So I placed a piece of copper about 5 inches long into a jar filled a third way up with distilled white vinegar. The first two weeks, nothing happened so I was told by a classmate in my Chemistry class to add table salt into the vinegar and a week later, the solution began to turn blue with the copper beginning to dissolve till it completed went away, leaving me with a turquoise blue liquid in the jar.What exactly happened here in terms of reactions?
Copper is far down the activity series and won't reduce the hydrogen in water or H+ ion acids, and in turn become oxidized to Cu^2+. This is why Cu metal won't produce hydrogen gas when treated with acids. That is, except nitric acid. But in the case of HNO3, the copper is oxidized by the nitrate ion, not H+. Initially, there was nothing which would affect the copper, or the thin layer of copper oxide on its surface. Vinegar is about 5% acetic acid, HC2H3O2 or CH3COOH, which is a weak acid, with very few H+ in solution. But when chloride ions where introduced from the dissolve sodium chloride, the CuO on the surface reacted producing [CuCl4]^2- ions and exposing fresh copper metal. You may notice that the solution initially is somewhat green. This is due to a mix of yellow [CuCl4]^2- and [Cu(H2O)6]^2+. Cu(s) + CuCl4^2- --> 2CuCl2^- .... forms ion with copper in the Cu(I) oxidation state This is called a comproportionation reaction, where an element in two different oxidation states react to form the element in an intermediate oxidation state. This is what causes the copper metal to dissolve. Cu(I) ions are oxidized by dissolved oxygen to Cu(II) ions and are hydrated, [Cu(H2O)6]^2+. This gives us the familiar blue solution typical of copper.
