[FishkaFishka]

Hello,

I have a chemistry assignment where I have to choose a chemical reaction and perform it in class. I chose to add copper pennies to ammonia, turning it light blue. I also have to provide the chemical equation for what is happening.


I have thought about it a long time on my own, and with the help of information available on the internet have come up with this:

2Cu + 8NH₃ + 2Н₂О + О₂ → 2[Cu(NH₃)₄] (OH)₂

Is this correct? And if not, what is the correct equation and why?

And, what exactly happens to the copper from the pennies when it reacts with the ammonia?

Thanks in advance.

[Corribus]

Think about the formal charges on each species in both the reactant and product side of your reaction equation. This should give you a clue as to what is happening.

(As another hint - you are not actually using pure ammonia here.)

[AWK]

Pure metallic copper reacts very slowly with oxygen in the air and the destruction of circulating coins is prohibited in many countries.
But you can do the experiment using a copper plate or a few pieces of wire. During school experiments, the time is usually limited to a maximum of several minutes and the experiment may fail without additional tricks.
In addition, you have to use fairly concentrated ammonia and heat the solution, and the ammonia smells quite unpleasant, i.e. the experiment must be performed in a fume hood or in the open air. Your reaction is formally accepted but is actually dissolving in ammonia a thin layer of basic copper carbonate Cu₂CO₃(OH)₂ on the surface of the coins or copper oxide (as in the proposed experiment). The acceleration of the reaction consists of creating a thicker layer of copper oxide on the copper plate or wire by enclosing in the oxidizing part of the burner flame (until obtaining a black surface, you have to hold the heated copper in a pair of tongs because copper conducts heat very well) and dissolving the thus obtained copper oxide in ammonia (concentrated ammonia diluted by volume twice, i.e. 1: 1). Coins can be used to demonstrate and discuss monetary copper alloys (in a few sentences).
Cu₂CO₃(OH)₂ + 8NH₃ = [Cu(NH₃)₄](OH)₂ + [Cu(NH₃)₄]CO₃
and in the case of accelerated experiment
CuO + 4NH₃ + H₂O = [Cu(NH₃)₄](OH)₂
You use ammonia water solution, usually written NH₃(aq), I advise you to mention it when discussing the reactions, but for the sake of simplicity, omit the reaction equations.

[FishkaFishka]

Thank you both for your replies.

AWK, I don’t mean to dissolve the coins completely, just to demonstrate how they turn the ammonia blue, which I tried out and it worked quite fast. The teacher doesn’t want anything to complicated or involved, as it’s really just a basic class, but she does want the chemical equation.

So with the above said, this equation

Cu2CO3(OH)2 + 8NH3 = [Cu(NH3)4](OH)2 + [Cu(NH3)4]CO3

Is still the one I should use?

[AWK]

This is a real equation for coins.

[FishkaFishka]

Okay. Thank you so much.