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Let's say you have a solution that is vinegar and it has salt (NaCl) dissolved in it. Then you place old dull pennies in it. Then you look at it 5 minutes later and the pennies are clean and looking like new again.

I understand acid can etch the copper compound but why is the salt necessary for this process and what exactly does it do?

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    $\begingroup$ I do not believe that the salt is necessary, but you should definitely try this at home. Copper(II) oxide is not soluble in water by itself, but Copper(I) oxide is soluble in acid. Vinegar is mostly water but a small percent of it is acetic acid. - You can find that under solubility on wikipedia. $\endgroup$ – Leonardo Dec 13 '12 at 3:52
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    $\begingroup$ The salt might increase the conductivity and facilitate a redox process. Or it could be there for the common ion effect. $\ce{CuCl2}$ is much more soluble than $\ce{CuO}$. @MDB Does the solution turn blue? $\endgroup$ – Ben Norris Dec 14 '12 at 2:19
  • $\begingroup$ I'd say the sodium chloride is just a catalyst to speed up the reaction given how dilute vinegar is. It forms CuCl2 in minute quantities which enables the neutralisation to speed up. $\endgroup$ – user2617804 Apr 24 '14 at 10:58
  • $\begingroup$ This experiment has been done literally millions of times by high school and middle school science students and the result is very consistent. Salt plus water plus vinegar is the combination which cleans the copper. THAT is an extremely strong experimental argument for why adding salt to vinegar shifts the equilibrium (dramatically). It's also common knowledge that HCl (hydrochloric acid) is stronger per mole than acetic acid. Try it yourself, it's very easy to verify. $\endgroup$ – user35135 Sep 21 '16 at 17:01
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The copper surface is oxidized, which slows down the formation of copper acetate. When you add chlorides to the solution, the chloride penetrates the oxide layer and creates a hole in that layer that allows acetate to react with the copper. To find more information on the effects of chlorides on the surface of copper look up copper corrosion and chlorides.

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  • $\begingroup$ Hi and welcome to chemistry.stackexchange.com. Feel free to take a tour of the site and check out the help center for further questions about it. $\endgroup$ – Jan Oct 7 '15 at 12:58
  • $\begingroup$ Hi Dale, I noticed that this reaction caused the release of gas. What is it's chemical composition? I assume it's not dangerous considering I've seen this experiment shown on multiple kid's science websites. $\endgroup$ – Morella Almånd Jun 21 '17 at 2:38
  • $\begingroup$ So salt only affects copper oxides? Or iron rust also? $\endgroup$ – endolith Dec 8 '20 at 20:50
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I think that what is happening, is that the salt adds sodium ions to the solution, thereby forcing the equlibrium in dissosiation of acetic acid to acetate and H+ ions further to the right.

It happens because Na gives it's electrons easier away to water than Cu does, and because acetic acid is a weak acid, normally giving only 1 of 100 H+ ions / acetate ions per 100 molecules of CH3COOH.

An exess of salt will keep the copper clean, even with no acid, and force Cu to first release the green Cu2+ from it's surface, and next for solid copper itself to oxidice, slowly.

So You can clean copper by just adding salt and water, even though the process is slow.I have tried this with copper nails. In a soultion of of water and salt, where salt remains solid at the bottom, the nails are still shiny orange, and the solution has become almost emerald green, after a couple of months.

I am not sure of the reaction mechanism here, but I have become more convinced that what I claim above really is happening.

Try doing this: Add liquid soap to two glasses, and pour in the same amount of water. Stir the solution. Add the same amount of vinegar as water and soap. Watch what happens. Then add salt (sodium chloride), but just to one of the glasses. Stir and wait for 15 minutes. Observe the very different results.

"You are the salt of the Earth ..."

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  • $\begingroup$ Salt lowers the pKa of acetic acid? Do you have a source for that knowledge? $\endgroup$ – Karl Apr 1 '16 at 20:51
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The salt is added to vinegar to speed up the process.

Addition of NaCl to vinegar shifts the equilibrium of acid dissociation, i.e. $\ce{CH3COOH <=> CH3COO- + H+}$, to right therefore concentration of $\ce{H+}$ & $\ce{CH3COO-}$ increases. Due to this, $\ce{NaCl}$ reacts with $\ce{CH3COOH}$ to give $\ce{HCl}$

$\ce{NaCl + CH3COOH <=> CH3COONa + HCl}$

Cooper Oxide (base) reacts with acids to form water and copper salts.

Acetic acid ($\ce{CH3COOH}$), which is a weak acid, will react slowly with Copper oxide as compared to $\ce{HCl}$ which is strong acid.

So when use vinegar-salt solution to clean penny, Copper Oxide parellely reacts with HCl and CH3COOH both and gives Copper salt and water.

$\ce{CuO + CH3COOH -> 2H2O + Cu(C2H3O2)2}$

$\ce{CuO + 2HCl -> H2O + CuCl2}$

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    $\begingroup$ I'm still not entirely convinced that adding NaCl will shift the equilibrium. It's definitely possible, but I don't see a good argument for why it should happen that way. $\endgroup$ – chipbuster Aug 13 '13 at 0:30
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    $\begingroup$ @chipbuster, I'd be strongly inclined to agree. Judging by pKa, HCl is around twelve orders of magnitude more acidic than HOAc, so even in a totally saturated solution the quantity of HCl generated would have to be miniscule. Unfortunately, I can only hazard a guess myself as to the reason that addition of salt accelerates the dissolution. If the solution is saturated, then the lower concentration of water itself could possibly drive the equilibrium forward by favoring dissolution and formation of water from the oxide in the process. That's entirely speculative, though. $\endgroup$ – Greg E. Aug 13 '13 at 0:51
  • $\begingroup$ There is a slight effect due to the Debye-Huckel effect. $\endgroup$ – ashu Aug 13 '13 at 0:59
  • $\begingroup$ To a first order approximation though, how strong would such an effect be? $\endgroup$ – chipbuster Aug 13 '13 at 6:50
  • $\begingroup$ still it will show enough effect because HCl is much more acidic than acetic acid. $\endgroup$ – ashu Aug 13 '13 at 8:46
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Copper(II) oxide is basic and reacts with acids to form a copper salt and water.

A strong acid, like HCl, reacts faster than a weak acid - like acetic acid (which is the main acid component of vinegar).

If one adds NaCl to vinegar, you get HCl:

$$\ce{CH3COOH +NaCl<=>CH3COONa +HCl}$$

Copper(II) chloride dissolves in the water.

So, the salt is added to speed up the process.

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    $\begingroup$ The equation above isn't correct. The sodium chloride is totally ionized, the acetic acid is only partially ionized and the amount of hydrogen ion in solution is thus relatively small. $\endgroup$ – Paul J. Gans Dec 16 '12 at 2:48
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    $\begingroup$ Thank you, @PaulJ.Gans Essentially what is required is more H+ in solution. I think that the addition of NaCl might shift the equilibrium of acetic acid dissociation towards the right is this equation: $$\ce{CH3COOH<=>H+ +CH3COO-}$$ so that you get more H+. $\endgroup$ – AJO_ Dec 16 '12 at 4:27
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    $\begingroup$ To a first approximation the addition of $\ce{NaCl}$ won't shift the equilibrium at all. There is a slight effect due to the Debye-Huckel effect, but that's mostly negligible. $\endgroup$ – Paul J. Gans Dec 17 '12 at 2:03

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