Oxidation-Reduction Reactions Chemistry Worksheet Page 21
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6.4 Voltaic Cells
Figure 6.6
Leclanché (or Dry) Cell
Carbon Rod (cathode)
+
Porous Barrier
2+
(Zinc ions, Zn
,
pass through)
Paste with MnO
,
2
NH
Cl, and ZnCl
4
2
Zinc (anode)
2+
from a paste containing NH
Cl, ZnCl
, and MnO
. The porous barrier allows Zn
4
2
2
ions to pass through, but it keeps the MnO
from coming into direct contact with the
2
zinc metal. In the center of the cell, a carbon rod that can conduct an electric current
acts as the cathode.
Electrolysis
Voltage, a measure of the strength of an electric current, represents the force that moves
electrons from the anode to the cathode in a voltaic cell. When a greater force (voltage)
is applied in the opposite direction, electrons can be pushed from what would normally
be the cathode toward the voltaic cell’s anode. This process is called electrolysis. In a
broader sense, electrolysis is the process by which a redox reaction is made to occur in
the nonspontaneous direction. For example, sodium metal reacts readily with chlorine
gas to form sodium chloride, but we do not expect sodium chloride, as it sits in our
saltshakers, to decompose into sodium metal and chlorine gas. We say the forward
reaction below is spontaneous, and the reverse reaction is nonspontaneous.
2Na(s) + Cl
(g) → 2NaCl(s)
2
Sodium metal and chlorine gas can, however, be formed from the electrolysis of salt,
in which an electric current is passed through the molten sodium chloride.
Electric current
2Na(l ) + Cl
2NaCl(l )
( g)
2
Electrolysis is used in industry to purify metals, such as copper and aluminum, and in
electroplating, the process used, for example, to deposit the chrome on the bumper of
a 1955 Chevy.
As you will see below, a similar process is used to refresh rechargeable batteries.
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