I'm doing an online chemistry course and we were directed to do a lab involving cleaning pennies. Basically it involves placing dull pennies in a solution of vinegar and salt to clean the pennies.

 

I know the pennies become dull due to the copper becoming oxidized from reacting with the acetic acid. I've done a bit of searching online as to why we add salt to the solution. I've seen comments that the chlorine in the salt mixed with the hydrogen in the acetic acid to make some HCl, however I've seen many comments saying this is untrue, but I've seen no alternatives as to the function of the salt.

 

My only guess is that the salt speeds up the reaction because of its ability to be fairly corrosive?

Some HCl will be formed by adding salt to vinegar, but in very small amounts (an equilibrium),

I think a mix of salt and vinegar cleans pennies due to the complexing ability of Cl-. Usually, the reaction between vinegar and copper oxide/sulfide is very slow due to the weak acidity of the vinegar. The vinegar dissolves the layer of oxides, forming [Cu(H2O)6]2+ and CH3COO-. What the chloride does, however, is allowing the formation of CuCl4(2-) within the solution. Thus, the equation can be written as: CuO(s)+2H+(aq)+4Cl-(aq)---->CuCl4(2-)+H2O. The chloride ions can also penetrate the oxide layer easily, as it penetrates the layer of Al2O3 that usually surrounds aluminium metal, causing it to react with water.