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Any spontaneous reduction-oxidation (redox) reaction can be used to generate electrical power. To do so, you
have to design a cell in such a way that electrons flow from the anode (Where
oxidation occurs) to the cathode (Where reduction occurs) through an external
circuit.
A typical cell is the zinc-copper one shown below.
- Zn(s) + Cu+2(aq) -> Zn+2(aq) + Cu(s)
If
you place a chunk of zinc metal in a solution of copper ions, you will end up
with zinc ions in solution and copper metal. While an interesting demo, it's not
useful for generating power since the electrons don't travel through a circuit
we can use
If you place a solid zinc electrode into a beaker filled with a solution of
zinc ions and a sold copper electrode into a seperate beaker full of copper
ions, you begin to have the makings of a cell. Here, the copper electrode will
be the cathode, and
the zinc the anode. If
you connect the two electrodes with a wire, you have an external circuit you can
draw power from. You also need a salt bridge, a way for charge balance
between the two beakers to be maintained: this is typically a glass tube filled
with a KCl solution and stoppered on both ends with a permeable plug. When
charge flows into one beaker (The copper beaker gains electrons, the zinc one
loses them) ions in the salt bridge migrate from one beaker to the other to keep
both solutions electrically neutral
Such a cell is often abbreviated as follows:
- Zn|Zn+2||Cu+2|Cu
The anode reaction is shown
on the left: zinc metal is oxidized into zinc ions. The double bar (||)
represents the salt bridge that seperates the two beakers, and the cathode
reaction (copper ions being reduced to copper metal) is shown on the right.
In the above cell, we can trace the movement of charge.
- Electrons are produced at the anode as the zinc is oxidized
- The electrons flow though a wire, which we can use for electrical energy
- The electrons move to the cathode, where copper ions are reduced.
- The right side beaker builds up negative charge. Cl- ions flow
from the salt bridge into the zinc solution and K+ ions flow into
the copper solution to keep charge balanced.
Example: In the cell below, what are the two half reactions and what
occurs at the anode and the cathode?
- Cr|Cr+3||Co+2|Co
Solution: The left hand side is the anode, the right is the cathode.
- At the anode, chromium metal is oxidized to chromium ions. The half
reaction is
Cr(s) -> Cr+3(aq) + 3e-
- At the cathode, cobalt ions are reduced to cobalt metal. The half
reaction is
Co+2(aq) + 2e- -> Co(s)
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