Re: How does copper from pennies plate onto steel in vinegar?

In the experiment you describe, the acetic acid from the vinegar dissolves 
copper ions into solution.  Then, a reduction-oxidation, or redox, occurs, where 
the iron in the nail gets oxidized and the copper gets reduced.  A few chemical 
reactions:

1)  Copper being dissolved, most likely oxide (CuO) on the penny being dissolved 
by acetic acid HAc:
CuO + HAc --> Cu(2+) + Ac(-) + OH(-)
The (-) and (+) indicate charges on the ions.  OH(-) is hydroxide ion which is 
basic.

2)  Now, the iron (Fe) oxidizing:
Fe --> Fe(2+) + 2e(-)
2e(-) represents two electrons.  

3)  The copper Cu(2+) reducing:
Cu(2+) + 2e(-) --> Cu

2) and 3) occur simultaneously at the nail.  

There is a very important consideration that allows the above to occur:  
electrochemical potential.  This type of potential (voltage) indicates whether a 
redox will occur spontaneously or not.  A positive potential means that the 
redox will occur; negative means it will not.  It turns out the the potentials 
for the above half reactions are +0.44 volts for Fe and +0.34 volts for Cu.  
Added together, this makes +0.78 volts, so the above experiment occurs 
spontaneously, and you get a copper-colored nail.