Right now I should be grading an 11th grade science test on redox reactions, but I’ve some doubts about the correctness of one of the answers, and I can’t find just the right information to confirm my suspicion.
The question is: How can we get a coat of zinc on an iron nail?
This being a redox-test the answer is of course through electrolysis, but with zinc to the left of hydrogen in the book’s simplified list of electrode potentials, one should expect production of hydrogen if trying this with an aqueous solution.
But even though I haven’t found as clear a text on the topic as I’d like, I get the impression that you can indeed plate iron with zinc from an aqueous solution, I just can’t figure out why.
Anyone have the explanation readily available, or a refutation?
The reaction for the hydrogen electrode should be 2H[sup]+[/sup] + 2e[sup]-[/sup] -> H[sub]2[/sub]
So zinc will react with acid to produce hydrogen. The concentration of acid in neutral water is 1E-7 M, so one might expect very slow oxidation of zinc in water.
For zinc to react with water itself, it needs to have a reduction potential below 2H[sub]2[/sub]Osub[/sub] +2e[sup]-[/sup] -> H[sub]2(g)[/sub] + 2OH[sup]-[/sup]sub[/sub]. That potential is -0.83.
Of course all this neglects any kinetic barriers that exist.
You can electrochemically deposit zinc, however the production of hydrogen is a problem. Thats why hot dipping is preferred. Problem with hydrogen formation:
It wastes coulombs - rather than them going into metal deposition,
2)it can change the crystal structure of the metal formation,
3)the hydrogen can adsorb into the substrate - changing its mechanical properties, and
4)the formation of OH[sup]-[/sup] at the surface (due to removal of H[sup]+[/sup]) can form the hydroxide.
In a similar way that alloy deposition is calculated using the Nernst equation, you can calculate the exchange-current densities and potential difference required to minimise hydrogen production. This has something to do with converging Tafel lines, but it was a while ago I did electrochemistry, so I can’t remember any more detail. Though this may give you some food for google.
I wonder if this is as much of a problem for basic plating baths. It’s true though, if I remember correctly the pH for our zinc bath was about 3. When ever the foreman wanted to strip off a coating of zinc he just turned off the current and let the barrel spin in the bath.