Video Transcript
Using the standard electrode
potentials shown in the table, determine which of the following metals are capable
of reducing La2O3 to La metal. (A) Al and Fe, (B) Ca and Al, (C)
Ca only, (D) Fe only, (E) Al only.
To answer this question, we need to
understand what the standard electrode potential means and how it can be used to
determine the best possible metal to reduce La2O3 to La.
La is the element lanthanum, and
La2O3 is lanthanum oxide, where lanthanum has a three plus charge. When we deal with standard
reduction potentials, we often are referring to reactions that occur within galvanic
cells. These cells often consist of two
half-cells connected by a salt bridge.
This diagram is a simplistic way to
represent such a galvanic cell. The two half-cell electrodes are
referred to as the anode and the cathode. An anode is the electrode of a
galvanic cell which provides electrons to the external circuit. The anode is the negative electrode
and is where oxidation occurs. The cathode is the electrode of a
galvanic cell that accepts electrons from the external circuit. It is the positive electrode and is
where reduction occurs. The flow of electrons between these
two half-cells is the basis for the production of an electrical current that is used
to power devices, as a simple example, a light bulb, perhaps.
The standard reduction potential of
many elements and substances has been determined. The standard reduction potential of
an electrode is the potential of an electrode to be reduced, measured in volts under
standard conditions and relative to the standard hydrogen electrode whose potential
is 0.00 volts. These standard reduction potential
values help us determine the direction of electron flow in a galvanic cell and which
electrode is the anode where oxidation will occur and which electrode is the cathode
where reduction will occur. The electrode with the more
positive potential has a greater tendency to accept electrons and be reduced. This electrode is the cathode. In other words, electrons will flow
towards the electrode with the more positive standard electrode potential.
We are asked to find the metals
which will reduce lanthanum ions to lanthanum metal. So, we can deduce that the
reduction half-reaction for lanthanum and its corresponding standard electrode
potential value of negative 2.52 volts must be the cathode. Now we need to find the element or
elements which have a more negative standard electrode potential than negative 2.52
volts. Those that are more negative are
capable of spontaneously reducing lanthanum ions to lanthanum metal. We can see that the value of
negative 2.868 volts is the only value more negative than lanthanum’s standard
reduction potential value. And this is for the element
calcium.
Calcium is a stronger reducing
agent than lanthanum, and so lanthanum would be reduced by calcium, and calcium
would be oxidized. This is the half-reaction that
would occur in the lanthanum half-cell. And this is the half-reaction that
would occur in the attached calcium half-cell. Notice the arrow has been flipped
left to right, as in such a cell calcium metal would be oxidized and not
reduced.
Finally, which of the metals are
capable of reducing La2O3 to La metal? The answer is (C), Ca only.
