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R-7
The cell potentials are measured as in the previous experiment, using a sensitive voltmeter. First, before
you start the experiment, you will determine which of the leads is connected to the more positive
terminal. To do this, recall that the cathode always has the more positive reduction potential when
compared to the anode in a voltaic cell. Let us assume that you set up your half-cells as shown in
Figure 2 of Experiment Q. Then connect the voltmeter to the electrodes, so that you get a positive
reading on the voltmeter (if your first connection gives a negative reading, reverse the leads to the
meter). This procedure allows you to identify which electrode is the cathode, since it will be the one
connected to the positive red lead. The other therefore, is the anode.
Part I. The Effect of Concentration on Electrode Potentials
In Part I you will prepare a series of Ag/Ag
+
half-cells using silver nitrate (AgNO3) solutions of different
concentrations. A strip of silver metal will be put into each solution, in turn, to form the test half-cells.
You will compare the potentials of these half-cells to a reference half-cell containing 1.00 M AgNO3
(NOTE: You will assume that a 1.00 M AgNO3 solution compared to a reference 1.00 M AgNO3
solution has a potential of 0.000 V). A plot of the oxidation half-cell potential against log ([Ag
+
] / 1.00
M), will establish a relationship between the E
ox
and log Q
ox
(since Q = [Ag
+
] / 1.00 M). The slope of
this graph is related to the Nernst equation constant.
Part II. The Determination of an Equilibrium Constant
In Part II, you will examine the relation between E° and an equilibrium constant. An equilibrium
constant for any cell can be obtained from E° values for the cell since:
E
cell
=
0.
0592
n
log K
eq
(13)
This relationship can be obtained from equation (1) by noting that at equilibrium E
cell
= 0.000 V and
that K
eq
is the special value for Q when all species are at equilibrium. We will study the
following reaction in this experiment:
2 Ag
+
(aq)
+ Cu
(s)
2 Ag
(s)
+ Cu
2+
(aq)
(14)
