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P-2
EXPERIMENTAL METHOD
In Part I, you will study a nonspontaneous redox reaction in which H
+
(aq)
is reduced to H
2(g)
and Cu is oxidized
to Cu
2+
(aq)
. Electrical energy from an external source is required to drive this nonspontaneous redox reaction.
The apparatus (see Figure 1) consists of an external source of electrical energy that provides DC voltage, an
ammeter to measure the current flow, and a variable resistance box to control the current flow (since
I
=
V/R). Electrolysis takes place in a beaker containing aqueous sulfuric acid. Reduction occurs at the
cathode, which in this case is an inert nichrome wire attached to the negative terminal of the power source.
Oxidation occurs at the anode, which is a piece of copper wire attached to the positive terminal of the power
source. You will measure the volume of hydrogen gas produced during the reaction at a known temperature
and pressure. Using the Ideal Gas Law, you will calculate the number of moles of hydrogen gas formed. The
number of moles of electrons transferred equals two times the moles of hydrogen gas formed (2H
+
(aq)
+ 2 e
-
H
2(g)
). If a relatively constant current is applied for a specific period of time, it is possible to calculate the
charge transferred in coulombs, Q = It. This information is used to calculate the number of coulombs of
charge associated with the transfer of one mole of electrons (i.e. the charge of one mole of electrons). This is
the value of one Faraday.
You will also measure the change in the mass of the copper anode. In this electrolysis, the reactions that
occur at the anode and the cathode both involve two electron processes. This requires that the moles of Cu
lost through the oxidation reaction (Cu
Cu
2+
+ 2 e
-
) must equal the moles of hydrogen gas formed by the
reduction reaction (2H
+
(aq)
+ 2 e
-
H
2(g)
). Using this as a basis, you will be able to estimate the atomic mass
of copper.
As you set up your apparatus, keep the following information about electrical circuits in mind:
1.
The resistance of a copper wire varies inversely with the square of the diameter of the wire.
2.
The resistance of the electrical circuit also varies inversely with the surface area of the wire in contact
with the solution.
3.
The greater is the separation between the electrodes in the solution, the greater the resistance in the
circuit.
