Page 130 - Basic physical chemistry for the atmospheric sciences

1 1 6 Basic physical chemistry

(6 . 1 )
and at the silver electrode the reduction half-reaction (6.2) takes place

2Ag + (aq) + 2e - -? 2Ag(s) (6.2)

u
Th s , the beaker on the right side of Figure 6. 1 is called the oxidation
half-cell, and the beaker on the left side the reduction half-cell.
The electrode at which the oxidation half-reaction takes place (i . e . ,
the copper electrode in Fig. 6. 1 ) is called the anode. Electrons flow
from the anode through the wire; therefore, the anode is considered to
be the negatively charged electrode. The electrode at which the reduc­
tion half-reaction occurs (i.e . , the silver electrode in Fig. 6. 1 ) is called
the cathode. Since electrons flow toward the cathode through the
wire, it is the positively charged electrode . Within the cell itself,
negatively charged ions (called anions) drift toward the anode in order

to neutralize the positive ions released into the solution by the oxida­
1
tion half-reaction ( s ee Fig. 6. ) . 4 Conversely , positively charged ions
(called cations) drift toward the cathode to keep that half-cell neutral
(see Fig. 6. 1 ) .

6.5 Strengths of oxidants and reductants; standard cell and
half-cell potentials
The two half-reactions of any redox reactions can be thought of as
occurring in the two half-cells of an electrochemical cell. We will
now show how this provides a quantitative method for comparing the
strengths of various oxidants and reductants and the spontaneous
direction in which a redox reaction will occur.
If the ammeter in Figure 6. 1 were replaced by a voltmeter, we could
measure the electric potential difference (in volts, indicated by V)
between the two electrodes of an electrochemical cell. Experiments
show that for any two metal electrodes (e.g. , Cu and Ag) , this potential
difference depends on the relative concentrations of Cu2 + ( aq) and
Ag + (aq) in the two solutions , as well as temperature , pressure, etc.
However , if the temperature is kept at 25°C, the pressure is constant
at 1 atm , and the concentrations of the two aqueous ions are kept
equal (say at 1 M ) , then, provided not too much current is drawn ,
any two metal electrodes generate a steady potential difference the
e • 0.46
magnitude of which depends on the nature of the electrodes ( . g .
V when the electrodes are Cu and Ag).