Page 134 - Basic physical chemistry for the atmospheric sciences

1 2 0 Basic physical chemistry
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Zn + ( aq) + 2e - � Zn(s)
the negative sign associated with the zinc electrode potential ( - 0. 76
V) can be interpreted as indicating that the spontaneous reaction pro­
t
ceeds from right to lef . Similarly, the fact that copper has a positive
electrode potential (0.34 V) indicates that the reduction reaction (6. 1 4)
proceeds spontaneously from left to right.
The greater the magnitude (with sign) of an electrode potential, the
greater is the driving force for the reduction half-reaction to take place
in that half-cell. For example, Zn and Cu have electrode potentials of
- 0.76 V and 0.34 V , respectivel y . Therefore, when paired together,
Cu will be involved in the reduction half-reaction; that is, it will be the
i
oxidant. Zinc will be involved in the oxidation half-reaction, that s , it
will be the reductant.
With the above sign convention , we can write

�en = � x + � ed (6. 1 9 )
where the magnitude and sign of �ed are the same as that for the
electrode potential of the reduction half-reaction , and �x has the same
magnitude but the opposite sign as the electrode potential it would
have were it to serve as a reduction half-reaction. If �en is positive,
the overall or net chemical reaction (obtained by adding the two half­
i
reactions) is spontaneous from left to right, with a driving force that s
proportional to t h e magnitude f �en· The following exercise should
o
make these points clear.
Exercise 6 . 6 . An electrochemical cell has electrodes made of zinc and
copper and operates under standard conditions. Which electrode is the
anode and which the cathode? Which way will electrons flow in the ex­
ternal (wire) portion of the circuit? What is the maximum electric po­
tential difference that this cell can generate? Will Zn(s) spontaneously
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2
)
reduce Cu + ( aq , or will Cu(s) spontaneously reduce Zn + (aq)?
Solution. As we have just seen, the electrode potentials of copper
and zinc are 0 . 3 4 V and - 0 .76 V, respectively . By convention, the
greater the value of the electrode potential the more likely it is that the
reduction half-reaction will take place at that electrode. Therefore, the
reduction half-reaction takes place at the copper electrode and the oxi­
dation half-reaction at the zinc electrode. Hence, by definition, the zinc
electrode is the anode and the copper electrode the cathode.
t
In the wire portion of the circuit , the electrons always flow from h e
anode (where electrons are released by the oxidation reaction) to the