[charlie20]

hi
Im investigating the catalysis of hydrogen peroxide decomposition by copper (II) chloirde, and have come up with an interesting phenomena that i dont quite uderstand! Can someone help?!
ok i have added 6ml of  aqueous, 80% saturated,  solution of copper (II) chloride to 60g 10% wt/wt concentration aqueous solution of hydrogen peroxide. The dark green copper (II) chloride catalyst immediately changed the clear peroxide blue, and oxygen was evolved.
But when i reduced the amount of catalyst added to 2ml, and then added this to the peroxde, the solution stayed green. Then after a minute or so the solution began to change blue, and also a strange brown substance appeared 'falling' at the top of the solution and then disappeared. Oxygen was then evolved and the solution turned totally to blue.
also i tried decreasing the amount of peroxide. Using 2ml of green aqueous copper (II) chloride added to 20g of 5% conc. peroxide the solution changed immediately to blue and oxygen was immediately evolved.
So whats going on? why does the catalyst to peroxide ratio affect the chemistry of the reaction? Why does the catalyst work and why does the solution change to blue in the first place?

Thanks for any help that you can give

charlie  

p.s. im an engineer not a chemist so keep any answers reasonably chemically simple

[charlie20]

i have been looking at the chemistry and i think the following chain mechanism may occur;
H₂O₂ <=>  H⁺ + HO₂^-
HO₂^- + Cu²⁺ -> *HO₂ + Cu⁺
*HO₂ <=> H⁺ + O₂^-
O₂^- + Cu²⁺ -> Cu⁺ + O₂
Where * denotes a free radical. So therefore copper is reduced. Does this explain the color change from green to blue? But then what happens as you increase the catalyst to peroxide ratio?
Cheers for any help


Charlie

[Albert]

Does this explain the color change from green to blue?

I'm sorry, the only thing I can tell you is that aqueous solutions of Cu²⁺ are pale blue, while those containing Cu⁺ are green.

Cu⁺ is unstable and subject to dismutation.

[charlie20]

Hi
I guess some more details of my problem might help;
- I am sure that the catalyst i inject in is copper (II) chloride - it says so on the bottle!
- the copper (II) chloride solution is blue when it is more diluted than the 80% saturated solution i used in the tests
- The resulting solution is blue, although there is also some greenish deposit at the bottom if it is left - is this dismutation [disproportionation?]
I really do think it has something to do with the equations above, i just dont know how!

Thanks

Charlie

[veljanovski]

Hi. I think i have a solution to you’re problem.

So what happens is:

Cu2+ + H2O2 -> CuO2 (dark brown – black) + 2H+
CuO2 + H2O2 -> Cu2+ + 2H2O + O2

Ok. This catalytic reaction and all reactions that are involved in peroxide decomposition are depending on concentration. The perfect conc. Is 1mmol cat. pro L.

So what happened? You have a green solution. It reacts with Peroxide at first quick it becomes blue, I think as a result of water (product of the second reaction). The Copper make a complex with water [Cu(H2O)6]2+ that is blue. That’s why the diluted Solutions are blue at the first place. When you dilute you’re cat. Than you make the same Reaction but it’s relly slow. That’s why you see the black substance that disappears after a while. I think is CuO2 this black substance.

Dragan

When you have more questions please ask me. It's an intersting subject for me too

[xiankai]

i wasnt even aware that CuO2 existed...

where can i find out more about that

[veljanovski]

I have the information from one german book. "Wasserstoffperoxyd" Autor: Willi Machu from 1951.
But I tried SciFinder and found 6000 papers for CuO2 so it is probably an alternative to finde something about it.

[hmx9123]

Copper chemistry can be very complicated.  You may want to google 'copper oxychloride' as well, to make sure that you're not perhaps forming this.