Oxidation-Reduction Reactions Chemistry Worksheet Page 20
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Chapter 6
Oxidation-Reduction Reactions
to the cathode, and because substances gain those electrons to become more negative
(or less positive), the cathode surroundings tend to become more negative. Thus cations
2+
are attracted to the cathode. In our voltaic cell, Cu
ions are reduced to uncharged
copper atoms at the copper strip, so metallic copper is the cathode. Because cations are
removed from the solution there and anions are not, the solution around the copper
cathode tends to become negative and attracts cations.
The component of the voltaic cell through which ions are able to flow is called the
electrolyte. For our voltaic cell, the zinc sulfate solution is the electrolyte in the anode
half‑cell, and the copper(II) sulfate solution is the electrolyte in the cathode half‑cell.
2+
As described above, the Zn/Zn
half‑cell tends to become positive due to the
2+
loss of electrons as uncharged zinc atoms are converted into Zn
ions, and the
2+
Cu/Cu
half‑cell tends to become negative due to the gain of electrons and the
2+
conversion of Cu
ions into uncharged copper atoms. This charge imbalance would
block the flow of electrons and stop the redox reaction if something were not done to
balance the growing charges. One way to keep the charge balanced is to introduce a
device called a salt bridge.
One type of salt bridge is a tube connecting the two solutions and filled with an
O
10
bjeCtive
unreactive ionic compound such as potassium nitrate in a semisolid support such as
gelatin or agar. For each negative charge lost at the anode due to the loss of an electron,
‒
an anion, such as NO
, moves from the salt bridge into the solution to replace it. For
3
example, when one zinc atom oxidizes at the anode and loses two electrons to form
2+
Zn
, two nitrates enter the solution to keep the solution electrically neutral overall.
For each negative charge gained at the cathode due to the gain of an electron, a cation,
+
such as K
ion, moves into the solution to keep the solution uncharged. For example,
each time a copper ion gains two electrons and forms an uncharged copper atom, two
2+
potassium ions enter the solution to replace the Cu
lost.
Dry Cells
Although a voltaic cell of the kind described above, using zinc and a solution of copper
ions, was used in early telegraph systems, there are problems with this sort of cell.
The greatest problem is that the cell cannot be easily moved because the electrolyte
solutions are likely to spill. The Leclanché cell, or dry cell, was developed in the 1860s
to solve this problem. It contained a paste, or semisolid, electrolyte. The reactions in
the dry cell can be thought of as consisting of the following half‑reactions (although
they are a bit more complicated than described here):
Zn(s) → Zn
2+
(aq) + 2e
‒
Anode oxidation:
+
‒
(s) + 2NH
(aq) + 2e
Cathode reduction:
2MnO
2
4
→ Mn
(s) + 2NH
(aq) + H
O
O(l )
2
3
3
2
+
Zn(s) + 2MnO
(s) + 2NH
Overall reaction:
(aq)
2
4
→ Zn
2+
(aq) + Mn
(s) + 2NH
(aq) + H
O
O(l )
2
3
3
2
These inexpensive and reliable cells have served as the typical “flashlight battery” for
many years. Their outer wrap surrounds a zinc metal cylinder that acts as the anode
(Figure 6.6). Inside the zinc cylinder is a porous barrier that separates the zinc metal
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