# What reaction occurs when etching copper to create circuit boards?

The components used for etching copper are acetic acid, hydrogen peroxide and sodium chloride. I think the equation may be similar to this:

$$\ce{2H3C2O2H + H2O2 + 2NaCl + Cu -> CuCl2 + 2H3C2O2Na + 2H2O}$$

Is this equation correct? There are a lot of bubbles created. What is the gas that is given off?

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– Jan
Nov 11 '15 at 19:18

Well your equation balances but that isn't the right reaction.

First lets break this down into half cell reactions.

Oxidation reaction

$\ce{Cu_{(s)} <-> Cu^{2+}_{(aq)} + 2e^{-}}$

Reduction reaction

$\ce{H2O2 + 2 H^+ + 2e^{-} -> 2 H2O_{(l)}}$

Decomposition reaction

Hydrogen peroxide spontaneously decomposes in acid solution.

$\ce{2H2O2 -> 2H2O_{(l)} + O2_{(g)}}$

Stoichiometric reaction between copper and hydrogen peroxide

So the stoichiometric reaction between copper and hydrogen peroxide is:

$\ce{H2O2 + 2 H^+ + Cu_{(s)} -> Cu^{2+}_{(aq)} + 2 H2O_{(l)}}$

the $\ce{2 H^+}$ ions come from the vinegar. (Let's use $\ce{HOAc}$ for vinegar).

$\ce{H2O2 + 2 HOAc + Cu_{(s)} -> Cu^{2+}_{(aq)} + 2OAc^{-} + 2H2O_{(l)}}$

Sodium chloride's Role

Finally the role of the $\ce{NaCl}$ plays in the etch. The $\ce{NaCl}$ must be fairly high to drive the $\ce{Cu^{2+}}$ to a copper chloride complex. This thus reduces the "free" $\ce{Cu^{2+}}$ in solution which keeps the electrochemical potential of copper high so that the solution keeps dissolving copper. (Think of the half-cell copper reaction as if it is in a battery. The voltage of the half-cell would drop as the copper builds up.)

• Before I edit it away amoung another minor typo I found, is there a reason why you write $\ce{Cu+^2}$ rather than $\ce{Cu^2+}$?
– Jan
Nov 12 '15 at 9:23
• Changed it. Didn't realize that there was such a strong convention, or that anyone would care...
– MaxW
Nov 12 '15 at 16:23
• Will, I'm not aware of any conventions, but I only ever saw $\ce{Cu^2+}$ (or $\ce{Cu++}$ from a particularly old biology teacher) so I thought I'ld better ask before doing things wrong ;)
– Jan
Nov 12 '15 at 16:35
• @Jan - LOL Pauling "The Nature of the Chemical Bond" used the $\ce{Cu^{++}}$ notation so your odd biology teacher was in good company....
– MaxW
Nov 12 '15 at 16:50
• I guess "++" was a good notation until the number of pluses required got larger ;)
– SasQ
Nov 20 '19 at 19:20