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It's fairly easy to figure out just what will happen in an an electrolytic
cell when you apply a specific amount of current. Since these type of cells
are often used for electroplating, it can be rather important to make sure you
only plate out as much gold or chrome as you need.
Simply look at the half reaction that you care about: if you want to plate
out gold
- Au+(aq) +e- -> Au(s)
it should be obvious
that you need one mole of electrons for every mole of gold you want to plate.
For chromium,
- Cr+3(aq) + 3e- -> Cr(s)
you need three
moles of electrons for each mole of metal.
There are a couple of simple relationships between common units like amperes
and volts and chemical units like moles of electrons. The most useful follow
- 1 mole e- = 96480 Coulombs
- 1 Coulomb = 1 Ampere*second
- 1 Joule = 1 Volt*Coulomb
With these relationships, you can compute
how much current you need to plate out a specific amount of metal
Example: You want to gold plate a ring with 0.100 grams of gold. If
you use house current (15 amps, 120 V) and a solution of Au+, how
long will it take?
Solution: To start, figure out how many moles of gold we have
- 0.100 g/197 g/mole = 5.08*10-4 moles of gold
Since gold
takes one electron per mole to be reduced, we need 5.08*10-4 moles of
electrons. Figure out how many Coulombs this is
- 5.08*10-4 moles e- * 96480 C/1 mole = 49.0
C
1 amp = 1C/s, and we're using 15 amp current, so we're putting in
15C/s and need 49.0 C, so
- 49.0C/(15C/s) = 3.27 seconds
Not very long: electroplating can be
very quick. |