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Question Video: Using Electrochemical Potentials to Select Metals Capable of Reducing Lanthanum Ions to Lanthanum Metal Chemistry • 10th Grade

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

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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.

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