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An electrolytic cell is one where a non-spontaneous reaction occurs
due to an external electric force placed on the cell. These are just volatic
cells run backwards: for example, the reaction
- Zn(s) + Cu+2(aq) -> Zn+2(aq) + Cu(s)
E0 = 1.101 V
- Zn+2(aq) + Cu(s) -> Zn(s) + Cu+2(aq)
E0 = -1.101 V
run forward (top reaction) is a voltaic cell
generating 1.101 V at standard conditions. The bottom reaction is just the
electrolytic cell: place a potential greater than 1.101 V on the cell and it
will run backwards. Electrolytic cells are used to electroplate metals, and when
you recharge a battery (volatic cell) you place an external current through the
battery, forming an electrolytic cell.
Reactions in an electrolytic cell might be surprising. If a reactant is
extremely hard to oxidize or reduce, you may well end up oxidizing or reducing
water instead.
At the anode, two possible reactions can occur:
- The oxidation of an anion: 2Br-(aq) > Br2(l) +
2e-
- The oxidation of water: 2H2O(l) -> O2(g) +
4H+(aq) + 4e- E0 = -1.229 V
The latter reaction occurs if the anion oxidation half reaction has a
potential of less than -1.229 V. For example, you cannot produce fluorine gas
from fluoride ion
- 2F-(aq) -> F2(g) + 2e-
E0 = -2.889V
in an electrolytic cell: the water will oxidize
first
At the cathode, you again have two possible reactions:
- Reduction of a cation: Fe+2(aq) +2e- -> Fe(s)
- Reduction of water to hydrogen: 2H2O(l) + 2e- ->
H2(g) + 2OH-(aq) E0 = -0.828 V
The latter reaction will occur if the cation reduction half reaction
has a potential of less than -0.828 V. For example, you cannot produce lithium
metal from lithium cation in a water solution,
- Li+(aq) + e- -> Li(s) E0
= -3.040 V
since the water will be reduced first.
Example: Can you reduce Mn+2 ions in a water solution to
Mn(s)? Can you oxidize Cu(s) to Cu+2 in water?
Solution: Check the half cell voltages for each reaction
- Mn+2(aq) + 2e- -> Mn(s)
E0 =-1.182 V
- Cu(s) -> Cu+2 + 2e- E0 =
-0.339 V
The manganese reaction has E0 = -1.182 V. Since
water is reduced with a potential of only -0.828 V, you cannot reduce
managanese ions to manganese metal in water. The copper reaction has
E0 = -0.339 V: water oxidizes at -1.229 V. In this case, copper will
oxidize first, so you can oxidize copper in water.
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