Lec XXII

 Chem 1200

Angel C. de Dios

Reactions XIII

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When the reaction inside an electrochemical cell is not spontaneous, we now have what we call an electrolytic cell. This is the opposite of a galvanic cell. It cannot operate on its own. It needs an external power source to drive the reaction. In the following example, at the cathode, sodium metal (an alkali metal - which is very electropositive) is produced from the sodium cation. What naturally occurs is the opposite. Sodium, being electropositive, loses an electron, to form the sodium ion, which has an octet configuration. What the cell is trying to achieve therefore is opposite to what happens naturally or spontaneously. On the other hand, at the anode of the following cell, chloride ions are being reduced to chlorine gas. Again, this is opposite to what halogens do. Chlorine needs an extra electron to achieve an octet configuration. Yet, in the cell below, we are trying to oxidize chloride ions back into chlorine gas.

In the above electrolytic cell, molten NaCl is used since it is only in the liquid phase that ions can move. Ions need to move toward the electrodes in order for the reaction to occur. It is true that ions can also move in an aqueous solution, but for NaCl, we cannot use an aqueous solution of NaCl. The following is the reason, water is a possible reactant.

The following shows the electrolysis of water.

Thus, we need to consider if the electrolysis of water could be a competing reaction.

First, at the cathode, where we are trying to produce a metallic element.

At the anode, we are often trying to produce a nonmetallic element:

Nernst equation at equilibrium:

Since all reactions have an equilibrium constant, all reactions have a potential. The potential is not exclusive to reduction-oxidation reaction. We see this in the following examples.

Here is another example:

Just to make sure that we are comfortable with concentration cells, here is another example. In this case, it is the concentration of an anion that can lead to an observed potential. In order for this cell to have a positive potential, the anion must have a higher concentration at the anode. The anion is a reactant at the anode, and it is a product at the cathode.

Short-hand notation

Ag | AgCl | Cl- (aq, 0.010 M) || Cl- (aq, 0.0010 M) | AgCl | Ag

The following are some instances of how we encounter the concepts in electrochemistry outside of this classroom.