 |
 |
1 of 21 |  |
 |
 |
Q-1
EXPERIMENT Q: VOLTAIC CELLS AND REDOX REACTIONS
OBJECTIVES
In Experiment P, the reactivities of Zn
(s)
, H
2(g)
, and Cu
(s)
were compared qualitatively to each other. In
this experiment, DC voltmeters will be used to quantitatively measure the potentials of a series of voltaic
cells. Voltaic half-cells used in this experiment are metal strips in solutions containing the corresponding
metal cation at 1.0 M concentration. The metals to be used are Ag, Cu, Pb, Cd, and Zn. Measurements
will be performed on a microscale level in order to minimize the amount of liquid waste produced. Your
observations will be summarized in the form of a displacement series based on increasing reduction
potential. The position of I2, in this series will also be determined. Finally, you will extend the scope of
your displacement series by qualitatively comparing the reactivity of the halogens I2, Br2, and Cl2 to each
other.
THEORETICAL CONSIDERATIONS
I. Quantitative Measurement of Electrode Potentials
Oxidation-reduction (redox) reactions involve the transfer of electrons. The number of electrons lost by
one species must equal the corresponding gain of electrons by another species. For example, consider the
reaction
Zn
(s)
+ 2 H
+
(aq)
Zn
2+
(aq)
+ H
2(g)
(1)
Electrons are transferred from Zn
(s)
to H
+
(aq)
within the solution. In this reaction, zinc loses electrons and is
oxidized to Zn
2+
. The oxidation of zinc provides electrons that are used in the reduction of the H
+
(aq)
.
Zinc is referred to as the reducing
agent (provides electrons for the reduction of H
+
(aq)
). Similarly, H
+
(aq)
gains electrons and is reduced to H
2(g)
. The reduction of H
+
(aq)
requires electrons that result in the
oxidation of the Zn
(s)
. H
+
(aq)
is referred to as the oxidizing
agent (i.e. attracts electrons resulting in the
oxidation of Zn).
Of two compounds, the stronger reducing agent is the one that has the greater tendency to lose electrons
(i.e. most easily oxidized). Experimentally, reaction (1) proceeds spontaneously in the direction written;
