Why do diluted sulfuric acid and hydrogen peroxide etch copper only when chloride ions are present?
I've mixed 30ml 10% sulfuric acid with 20ml 3% hydrogen peroxide and put a drop of this solution onto copper foil. The solution quickly removed the oxide layer from the copper but did not dissolve it any further within the hour that I left the drop on the copper. When I wiped the solution off with a white paper towel, I couldn't see any blue or green tint, indicating that there was (almost) no copper dissolved in the solution.
After that, I added (roughly) 100mg of sodium chloride to the solution and again put a drop of it onto the copper foil. (To clarify, I put 100mg sodium chloride into the entire remaining ~50ml of solution.) It began to oxidize and dissolve the copper, turning its surface brown. When I later wiped the solution off, the paper towel was stained greenish-blue.
A dilute mixture of hydrochloric acid and hydrogen peroxide also readily dissolves copper.
I've been unable to find any explanation for this even after an hour of googling. I've found this chemistry stack exchange answer that deals with a similar reaction; however, it doesn't explain the role of the chloride ions.
Why does this solution only dissolve copper when chloride ions are present? (That is, how does the chloride help oxidizing the copper and why isn't hydrogen peroxide enough?)
I've got three theories but I have no idea which of them (if any) is correct. 938
(I'm sadly not a chemist.)
1. The hydrogen peroxide oxidizes chloride to $\text{Cl}_2$, which then attacks the copper. The problem with this theory is that hydrogen peroxide has $E^0=1.776V$ in acidic solutions while chlorine only has $E^0=1.396V$, meaning that the peroxide should attack the copper more readily than the chlorine, yet it evidently doesn't.
2. The $\text{Cl}_2$ produced as in theory #1 is further oxidized to form $\text{HClO}$ (or even further to $\text{HClO}_n$), which then oxidizes the copper. This sounds like the most likely reaction to me.
3. The chloride ions somehow catalyze the decomposition of hydrogen peroxide into radicals, which then attack the copper. 662