woelen
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Anhydrous magnesium perchlorate: is this a covalent compound?
I have some anhydrous magnesium perchlorate. This material seems to be used as a very strong drying agent. When it is added to water, then it
dissolves and some heat is produced, but not very much. It is more like CaCl2 than like P4O10 when it is added to water.
But more interesting is when it is added to acetone. I have dry acetone and in this the solid also dissolves easily. When it dissolves in acetone,
also some heat is produced, similar to when it is added to water. The solution in acetone is clear and colorless. In total I dissolved 0.5 grams of
Mg(ClO4)2 in 5 ml of acetone.
I attempted to electrolyse this little amount of liquid with 2 platinum electrodes, which are connected to a 13.8 V power supply in series with a 4.7
Ohm ceramic power resistor. When this is done, only appr. 100 uA of current is flowing through the cell and no visible things happen. When I add a few
drops of water, then immediately the current goes up to 250 mA (more than 2500 as much) and bubbles of gas are produced. When I add a few more drops
of water, then the current rises a little more to appr. 300 mA, but the rise is not as dramatic as when the first drop of water was added. After some
time, a white solid had collected at the bottom.
This makes me think that Mg(ClO4)2 dissolves as a covalent entity in acetone, but on the other hand I would be surprised to see a strongly
electropositive metal combined with a strongly electronegative entity like ClO4(-) to form a covalent compound. What would your ideas be about this?
When the liquid is shaken, the white solid which had collected at the bottom redissolves again. I think that this white solid is Mg(OH)2 and that it
redissolves again, because acid has formed at the anode, which dissolves the Mg(OH)2 when shaking.
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As a side note, I also noticed something else. After adding a few drops of water, gas was produced, but only at the anode. The cathode did not evolve
any gas. Could it be that acetone is being reduced to isopropyl alcohol?
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kmno4
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According to literature**, Mg(2+) is very weakly solvated by acetone, so it is possible that solvated ion triplet "Mg(ClO4)2" exists in acetone
solution. By the way, perchlorates of Ca, Ba, Sr are also very good soluble in acetone.
Thermal decomposition (it does not fuse) of anhydrous Mg(ClO4)2 gives MgO as residue, so it may be more covalent than, for example, KClO4 (but it is
rather obvious).
**
http://dx.doi.org/10.1021/ja01013a012
http://dx.doi.org/10.1039/F29726800821
[Edited on 9-9-2009 by kmno4]
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woelen
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If I understand it well, what you are saying is that ions stick together in the acetone solution, such that the groups of ions as a whole are neutral?
But what is the difference with simply Mg(ClO4)2 being in solution and no ions at all? If the ions stick together, then we do have Mg(ClO4)2-entities,
isn't it?
I only can read the abstract and first page of the posted links, for the rest I need login credentials, so I only could get the idea of ions sticking
together.
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kmno4
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http://www.sciencemadness.org/talk/viewthread.php?tid=11975&...
Something about ion pairs.
CuCl2 also dissolves in acetone but gives not conducting solution but try to add some water.....
What is crystalline structure of Mg(ClO4)2 - I not know, no such data are available for me.
Single molecules of ionic compound are very polar and coordinate everything around. Such molecules exist only
in gas phase.
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JohnWW
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Mg(ClO4)2 would certainly be an ionic salt, as evidenced by its exothermic dissolution in water. Perchlorate, being the resonance-stabilized anion of
a very strong oxy-acid, is one of the anions that are the very least liable to form complexes with metal cations (along with chlorate, iodate,
bromate, perbromate, periodate, permanganate, sulfate, chromate, perxenate, nitrate, as other examples), which would increase solubility in inert
organic solvents.
[Edited on 9-9-09 by JohnWW]
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DerAlte
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@Woelen
See http://www.iupac.org/publications/pac/1986/pdf/5811x1535.pdf re acetone for all one would ever want to know about this solvent (and more!)
Regards, Der Alte
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