What kind of acid base reaction is cleaning a penny?

Question

1. what kind of chemical make-up is a penny made dirty by oxidation? is it acid or base?

I tried cleaning a penny with following stuff :

vinegar

baking soda

water

2. what kind of acid-base neutralization reaction is happening when these products are cleaning the penny?

Answer 1

Besides dirt, oil and assorted grime, on the surface of a penny is a small amount of copper (II) oxide, CuO.  Most coin collectors advise against trying to clean old coins.  It actually diminishes their value.

Copper (II) oxide is a basic anhydride and reacts like a base with acid.

CuO(s) + HCl(aq) --> CuCl2(aq) + H2O(l)

But, as you can see from the equation, it will form copper (II) chloride which is water soluble.  Therefore you are actually removing a small portion of copper from the penny.

The same would be true for acetic acid (vinegar), which would form copper (II) acetate in solution.