Copper Carbonate BASIC??

[xeluc]

One of my science teachers wanted me to synthesize a lot of copper compounds to show the color variety form one element and I went to synthesize Copper Carbonate via electrolysis. I looked for the msds for Copper Carbonate and for the formula it gives me CuCO3 Cu(OH)2 . So why is Hydroxide mixed in there? Is it impossible to synthesize pure CuCO3? More importantly, Did I synthesize this Copper Carbonate basic or jsut Cu(OH)2? Specs. Copper anode/cathode. 2 beakers with salt bridge NaHCO3 solution. 12 Votls... I have a bright blue precipitate.

[woelen]

Looking at your setup, I expect you made basic copper carbonate. I have a picture of this stuff on my website. Compare it with the color of your precipitate.

 

http://woelen.scheikunde.net/science/chem/compounds/copper_carbonate_basic.html

 

If it is purely blue, then you probably have copper hydroxide, but in the presence of bicarbonate I would expect the basic carbonate.

 

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Many metal ions, especially many transition metals, but also bismuth, lead and antimony, have a strong tendency to hydrolyse. The main mechanism is that the metal ion has water molecules coordinated around it and that these water molecules loose H(+) ions. For copper:

 

[Cu(H2O)4](2+) <--> [Cu(H2O)3(OH)](+) + H(+)

 

For some salts this goes even further, with a second or a third water molecule loosing H(+).

 

When such solutions are evaporated, they tend to loose acid and a basic salts remains in solution, which finally precipitates.

 

When the counter ion itself already is basic, such as carbonate, then the hydrolysis reaction is favored even more.

 

Stating it very simplistically, when carbonate and copper (II) ions are brought together, then the copper (II) ions tend to hydrolyse more strongly and then there almost only is [Cu(H2O)3(OH)](+), which with carbonate gives rise to formation of precipitates like [Cu(OH)]2CO3.xH2O on precipitation. On heating, the water is lost and the carbonate and hydroxide are retained.

 

This explanation is somewhat simple (reality is more complex), but it certainly explains the basic idea behind the formation of basic carbonates.

 

Because of the alkalinity of carbonate, most transition metals only form basic carbonates. The exceptions are FeCO3, MnCO3 and CrCO3. All other ions form basic carbonates, and some ions do not form carbonates at all, such as Fe(3+), Cr(3+). With these, hydrolysis is so strong, that in the presence of carbonate they simply precipitate as Fe(OH)3 and Cr(OH)3.

[xeluc]

No its not purely blue. Theres a light blue light precipitate and a greenish more compact precipitate.... Maybe thats the carbonate... Its in solutiopn still.. im gettign rid of the carbonate ions by decanting... if the green is carbonate than it looks like i can get a decently pure sample of carbonate since the hydroxide floats arounbd more..

[Tetrahedrite]

Basic copper carbonate is light BLUE, not green, when not well crystalline. The amorphous variety of this compound is found in nature and is known as georgeite, will the crystalline variety is the extremely common mineral malachite (which is green).

[woelen]

Basic copper carbonate is light BLUE, not green, when not well crystalline. The amorphous variety of this compound is found in nature and is known as georgeite, will the crystalline variety is the extremely common mineral malachite (which is green).

I would say, it is somewhat inbetween. The sample I have is quite pure basic copper carbonate (see link, two links above) and this is blue/green, I would call this color cyan.

 

Compare the color of basic copper carbonate with the color of copper sulfate or copper nitrate. Then you see that the latter two are really blue.