Reaction between Copper, Hydrogen Peroxide, and Vinegar

[Ben-P]

Hey there PF,

I'm trying to determine the reaction equation for a little home experiment I've been conducting. The setup is very minimal: copper metal in a solution of 3% H₂O₂ and 5% CH₃COOH. In reality, I'm probably missing something fairly straightforward.

The reaction occurs very slowly, but within a few minutes it becomes very clear that this is a redox reaction, seeing as solid copper is dissolving into solution. As such, the overall reaction can generally be determined by examining the half-reactions.

The first of these half-reactions is very obvious:
Cu_(s) → Cu²⁺_(aq) + 2e^-
(It forms the cupric ion because the product is copper acetate, Cu(CH₃COO)₂).

The second reaction, however, is slightly more annoying because H₂O₂ can form a few different combinations of products. Seeing as it is the only available oxidizing agent, I assumed the equation was something like this:
H₂O_2(aq) + 2e^- + 2H⁺_(aq) → 2H₂O_(l)
(Hydrogen ions are donated by CH₃COOH, and as they are consumed, copper (II) ions are formed, thus the net charge of the solution remains 0)

Thus, by adding these two equations together, I assumed the overall equation to be
Cu + H₂O₂ + 2CH₃COOH → Cu(CH₃COO)₂ + 2H₂O.

All seemed fine, until I looked at the solution again, and realized that gas was being evolved (oxygen, not hydrogen, as determined by re-ignition of a splint). My first guess was perhaps that the freshly-etched copper was acting as a catalyst for the decomposition of hydrogen peroxide. However, after removing a piece from the solution, rinsing it quickly, and placing it in a solution of just hydrogen peroxide, I wasn't able to notice any gas being evolved.

Does anyone know the actual equation, or have suggestions for possible side reactions occurring?

 

[NascentOxygen]

Hi Ben-P,http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif


Does a mix of vinegar and peroxide, without the copper, liberate any gas?

Does the gas appear to be forming on the copper, or coming out of solution everywhere?

 

[Ben-P]

Thanks for your response, NascentOxygen!

Does a mix of vinegar and peroxide, without the copper, liberate any gas?

No gas is evolved by the mixture of these two solutions.

Does the gas appear to be forming on the copper, or coming out of solution everywhere?

This is difficult to distinguish; during the entire reaction (as the solution goes from clear to deep blue), gas appears to form on or near the metallic copper. However, after several hours, gas is no longer being evolved on or near the copper, but gas continues to be slowly evolved by the solution.

After some quick researching (that I didn't have time for when first posting), it is clear that the cupric ion is actually known to be a catalyst for the decomposition of hydrogen peroxide, thus explaining the continual evolution of gas by the solution (probably on impurities, such as dust, acting as nucleation sites). In addition to this, during the reaction, the copper metal probably also acts as an excellent nucleation site for gas to form, thus making it appear as though the metal itself is the catalyst.

An interesting question still remains, however, and I haven't been quite able to find an available (open-access) paper discussing the subject: how is it that the cupric ion catalyzes the decomposition of hydrogen peroxide? Is the mechanism understood?

 

[NascentOxygen]

the cupric ion is actually known to be a catalyst for the decomposition of hydrogen peroxide,

Then you are way ahead of me! Glad you got this sorted.

Good luck with your experimental studies.

 

[johnthedream]

So I actually carried out this experiment myself..I'm not much of a major at chemistry (im in high school)but I did discovered that when the copper was submerged in the solution of vinegar and hydrogen peroxide at room temperature.. There seemed to be no reaction going o
n,so I
heated the the mixture.After heating there seemed to be some effervescence and a gas coming off which I guess was oxygen.Where did it
from?I guess it came from the decomposition of hydrogen peroxide
Cu+H202------>heat =O2+H20
Then the copper would react with the vinegar giving you the acetate,all of this may be summarized by one equation
Cu+H202+CH3COO------>heat =Cu(CH3COO)2+H20+O2

 

[NascentOxygen]

The sequence may be that Cu metal slowly reacts with the acid to form the cupric ion. This ion being a catalyst for the decomposition of the peroxide then starts to liberate the gas.

You could try separately warming Cu with the vinegar to form some cupric ions, then decant some of this solution into a test tube of peroxide to confirm this.