Re: Effect of salt and vinegar to tarnish on pennies

Basically the vinegar (5% solution of acetic acid=H3C-COOH*) is reacting with the copper penny to form copper acetate (Cu [H3C-COO]2). The red H (starred if the color is not transmitted) represents the acidic hydrogen of the acid. The equation is :

Cu + 2 H3C-COOH* ---->  Cu(H3C-COO)2 +H2(gas)
      vinegar         copper acetate

This reaction leaves a shiny layer of fresh copper behind, which you see as 'shiny & bright'. If you observe carefully you may see tiny bubbles of hydrogen gas. Eventually the solution will turn blue from the color of the dissolved copper acetate.

Things are actually more complicated. The surface of the penny is likely coated with copper II oxide (other names are: cupric oxide or copper oxide black, CuO, has a brownish black color) or copper I oxide (other name is: cuprous oxide, Cu2O, has a reddish brown color) . These oxides are soluble in acids and so may be dissolved by the vinegar (5% acetic acid solution). The copper I oxide is insoluble in water. The added salt (NaCl) may adjust the ionic strength of the weak acetic acid solution to promote the dissolution of the of these oxides (gives a faster reaction).

Information on the acid+metal reaction is reviewed in general chemistry texts. Information on the copper oxides & their solubility was taken from: Hawley's Condensed Chemical Dictionary, 11th ed., Van Nostrand Reinhold, NY, 1987.