Simple question

[abskebabs]

This is only my 2nd thread in the chemistry section, and I have a simple question for the bright chemists out there(you know who you are;) ). Why do transition metals not form carbonates. I know it is roughly to do witht heir charge density and polarising power but I know there is a much more concise answer to this question.

Thanks in advance:-)

[woelen]

What you are saying is only true for some transition metals. There are some carbonates of transition metals, e.g. FeCO3, Ag2CO3. Most transition metals, however form so-called basic carbonates, such as Cu(OH)2.CuCO3, Ni(OH)2.NiCO3. These are mixed hydroxides/carbonates. Only a few do not form carbonates at all, e.g. Cr(III) and Fe(III).

 

This all has to do with the acid properties of the cations. The higher the oxidation state of a metal, the more acidic its ion is. E.g. CrO is purely basic, Cr2O3 is amphoteric (can exhibit some acidic properties as well), and CrO3 is purely acidic and forms H2CrO4 (and H2Cr2O7) in water.

 

Cations, which exhibit acidic properties, hence cannot form carbonates from their aqueous solutions. E.g. when a solution of CrCl3 and Na2CO3 are mixed, then CO2 bubbles out of solution and Cr(OH)3 is formed. The same is true for iron (III). For copper (II) and nickel (II) this effect is less pronounced, but still, some carbonate is converted to bicarbonate or CO2 and a mixed hydroxide/carbonate is precipitated.

 

All this does not say that carbonates of metal-ions like Fe(III) or Cr(III) cannot exist. It only says that these carbonates cannot be prepared from aqueous solution. IF a synthetic route of preparation of these carbonates could be found, then they might exist. But, I do not know of any such preparative route.

 

For similar reasons, no chromium sulfide and aluminium sulfide can be prepared. When you bring together a solution of sodium sulfide and chromium chloride, then Cr(OH)3 is precipitated, and H2S escapes from the solution. However, Cr2S3 can be made from the finely powdered elements. So, although Cr2S3 cannot be prepared from aqueous solutions, it can be made by means of other preparative routes and this compound is perfectly stable.

[Gilded]

They don't? At least there's iron and nickel carbonates, did you have any more particular metals in mind? Quite a few transition metals (in the loosely defined sense) also exist in oxidation states suitable for carbonate formation.

 

Edit: Damn, have to learn to write faster, woelen beat me to it.

Edit2: "However, Cr2S3 can be made from the finely powdered elements." And so can aluminum sulfide, actually, I've tried it myself. Quite a violent reaction too.

[abskebabs]

My, my it's been a while since I posted but I can't believe how wrong I was. I think it would have been more correct to say that generally metals with a higher charge density(like Al3+, Fe3+, etc) tend not to form carbonates in aqueos solutions. Is that a correct statement, or am I wrong again:-( ?