Oxidation-Reduction Reactions Chemistry Worksheet Page 19
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6.4 Voltaic Cells
A clever arrangement of the reaction components allows us to harness this reaction to
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produce electrical energy. The setup, shown in Figure 6.5, keeps the two half‑reactions
separated, causing the electrons released in the oxidation half of the reaction to pass
through a wire connecting the two halves of the apparatus. The proper name for a
set‑up of this type is a voltaic cell. Strictly speaking, a battery is a series of voltaic cells
joined in such a way that they work together.
Figure 6.5
A Voltaic Cell
This diagram shows the key
components of a voltaic cell
2+
composed of the Zn/Zn
and
2+
Cu/Cu
half-cells.
e
−
e
−
KNO
Salt
3
(−)
(+)
Bridge
Positive
Negative
Zn
Cu
electrode
electrode
(anode)
(cathode)
SO
2
−
4
Cu
2+
Zn
2+
K
+
NO
−
3
SO
2
−
4
Electrolyte
Site of Oxidation
Site of Reduction
Zn
Zn
+ 2e
Cu
+ 2e
Cu
2+
2+
−
−
A voltaic cell, then, is composed of two separate half‑cells. In
the case of the zinc and copper(II) redox reaction, the first half‑cell
consists of a strip of zinc metal in a solution of zinc sulfate. The second
half‑cell consists of a strip of copper metal in a solution of copper(II)
2+
sulfate. In the Zn/Zn
half‑cell, zinc atoms lose two electrons and
are converted to zinc ions. The two electrons pass through the wire to
2+
2+
the Cu/Cu
half‑cell, where Cu
ions gain the two electrons to form
Messages were originally sent through
uncharged copper atoms. The zinc metal and copper metal strips are
telegraph wires by voltaic cells composed of
called electrodes, the general name for electrical conductors placed
2+
2+
Zn/Zn
and Cu/Cu
half-cells.
in half‑cells of voltaic cells.
The electrode at which oxidation occurs in a voltaic cell is called the anode. Because
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electrons are lost in oxidation, the anode is the source of electrons. For this reason, the
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anode of a voltaic cell is designated the negative electrode. Because electrons are lost,
forming more positive (or less negative) species at the anode, the anode surroundings
tend to become more positive. Thus anions are attracted to the anode. In our voltaic
2+
cell, Zn is oxidized to Zn
at the zinc electrode, so this electrode is the anode. The
2+
solution around the zinc metal becomes positive due to excess Zn
ions, so anions are
attracted to the zinc electrode.
The cathode is the electrode in a voltaic cell at which reduction occurs. By convention,
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the cathode is designated the positive electrode. Because electrons flow along the wire
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