Chemistry - Explanations of the thermal stability of Group 2 carbonates...?

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jacie
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Hi,
I hope someone can help me with this. I am a little stuck when it comes to explaining the thermal stability of Gr2 carbonates (as the thread title suggests obv).
I need to be able to explain the trend and suggest how the thermal stability of sodium carbonate compares with that of magnesium carbonate.
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elpaw
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(Original post by jacie)
Hi,
I hope someone can help me with this. I am a little stuck when it comes to explaining the thermal stability of Gr2 carbonates (as the thread title suggests obv).
I need to be able to explain the trend and suggest how the thermal stability of sodium carbonate compares with that of magnesium carbonate.
the bigger the ions, the less polarising they are, and so the more stable their salts are. therefore magnesium carbonate is more stable than sodium carbnate.
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jacie
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Ok cool, thats simple enough but what exactly does thermal stability refer to about the salts? is it whether they react exo or endothermically?
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elpaw
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(Original post by jacie)
Ok cool, thats simple enough but what exactly does thermal stability refer to about the salts? is it whether they react exo or endothermically?
it means how easily they decompose when heated
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jacie
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Ok great, makes sense now. Thanks
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jacie
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Oh...erm...me still thick, can u just tell me how thermal stability relates to the gr2 carbonates cos they all carry the same 2+ charge....have i got to talk bout the principle energy crap?
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elpaw
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have you done anythiong about polarisation in salts? the smaller the group 2 ion, the more it will distort the carbonate ion; so sodium, as the smallest GII ion, will distort the most. because of this distortion, the salt will be more likely to decompose, as the salts would "like" to be a more normal shape.
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instincts
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(Original post by elpaw)
have you done anythiong about polarisation in salts? the smaller the group 2 ion, the more it will distort the carbonate ion; so sodium, as the smallest GII ion, will distort the most. because of this distortion, the salt will be more likely to decompose, as the salts would "like" to be a more normal shape.

Is sodium in group 2
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elpaw
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(Original post by instincts)
Is sodium in group 2
so why is jacie asking about GII carbonates?
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LS.
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Group II carbonates become less soluble moving down the group...the pattern is generally repeated for most group II ions where the negative ion has a double charge.

The carbonates become more difficult to decompose when you go down the group; so the thermal stability of calcium carbonate is greater than that of magnesium carbonate
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jacie
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Ok i think that makes sense now.

I was asking bout Gr2 carbonates cos i need to know about them??
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elpaw
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(Original post by jacie)
Ok i think that makes sense now.

I was asking bout Gr2 carbonates cos i need to know about them??
its just that then you asked about sodium and magnesium, which are in group 1.
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hornblower
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(Original post by elpaw)
its just that then you asked about sodium and magnesium, which are in group 1.
Magnesium is in Group II.
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elpaw
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(Original post by hornblower)
Magnesium is in Group II.
im just confused now, doesh she want to know about thermal stability down a group? or accross a period??
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hornblower
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(Original post by elpaw)
im just confused now, doesh she want to know about thermal stability down a group? or accross a period??
Shouldn't it be down Group II? You were right about the polarising power of the cation. Weren't all Gp I carbonates thermodynamically stable? So, sodium carbonate is stable when heated, whereas magnesium carbonate isnt.
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jacie
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I needed to know bout the trend down Gr2 but needed to compare it with a Gr1 element.
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john !!
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Well as you go down both gropus their thermal stabilities when they are carbonates increases for the reason stated above - larger ionic radius, weaker pull of nucleus to the carbonate's electrons, less distortion and more energy required to break it apart.

The different between group I and II is that group 1 nucleuses have a lower nuclear charge (the cations in group one are 1+ and in group 2 they are 2+) so there again is less polarising power of the cation as you go towards the left of the table. That is why Group 1 carbonates are more thermally stable than group 2 carbonates.

There's something else about small group 1 carbonates, they decompose differently but I can't remember how.
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jacie
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Right, i think it is making more sense now.

Thanks everyone for your help.
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john !!
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Just remember that a highly polarising anion (positive ion) usually has a:
- high nuclear charge (2+ or 3+ like Al^3+)
- small ionic radius (like Al or Be)

and then from that work out the relative distortion, and that more distortion means a weaker bond, which means less thermal energy is needed to break that bond.

Remember when the question asks for only one group, do not mention nuclear charge because theya re all 2+.
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