Sciencemadness Discussion Board

Reduction of Copper(II) oxide with Monoammonium phosphate

Eddygp - 9-6-2012 at 04:04

I didn't really expect this reaction to happen, but I heated some crystals of monoammonium phosphate on a cupric oxide "wool", and when the salt melted, the cupric oxide turned to copper and some gases were produced. What was produced? Why did this happen?

CuO + NH4H2PO4 = Cu + H2O + (N2?) + (?)

weiming1998 - 9-6-2012 at 05:07

Quote: Originally posted by Eddygp  
I didn't really expect this reaction to happen, but I heated some crystals of monoammonium phosphate on a cupric oxide "wool", and when the salt melted, the cupric oxide turned to copper and some gases were produced. What was produced? Why did this happen?

CuO + NH4H2PO4 = Cu + H2O + (N2?) + (?)


1, Are you sure that Cu, instead of Cu2O is formed? Cu2O seems far more likely for a comparatively weak reducing agent like NH4+, and Cu2O has the same colour. To test, try and dissolve your red "copper" in concentrated NH3(aq) or HCl(aq). http://en.wikipedia.org/wiki/Copper(I)_oxide Taken from Wikipedia, Cu2O dissolves in both to form soluble complexes, while elemental copper won't react with either of them in a short time unless oxidants are added.

As for the equation, you are right about N2 forming. Ammonium salts paired with an oxidizing anion does decompose into nitrogen compounds in higher oxidation states. If the temperature is high, reducing agents are present, or if your oxidizing anion is weak, N2 is formed. Example: Ammonium sulfate decomposes into SO2, N2, H2O and some ammonia is also formed. http://en.wikipedia.org/wiki/Ammonium_sulfate

If a stronger oxidizing anion is utilized, lower temperatures, or if no reducing agents are present, NOx can form.
Example: NH4NO3 decomposes between 180 and 240 degrees Celsius non-explosively to form mostly N2O, with small amounts of NO, NO2, etc present.

CuO is a weak oxidant (at least compared to NO3-), so we can deduct that mostly N2 is formed. So, taking ideas from the decomposition of ammonium salts, we can hypothesize that the two idealized equations (depends on if Cu or Cu2O is formed) are:
1, 3CuO+2NH4H2PO4===>3Cu+3H2O+N2+2H3PO4
2, 6CuO+2NH4H2PO4===>3Cu2O+3H2O+N2+2H3PO4
In reality, a mix of compounds (such as NH3, (HPO3)n, H2P4O7, NOx, etc, can be formed also.

Eddygp - 9-6-2012 at 06:01

First one you mentioned is most probable: I have samples of cupric and cuprous oxides and the colour is definitely different from both. The copper I have obtained has a coppery-golden colour.

weiming1998 - 9-6-2012 at 06:41

Quote: Originally posted by Eddygp  
First one you mentioned is most probable: I have samples of cupric and cuprous oxides and the colour is definitely different from both. The copper I have obtained has a coppery-golden colour.


Not necessarily. Cu2O can have a range of different colours from orange to brown to light-yellow, so colour can't specifically tell you the compound. I suggest you try the NH3 or HCl test I told you earlier.

Eddygp - 9-6-2012 at 06:45

OK. Is household ammonia too dilute for that?

[Edited on 9-6-2012 by Eddygp]

blogfast25 - 9-6-2012 at 07:10

If copper was indeed formed, try and reacting it with nitric acid. Even 35 % yields quite a vigorous reaction and deep blue copper (II) nitrate.

Eddygp - 9-6-2012 at 07:22

It did not dissolve in ammonia. @blogfast, I do not have nitric acid at the moment... :(

blogfast25 - 9-6-2012 at 10:23

Try HCl + H2O2?

AJKOER - 9-6-2012 at 15:25

I believe the reaction is better described as a two step process. First, Wiki notes that the decomposition of Ammonium dihydrogen phosphate proceeds as follows, to quote:

"Solid monoammonium phosphate show a dissociation pressure of ammonia of 0.05 mmHg at 125°C based on the decomposition reaction as follows:[2]

NH4H2PO4(s) <==> NH3(g) + H3PO4(l) "

Link: http://en.wikipedia.org/wiki/Ammonium_dihydrogen_phosphate

Second, the know reaction of NH3 and CuO:

2 NH3 (g) + 3 CuO (s) ---> N2(g) + 3 Cu(s) + 3 H2O(g)

See for example "Chemical Principles", by Steven S. Zumdahl, Donald J. DeCoste, page 79.

Link: http://books.google.com/books?id=hsuV9JTGaP8C&pg=PA79&am...

So the net reaction is:

2 NH4H2PO4(s) + 3 CuO ---> 2 H3PO4(l) + N2(g) + 3 Cu(s) + 3 H2O(g)

as was previously suggested.