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Thread: Solubility

  1. #1 Solubility 
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    Hi,

    I was wondering why all metal nitrates are water soluble whereas metal sulfides are not.

    I read that solution formation occurs in 3 steps:

    1) Expanding the solute.
    2) Expanding the solvent.
    3) Allowing solvent and solute to interract.

    Obviously the 2nd step is the same for both of them. So, what is the difference then? Is breaking a metal nitrate easier than breaking a metal sulfide? In this case, why? Or does a metal nitrate interact more with water?

    Thanks.


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  3. #2  
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    Just to see if we're on same page, and I'm about to go night night, could you give specific example of a metal nitrate and a metal sulfide?


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    Well, according to this site

    http://www.ualberta.ca/~jplambec/che/p101/p01183.htm

    Metal nitrates are soluble almost without exception.
    and

    Metal sulfides are generally insoluble in water
    So, mmm Ca(NO3)2 and CaS (to avoid Na, K and NH4 which are almost always soluble).
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  5. #4  
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    Okay, excellent.

    Well, the metal nitrates are more ionic in character, more saltlike, more polar, than the metal sulfides. This is because the NO3 species is composed of 3 pretty electronegative species, which and the fact that they're in the 2nd row makes them even more capable of ripping away electrons from whatever they're attached to because of the uniquess principle.

    A similar phenomenon is observed betwen chlorine and flourine. Chlorine actually has similar electron affinity than fluorine, but fluorine is more electronegative than chlorine because of its small size. So pretty much whatever fluorine is attached to is going to lost face time with its electron significantly. (However, it should be noted that XF is generally insoluble in water despite its polarity because of the intensity of the bond).

    When you're dealing with a MS, you're going to have a bond that is more covalent than the metal nitrates, and sulfur is very different reactively than oxygen. Notice, I'm not calling these covalent species, just more covalent in character.

    When you're in general chemistry these things are made very black and white so you can get the basic ideas, but in reality, in most cases, this simplification does not do the new model justice.

    Like dissolves like, and we're talking about water, so MNO3 is more like NaCl than MS, which is considerably less polar.

    The size of the atoms also come into account, and that these are all very general terms.

    I hope that answers your question.
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  6. #5  
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    Yes I understand thanks.

    But about metal carbonates which are also insoluble, it's the same? Cause they do look like nitrates after all (NO3 and CO3).

    mmm... after all, C and N are not so different in electronegativity (2.55 and 3.04)

    So, does this 0.5 diffference accounts for insolubility of carbonates?
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    0.5 is considered to be pretty significant on a scale with 4 as the max. And nitrogen is considered an electronegative atom while carbon is considered relatively neutral (and similar to Hydrogen).

    The 3 oxygens have a hell of a pull, but they're all happy pulling on the poor little carbon so the bonds in the carbonates are more covalent in character than an NaCl. The bond is between the carbon and the metal. Also, I think the carbonate may have resonance structures that would make it generally even less soluble.
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    Ok, one last question.

    How do you calculate the bond polarity of a metal with a radical?

    For NaCl it's easy: EN of Cl - En of Na, and the greater the ∆EN, the more polar a bond is in general.

    But with radical (e.g. NO3...etc) how can you calculate the EN?
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  9. #8  
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    There is no real way to calculate it, not one that I'm aware of anyway. I just know how species behave and interact with each other individually. There may be, but usually dealing with more than 2 species and calculating anything physical the numbers quickly become less reliable and more advanced and involved calculations end up being required.

    Qualitatively, the key is the atom of the polyatomic ion that is attached to the metal and the nature of the other species involved. The key is polarity, the more polar the bond, the more likely it will dissolve in water. Other than that, an actual explaination on what to look before has too much information involved for me to go into here. You'll get to it in advanced chemistry courses.
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  10. #9  
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    I think there are a few other considerations that need to be mentioned here, though I do not know what level of chemistry people are exposed to here, so i will just mention them breifly and if you want more explination, then just ask.

    1) I think considerations of a hard/soft-acid/base nature can be made here as well. That is most metals are fairly soft and so is sulfur, while nitrogen is a fairly hard atom. SO one would exect the metal-sulfur bond to be less labile than the metal-nitrogen or metal-carbon bond

    2) I also think that pi-backbonding plays a significan role from some of these salts. In particular, carbonate may be of the correct geometry to undergo pi backbonding and this would explain why metal-carbon is so stable, when it is not really expected to be so.

    I think that is all I have to add to this.

    Toodles
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