Sciencemadness Discussion Board

Dissolving copper hydroxycarbonate in ammonia solution

SnailsAttack - 24-2-2023 at 05:03

You guys know how copper hydroxide dissolves in ammonia solution to yield the copper hydroxamine (Cu(OH)₂·4NH₃) complex (water ligands omitted) (referred to as "tetraamminediaquacopper hydroxide" in the literature), like this:

Cu(OH)₂(s) + 4NH₃(aq) -> Cu(OH)₂·4NH₃(s)

What about copper hydroxycarbonate (Cu₂(OH)₂CO₃)? Dissolving it in ammonia solution doesn't release carbon dioxide, so how exactly is the carbonate complexing with the ammonia?

I have 4 theories:
Quote:
Theory 1 - When dissolved in ammonia solution, the copper hydroxycarbonate forms a direct adduct with ammonia:

Cu₂(OH)₂CO₃(s) + XNH₃(aq) -> Cu₂(OH)₂CO₃·XNH₃(aq)


Theory 2 - When dissolved in ammonia solution, the copper hydroxycarbonate splits to copper hydroxamine (Cu(OH)₂·4NH₃) and copper carbamine (CuCO₃·XNH₃):

Cu₂(OH)₂CO₃(s) + (4+X)NH₃(aq) -> Cu(OH)₂·4NH₃(aq) + CuCO₃·XNH₃(aq)


Theory 3 - When dissolved in ammonia solution, the copper hydroxycarbonate splits to copper hydroxamine and ammonium bi/carbonate:

Cu₂(OH)₂CO₃(s) + 10NH₃(aq) + 2H₂O(l) -> 2Cu(OH)₂·4NH₃(aq) + (NH₄)₂CO₃(aq)
Cu₂(OH)₂CO₃(s) + 9NH₃(aq) + 2H₂O(l) -> 2Cu(OH)₂·4NH₃(aq) + NH₄HCO₃(aq)


Theory 4 - When dissolved in ammonia solution, the copper hydroxycarbonate splits to copper hydroxamine and ammonium cuprocarbonate ((NH₄)₂Cu(CO₃)₂):

2Cu₂(OH)₂CO₃(s) + 14NH₃(aq) + 2H₂O(l) -> 3Cu(OH)₂·4NH₃(aq) + (NH₄)₂Cu(CO₃)₂(aq)

Theory 2 seems the least plausible to me because copper carbonate (CuCO₃) is unstable in the presence of water, but could perhaps be stabilized in solution as CuCO₃·XNH₃ by the ammonia ligands.

The cuprocarbonate anion (Cu(CO₃)₂²⁻) (referred to as "dicarbonatocuprate" in the literature) mentioned in theory 4 is not well known, but does exist. Its color is identical (as far as I can tell) to the copper hydroxamine complex, and can be synthesized as sodium cuprocarbonate (Na₂Cu(CO₃)₂) by dissolving copper hydroxycarbonate in sodium carbonate solution. The mineral form of this compound is called "juangodoyite" or the trihydrate, "chalconatronite". I don't know if an ammonium salt of cuprocarbonate exists, but I see no reason why it wouldn't.

Anyway, tell me what you guys think, or if you have any ideas for tests to narrow down the possible reactions. And if you have another reaction theory, keep it to yourself. No I'm just kidding haha.

[Edited on 2/24/2023 by SnailsAttack]

DraconicAcid - 24-2-2023 at 10:17

The ammonia is going to coordinate directly to the copper. The counterions simply are left dangling in solution.

CuX(s) + 4 NH3 --> [Cu(NH3)4]2+(aq) + X2-

This will be true regardless of whether X is a pair of hydroxide, a carbonate, or a mixture of the two.

Carbonate can coordinate to copper at high concentrations, but ammonia is a much stronger ligand.

SnailsAttack - 24-2-2023 at 22:46

Quote: Originally posted by DraconicAcid  
The ammonia is going to coordinate directly to the copper. The counterions simply are left dangling in solution.

CuX(s) + 4 NH3 --> [Cu(NH3)4]2+(aq) + X2-

This will be true regardless of whether X is a pair of hydroxide, a carbonate, or a mixture of the two.

Carbonate can coordinate to copper at high concentrations, but ammonia is a much stronger ligand.

You're right, I overcomplicated this. It makes more sense if I use ionic notation. I think the following formula is what actually occurs when copper hydroxycarbonate dissolves in ammonia solution:

Cu₂(OH)₂CO₃(s) + 8NH₃(aq) -> 2Cu(NH₃)₄²⁺(aq) + 2OH⁻(aq) + CO₃²⁻(aq)

And the adduct would just be Cu₂(OH)₂CO₃·8NH₃.