Penny Experiment, trial 1

When I was in middle/high school (I don't remember which) we dropped pennies into sulfuric acid to watch them react. I remembered this to be a lot of fun, and at the beginning of this semester I thought I'd try an "at-home" version with vinegar to see if dilute acetic acid would do the same thing. I expected it to, as hydronium should still be present in solution, but I expected the kinetic to be severely limited since there wouldn't be as much hydronium in solution.

So, run 1: I filled a ceramic coffee mug with "5% acidity" vinegar, and placed a penny (post 1984) into the acid and let it sit. Two months or so later (I probably should have documented this more rigorously, but it was just a curiosity on my part), water had evaporated but the penny appeared to be identical aside from some black grime (whatever that grime is made of) that was more easily removed. So, I had a shiny penny.

Run 2: I'm trying to stick with the "easily in reach within your home" type chemicals for pop-experimental purposes. It was suggested to me that I add salt in order to form a little HCl in solution. Seeing as the other experiment didn't work at all I thought why not, give it a try and see what happens.

I filled the coffee cup with fresh vinegar, and then poured salt to fill the coffee cup ~ 1/2 way. This would ensure a saturated solution of salt in vinegar, thereby possibly driving the reaction to form HCl. Then I dropped the same penny in. ~ 2 weeks after I started the experiment, I noticed that some dark splotches were forming on the salt. When I pulled the penny out to examine it, I thought I saw some zinc, but I wasn't positive if it was just me looking for it, so I through the penny back in. Just today (some odd 2 weeks after the last check) I pulled the penny out and sure enough: holes had been eaten through the copper, and some of the copper surrounding the holes was easily removed as the zinc underneath had been oxidized.

Very neat! The salt certainly sped up the reaction (though I'm not sure if it's acting as a catalyst), from my rough qualitative memory watching an at-home experiment when the fancy strikes me. There was also a pretty cool crystal layer that had formed as the vinegar had evaporated again. I think the evaporation likely helps in reacting with the penny, as the concentration of acid is increasing as water evaporates. I'm somewhat curious about the composition of the crystals (Sodium acetate? Sodium Chloride? Sodium Iodide, as it was iodized salt?). The primary thing that's mystifying me at the moment is: Why did the salt actually help in this reaction? HCl, being a strong acid, would dissociate completely, and so would likely not form to an appreciable amount -- I would expect this to hold even in a saturated solution of NaCl. If anything, I would think that the ionic atmosphere would play a larger role in interfering with the equilibrium. Of course, I also used the same penny after the last one, so I'm thinking I need to redo all this with a new penny at least. And, so that I don't have to use a pH meter (or titration... but for this I'd likely deal with the error in the meter) to find how much hydronium I have at the end, I'm going to try covering the coffee mug so that no water evaporates. Still, kind of some interesting preliminary results.