Sciencemadness Discussion Board

Aqueous Copper(II) Nitrate and Magnesium Ribbon

Volanschemia - 23-1-2015 at 20:19

Hi everyone.
I just did the famous silver tree reaction (adding copper metal to a solution of silver(I) nitrate which forms silver metal and copper(II) nitrate). I filtered off the copper(II) nitrate solution and kept the silver crystals and just as I was about to discard the solution I thought, what if I put a more reactive metal than copper in? Would it make a copper tree? I had some magnesium ribbon laying around so chucked a few bits in.
Over the course of about 5 mins the solution started to turn GREEN (not clear as I had expected) and bubbles started to evolve. Also the magnesium ribbon began to get coated with a black substance which I am guessing is the copper precipitate, although it is more black than red (possibly been oxidized by magnesium nitrate to copper(II) oxide???).
Just wondering why the solution has turned green and why it is forming bubbles.
It is still reacting at the moment so it might turn clear eventually. I will post back here with an update and some pictures soon.
Thanks!

DraconicAcid - 23-1-2015 at 21:13

A solution of copper ions is acidic, so you will get the reaction:
Cu2+ + Mg + 2 H2O -> Mg2+ + H2 + Cu(OH)2

The black is probably finely divided copper, though.

Volanschemia - 23-1-2015 at 21:53

Quote: Originally posted by DraconicAcid  
A solution of copper ions is acidic, so you will get the reaction:
Cu2+ + Mg + 2 H2O -> Mg2+ + H2 + Cu(OH)2

The black is probably finely divided copper, though.


Thanks for your answer.

So there is two reactions happening?
The one above and Mg + Cu(NO3)2 = Mg(NO3)2 + Cu

So what is causing the green colour? As far as I know Mn2+ is pink.

diddi - 23-1-2015 at 22:42

green is most likely due to a complex involving Cu2+
Cu metal is black when precipitated, so it is not CuO
Because Mg is quite reactive (E° =-2.38) , it is capable of displacing H+ from H2O
so there are 2 competing reactions, and the bulk of the Mg will be consumed by the H2O I would expect

Volanschemia - 23-1-2015 at 23:00

Thanks for your reply diddi.
I completely forgot about Mg's reactivity with water.
What would the Cu2+ be complexing with?
It seems there is actually three reactions happening, because there is a blue precipitate forming on the bottom of the beaker.
I'm guessing it is the reaction DraconicAcid mentioned.
There's a lot going on!
I will wait for the reaction to cease then see what the products are.

[Edited on 24-1-2015 by TheAustralianScientist]

DraconicAcid - 24-1-2015 at 01:05

Quote: Originally posted by TheAustralianScientist  


So what is causing the green colour? As far as I know Mn2+ is pink.


But there is no manganese in solution- magnesium is colourless. The green colour would be some copper compound.

Volanschemia - 24-1-2015 at 01:20

Quote: Originally posted by DraconicAcid  
Quote: Originally posted by TheAustralianScientist  


So what is causing the green colour? As far as I know Mn2+ is pink.


But there is no manganese in solution- magnesium is colourless. The green colour would be some copper compound.


Sorry about that! :o
I'm sure it's not the first time someone confused Manganese and Magnesium.
Any idea what copper compound might be. As far as I know the only ions the copper could react with are NO3- or OH-, both of which form blue compounds with copper.
The only green copper compound I know of is CuCl2 and there was no Cl- in there.

diddi - 24-1-2015 at 01:29

the attachment of various ligands will effect the colour you normally expect, by altering the energy levels in the d-orbital split. possible ligands here are OH-, H2O, NO3-
Cu2+ is typically 4-coordinate and most structures are approximately square planer. so there could be some strange complex happening. cant say for sure.
and dont forget there are still Ag+ ions in there


[Edited on 24-1-2015 by diddi]

Dan Vizine - 25-1-2015 at 18:38

Just curious, what other metals may be in your reagents? Was the copper chemically pure or from a source like wires or pipes?

Many chemists have famously been misled by assumptions based on the colors they observed which were actually from unexpected or undetected minor impurities.

The black is the copper, no doubt. The bubbles are hydrogen, your faith in the electromotive series is reinforced. But, I suspect a metallic impurity. Did you use tap water?

No denying diddi's point. Even though Jahn-Teller distortion is often though of as being about Oh molecules, it is important to copper, also. Those energy level shifts can be pretty fluid.



So, it's hard to say for sure if it's one thing or the other or both.

[Edited on 26-1-2015 by Dan Vizine]

Volanschemia - 25-1-2015 at 19:37

I did use copper wire from electrical cables so there could be some metal impurites. There could also have been some CuO in there from oxidation of the copper wire by AgNO3.
There is a light blue precipitate on the bottom of the beaker which I'm guessing is Copper(II) Hydroxide (will test later by heating to 60 degrees C), so DraconicAcid's reaction happened as well.
Quote: Originally posted by Dan Vizine  
Just curious, what other metals may be in your reagents? Was the copper chemically pure or from a source like wires or pipes?

Many chemists have famously been misled by assumptions based on the colors they observed which were actually from unexpected or undetected minor impurities.

The black is the copper, no doubt. The bubbles are hydrogen, your faith in the electromotive series is reinforced. But, I suspect a metallic impurity. Did you use tap water?

No denying diddi's point. Even though Jahn-Teller distortion is often though of as being about Oh molecules, it is important to copper, also. Those energy level shifts can be pretty fluid.



So, it's hard to say for sure if it's one thing or the other or both.

[Edited on 26-1-2015 by Dan Vizine]

Volanschemia - 25-1-2015 at 22:10

Here are some pictures.
The first two show the green colour during the reaction.
The second two show the reaction is finished and the solution is now colourless and there is a blue precipitate on the bottom of the beaker as well as some copper powder. (Am I right in thinking that the disappearance of the green suggests a complex?)
The last one shows the products on a filter paper. I will test the blue precipitate when it dries to make sure it is Copper(II) Hydroxide, but I don't know of any other insoluble blue Copper(II) compounds.

About 15 mins after addition of Mg (2).JPG - 149kB12 hours later.JPG - 91kBCopper on the bottom.JPG - 100kBProducts on Filter Paper.JPG - 175kBAbout 15 mins after addition of Mg.JPG - 114kBAbout 3 mins after addition of Mg.JPG - 128kB

diddi - 25-1-2015 at 22:34

looking here:
http://www.docbrown.info/page07/transition09Cu.htm

I see that a Cu(Cl-)4 complex is yellow/brown. in blue copper sln, this would appear green
so if there is any salt contamination from tap water etc, that could explain the colour.

you have mixed products in four filter paper, btw. will need recrystallization possible in acidic environment to separate them?

Volanschemia - 25-1-2015 at 22:43

I used distilled water in the reaction so I don't see how any Cl- ions could have gotten in there. Products on the filter paper would include:
Copper(II) Hydroxide
Copper(II) Nitrate (trace)
Silver(II) Nitrate (trace)
Magnesium(II) Nitrate
Magnesium Metal
Copper Metal

What do you mean by an acidic recrystallization?
Quote: Originally posted by diddi  
looking here:
http://www.docbrown.info/page07/transition09Cu.htm

I see that a Cu(Cl-)4 complex is yellow/brown. in blue copper sln, this would appear green
so if there is any salt contamination from tap water etc, that could explain the colour.

you have mixed products in four filter paper, btw. will need recrystallization possible in acidic environment to separate them?

diddi - 25-1-2015 at 23:25

recrystallization from acids is often a useful way of altering the solubility of compounds, so it may be that you can solvate one solid and leave others behind.

Volanschemia - 25-1-2015 at 23:32

Quote: Originally posted by diddi  
recrystallization from acids is often a useful way of altering the solubility of compounds, so it may be that you can solvate one solid and leave others behind.


So how would I go about doing that?
Would the Copper(II) Hydroxide dissolve in acid?
What acid would I use?

diddi - 25-1-2015 at 23:41

readily: OH- + H+ = H2O
not sure about getting it back again without complexes tho. have to think about that. try looking up solubilities

Volanschemia - 26-1-2015 at 00:00

The problem with acid recrystallization is than if I add everything to an acid, any remaining NO3- ions will react to form Nitric Acid which will dissolve the copper! So everything will dissolve which isn't the outcome we're after.

2HCl(aq) + Mg(NO3)2(aq) = MgCl2(aq) + 2HNO3(aq)

HCl + Cu(OH)2 = CuCl2 + H2O