How does the method of making a hydrated salt differ to making an anhydrous salt ?

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lhh2003
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Say I want to make MgCl2 AND MgCl2 .6 H2O and proceed to do this by splitting a sample of MgCl2(aq) formed by reacting MgCO3 and HCl in half.

Would I just heat both samples until crystals form, but heat the filtrate even further which I want to form MgCl2 so that the water of crystallisation evaporates ?

It says that heating causes the crystals to form. Why is that ?
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Arctic Kitten
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(Original post by lhh2003)
Say I want to make MgCl2 AND MgCl2 .6 H2O and proceed to do this by splitting a sample of MgCl2(aq) formed by reacting MgCO3 and HCl in half.

Would I just heat both samples until crystals form, but heat the filtrate even further which I want to form MgCl2 so that the water of crystallisation evaporates ?

It says that heating causes the crystals to form. Why is that ?
Yes, heating makes the water evaporate. If they evaporate just enough (ie, only water not bounded with the molecules evaporate), only MgCl2 .6 H2O will form.
If you heat it even further, the water inside the MgCl2 .6 H2O will start to evaporate as well, and you will get a mixture of MgCl2 .6 H2O and MgCl2. Continue heating, all water evaporates and you are left with MgCl2.
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charco
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(Original post by lhh2003)
Say I want to make MgCl2 AND MgCl2 .6 H2O and proceed to do this by splitting a sample of MgCl2(aq) formed by reacting MgCO3 and HCl in half.

Would I just heat both samples until crystals form, but heat the filtrate even further which I want to form MgCl2 so that the water of crystallisation evaporates ?

It says that heating causes the crystals to form. Why is that ?
The previous post is not correct.

Heating many hydrated salts does not give the anhydrous salt as there may be decomposition or further reaction caused by the combination of a high density cation and the water molecules at high temperature.

Magnesium 2+ ions are small and have a double charge, this gives them a relatively high charge density.

The small ion causes polarisation of the water molecules with loss of hydrogen ions and the final product contains both the chloride and the basic salt, Mg(OH)Cl.

Consequently, evaporation is not a good method of preparing the anhydrous salt. Direct reaction between chlorine and magnesium yields the desired compound.
Last edited by charco; 5 months ago
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lhh2003
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(Original post by charco)
The previous post is not correct.

Heating many hydrated salts does not give the anhydrous salt as there may be decomposition or further reaction caused by the combination of a high density cation and the water molecules at high temperature.

Magnesium 2+ ions are small and have a double charge, this gives them a relatively high charge density.

The small ion causes polarisation of the water molecules with loss of hydrogen ions and the final product contains the basic salt, Mg(OH)Cl.
Ahh , if only it was, that would have explained why slow gentle heating gives crystals but quick and intense heating gives powder..

So what is the correct method to extract hydrous and anhydrous samples in the scenario described above ?
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charco
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(Original post by lhh2003)
Ahh , if only it was, that would have explained why slow gentle heating gives crystals but quick and intense heating gives powder..

So what is the correct method to extract hydrous and anhydrous samples in the scenario described above ?
Sorry if I wasn't clear.
Evaporation at room temperature will give the hydrated salt.
Anhydrous salt should be prepared by direct reaction between the elements.
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lhh2003
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(Original post by charco)
Sorry if I wasn't clear.
Evaporation at room temperature will give the hydrated salt.
Anhydrous salt should be prepared by direct reaction between the elements.
So in other words, react the MgCO3 with HCl(aq) to make a hydrous salt but react it with HCl(g) to make an anhydrous salt ?
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charco
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(Original post by lhh2003)
So in other words, react the MgCO3 with HCl(aq) to make a hydrous salt but react it with HCl(g) to make an anhydrous salt ?
No, read my post again!
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charco
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Here is a reference:

Name:  MgCl2 Shriver and Atkins' Inorganic Chemistry.png
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lhh2003
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(Original post by charco)
No, read my post again!
I don't quite understand what you mean in this context by a "direct reaction" .
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charco
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(Original post by lhh2003)
I don't quite understand what you mean in this context by a "direct reaction" .
"Evaporation at room temperature will give the hydrated salt.
Anhydrous salt should be prepared by direct reaction between the elements"

element + element --> Compound
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lhh2003
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(Original post by charco)
"Evaporation at room temperature will give the hydrated salt.
Anhydrous salt should be prepared by direct reaction between the elements"

element + element --> Compound
So to make MgCl2 , I would react Mg + Cl2

And to make MgSO4 , what would I do here ? React Mg with S with O2 ?
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(Original post by lhh2003)
So to make MgCl2 , I would react Mg + Cl2

And to make MgSO4 , what would I do here ? React Mg with S with O2 ?
No, the sulfate is stable to heat, so you CAN prepare the anhydrous magnesium sulfate by heating the heptahydrate for several hours in an oven at 120ºC
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lhh2003
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(Original post by charco)
No, the sulfate is stable to heat, so you CAN prepare the anhydrous magnesium sulfate by heating the heptahydrate for several hours in an oven at 120ºC
I am in Year 12 so have not encountered this before, but what is heptahydrate ? And also do you know this knowledge through logic or through memory ? Like is this just recall or did you go through a logical process to figure this out ?
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(Original post by lhh2003)
I am in Year 12 so have not encountered this before, but what is heptahydrate ? And also do you know this knowledge through logic or through memory ? Like is this just recall or did you go through a logical process to figure this out ?
hepta = 7
hydrate = water of crystallisation molecules
heptahydrate = .7H2O
magnesium sulfate heptahydrate = MgSO4.7H2O

I have learned it. With inorganic compounds it's mainly a matter of learning. There is logical explanation as to why it happens, but you probably couldn't work it out beforehand.

For example, I know that cations with high charge density are difficult/impossible to prepare as anhydrous salts from solution. I know that Mg2+ has a high charge density, therefore ...

I know about the reaction with chlorides from the behaviour of lithium. So naturally I can extrapolate the behaviour to magnesium (there is a diagonal relationship in the periodic table between Li and Mg, due to the similar cationic charge density.

Learning, understanding, extrapolating.
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