Friday, October 24, 2014

Nickel and Silver Platting Calculation

A 0.500 amp current flowing for exactly one hour is passed through a solution of nickel (II) ions.

  1. How many grams of nickel metal will plate out?
  2. If this cell is hooked in series with another cell containing Ag+ ions (in series the same current passes through both cells), how many grams of Ag will plate at the same time?

First we need to know how much charge passed through the cell(s):
0.500 A x 3600 s = 1800 C

Now how many moles of electrons was that?

(1800 C x 1 mol e-) / 96500 C = 0.0187 mole e-

Since nickel (II) has a charge of +2, one atom of nickel will require 2 e- to plate:

0.0187 mole e- x 1 mole Ni / 2 mole e- = 0.00935 mole Ni

This is 0.549 grams
Since silver is plated by the same amount of charge, but each atom of silver requires only one electron to plate (Ag ions are +1), we should expect twice as many moles of silver atoms to plate:

0.0187 mole e- x 1 mole Ag /1 mole e- = 0.0187 mole Ag

Thus 2.02 grams of silver are produced. Note that the ratio based on charge affects the moles of silver and nickel. Their atomic masses influence the final mass plated.

Plating experiments are rather boring and this one is no exception. There is not much to do as the process takes place (especially if your calculator is recording the data...) and there is generally not much to look at. However, there is time for a fascinating diversion. You know from the activity series that zinc, which is above copper, will be displaced by copper in solution. The voltage for this process is 1.10 v as you saw in the previous experiment. The opposite process, zinc displacing copper is non-spontaneous. Or is it?

For many years chemistry teachers have done a demonstration in which they turn ordinary pennies into "gold". Traditionally the process has been done in a hot, very alkaline solution containing the ion Zn(OH)42- and some solid zinc. This combination of materials caused zinc metal to spontaneously plate on the copper. When the "silver" pennies were subsequently heated in a flame, the thin coating of zinc alloyed with the copper, producing yellow brass or "gold".

Recently a pair of chemists investigated this process more closely. Their report appeared in the Journal of Chemical Education. They found that the process was very different from what had been assumed. In fact, the very alkaline solution was not required at all! Copper placed in a heated solution of Zn2+ and a small amount of metallic zinc will become plated with zinc metal!! How is this? According to their research the process is driven by the alloying which occurs at the very surface of the copper. Due to thermal agitation in a heated solution, some zinc ions manage to work their way into the spaces between the copper atoms and form a very thin layer of brass.

The voltage for zinc metal plating on brass (as opposed to copper) is positive! Thus the "silver" appearance of the penny after heating is actually a type of brass (silver brass).
When heated in a flame, the silver brass is converted to yellow brass ("gold"). You can do this yourself, using the plating solution and some granular zinc.

Bring two of the shiniest (cleanest) pennies you can find. Place them in the beaker of solution provided on the hot plate and boil the solution. It takes about 10-15 minutes to plate a uniform layer of silver brass. Turning the pennies occasionally will help. When they are covered in silver brass, remove them and rinse in water. You can keep one "silver" penny and heat the other very carefully in a cool burner flame to change it to "gold". Too hot a flame will melt the brass coating, so be careful. This is just for fun, but it also points up the fact that not everything which we take for granted is happening they way we think. There are plenty of puzzles left to solve.