fe3+/Cu2+ + ·OH + X- (15)
where X = Cl, ONO, and SCN. "
Then, as above, for example:
·OH + HOCl = H2O + ·ClO
·ClO + ·ClO = Cl2O2
Cl2O2 + H2O = ClO3- + Cl- + 2H+
---------------------------------
In closing, the cited reaction:
MgCl2 (aq) + 3 O2 --> Mg(ClO3)2 (aq)
is, at best, a likely misleading over simplication of underlying processes omitting key intermediates and catalytic detail.
[Edited on 17-10-2016 by AJKOER]
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