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Eddygp
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[*] posted on 19-5-2012 at 08:29
Unknown reaction


I had thought about reacting sodium bicarbonate and magnesium sulfate in aqueous solution. I had expected it to form sodium sulfate and magnesium bicarbonate or some sort of carbonate and bisulfate. However, I was surprised when I saw some CO2 bubbles forming in the solution. What did I make?
Thanks in advance.




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[*] posted on 19-5-2012 at 08:40


Magnesium bicarbonate forms but is unstable, easily decomposing to magnesium carbonate and carbon dioxide.



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plante1999
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[*] posted on 19-5-2012 at 08:41


Quote: Originally posted by LanthanumK  
Magnesium bicarbonate forms but is unstable, easily decomposing to magnesium carbonate and carbon dioxide.

You forgot the water!

2NaHCO3 = MgSO4 -) MgCO3 + Na2SO4 + CO2 + H20




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Eddygp
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[*] posted on 19-5-2012 at 08:50


But... is Na2SO4 hence more stable than NaHCO3?



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[*] posted on 19-5-2012 at 08:56


but will sodium sulfate really be formed as well?the same hard rock stuff left over after nitric acid synthesis?just like that?
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[*] posted on 19-5-2012 at 10:47


Quote: Originally posted by plante1999  
Quote: Originally posted by LanthanumK  
Magnesium bicarbonate forms but is unstable, easily decomposing to magnesium carbonate and carbon dioxide.

You forgot the water!

2NaHCO3 = MgSO4 -) MgCO3 + Na2SO4 + CO2 + H2O


That is the correct reaction. I use this all the time to make magnesium carbonate as a precursor to all the other magnesium salts you could ever want.

Realize that the sodium sulfate will be in the aqueous solution since sodium salts are incredibly soluble. You could filter off the Magnesium carb and then crystallize the mother liquor left over for sodium sulfate if you wanted.




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[*] posted on 19-5-2012 at 11:32


Umm... I do not have any precipitate :( It's all dissolved...
EDIT: Sorry, it may be MgCO3·5H2O, which is soluble

[Edited on 19-5-2012 by Eddygp]




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[*] posted on 19-5-2012 at 11:34


Quote: Originally posted by cyanureeves  
but will sodium sulfate really be formed as well?the same hard rock stuff left over after nitric acid synthesis?just like that?

No, it is impossible for the sodium sulfate to form in such a reaction. This is aqueous precipitation chemistry and thus the reaction equation that plante1999 wrote is correspondingly wrong.

The reason why no sodium sulfate can form is because this salt dissociates in water. Furthermore, it can not be directly crystallized from aqueous solutions unless heating to high temperatures to drive off the water from its hydrates. Therefore, the only sodium compound that could form in this reaction, but only if the solutions are concentrated enough to reach saturation, is the sodium sulfate decahydrate. This only has moderate solubility at ambient temperatures and can easily form large crystals. Binary sulfates of the two metals also exist, but should not form under proper reaction stoichiometry.




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[*] posted on 19-5-2012 at 16:05


Quote: Originally posted by Nicodem  
Quote: Originally posted by cyanureeves  
but will sodium sulfate really be formed as well?the same hard rock stuff left over after nitric acid synthesis?just like that?

No, it is impossible for the sodium sulfate to form in such a reaction. This is aqueous precipitation chemistry and thus the reaction equation that plante1999 wrote is correspondingly wrong.

The reason why no sodium sulfate can form is because this salt dissociates in water. Furthermore, it can not be directly crystallized from aqueous solutions unless heating to high temperatures to drive off the water from its hydrates. Therefore, the only sodium compound that could form in this reaction, but only if the solutions are concentrated enough to reach saturation, is the sodium sulfate decahydrate. This only has moderate solubility at ambient temperatures and can easily form large crystals. Binary sulfates of the two metals also exist, but should not form under proper reaction stoichiometry.



I did not bother to wright the hydratation of each reactant....

When I do a synthesis I calculate it since It could make all the reaction go wrong if stoichiometry is not respected.




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[*] posted on 19-5-2012 at 16:35


Since the reaction is in aqueous solution, just combining solutions of NaHCO3 and MgSO4 will result in a mix of Mg2+ ions, HCO3- ions, SO4- ions, and Na+ ions. I tried to find data on the solubility of Mg(HCO3)2 but I can't (all I got was some scam cancer treatment). Mg(HCO3)2 is unstable, but the HCO3- ion itself is fairly stable, and won't decompose in aqueous solution at normal pH. So just combining dilute aqueous solutions of MgSO4 and NaHCO3 will just result in a pool of ions. Combining concentrated/saturated solutions, however, might cause some Mg(HCO3)2 to fall out of solution if its solubility is lower than sodium bicarbonate's, these might decompose to MgCO3, resulting in the bubbles you see. If Mg(HCO3)2's solubility is higher than NaHCO3, however, limited or no reactions occur unless you pour water directly on to solid MgSO4 and NaHCO3.

Magnesium carbonate pentahydrate, although slightly soluble, is still relatively insoluble (http://en.wikipedia.org/wiki/Magnesium_carbonate), so precipitation will still happen, maybe just too little to see compared to the CO2 gases with a much larger volume.
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[*] posted on 19-5-2012 at 16:56


Quote: Originally posted by sargent1015  
Quote: Originally posted by plante1999  
Quote: Originally posted by LanthanumK  
Magnesium bicarbonate forms but is unstable, easily decomposing to magnesium carbonate and carbon dioxide.

You forgot the water!

2NaHCO3 = MgSO4 -) MgCO3 + Na2SO4 + CO2 + H2O


That is the correct reaction.


That is not the correct reaction. The correct equation would be two steps:

(2)NaHCO3 + MgSO4 --> Mg(HCO3)2 + Na2SO4

Then

Mg(HCO3)2 --> MgCO3 + H2O + CO2


Nicodem was talking about the solubility of sodium sulfate in a hydrated form. I thought that when a salt with waters of crystalization goes into solution it gives the water to the solution and becomes ions. Is he confused or am I?:o




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[*] posted on 20-5-2012 at 07:04


Quote: Originally posted by 99chemicals  

(2)NaCO3 + MgSO4 --> Mg(HCO3)2 + Na2SO4

Then

Mg(HCO3)2 --> MgCO3 + H2O + CO2


Nicodem was talking about the solubility of sodium sulfate in a hydrated form. I thought that when a salt with waters of crystalization goes into solution it gives the water to the solution and becomes ions. Is he confused or am I?:o



My bad, I was thinking about the slightly different reaction:

MgSO4 + Na2CO3 --> MgCO3 + Na2SO4
(which is however not quite right, see link below)

The sodium carbonate can be generated in situ by heating up the sodium bicarb reaction with magnesium sulphate. THIS is what I did to make my magnesium carbonate.

Alternatively, you could just make sodium carbonate ahead of time and use it.

Check out this link for the reaction/procedure
http://www.nuffieldfoundation.org/practical-chemistry/making...




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Eddygp
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[*] posted on 22-5-2012 at 08:56


I will probably crystallize it first and then... try to see what it is.



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