An electrochemical cell is assembled that plates out copper from a concentrated Cu2+(aq) solution and dissolves copper from an electrode into a relatively dilute solution of copper ion. The potential that the cell produces can be compared with the voltage predicted by the Nernst equation.

The apparatus consists of a pair of electrodes that are vertical and co-planar, and fit easily into the container (see figure). The electrodes should be separated from each other by about 2" and be soldered to thick, insulated copper wires (14 or 16 gauge household electrical system wiring works well). Cover the exposed solder with paint or epoxy. Tie the wires together so as to maintain the spacing and orientation of the electrodes. Place a hook on the output wires so that when the hook is placed over the lip of the container, the lower electrode will be suspended about 1 cm from the bottom of the container.

Make a mark on the container that will be half way between the two electrodes when they are hooked over the container lip.

Place the container on the ring stand and pour in enough 0.01 M CuSO4 solution to reach the mark. Attach the iron ring to the ring stand and place the separatory funnel in it so that the tubing just reaches the bottom of the container. Pour the 1 M CuSO4 solution through the funnel into the separatory funnel. Open the stopcock slowly, and add the 1 M CuSO4 solution so as to layer it below the 0.01 M CuSO4 solution until the interface between the solutions reaches the mark. Remove the separatory funnel and iron ring. Slide the electrode assembly into the container. Connect the millivolt meter and measure the potential. If the meter can measure current at the sub-milliamp level, make that measurement as well.

This is an interesting electrochemical cell because it is capable of doing electrical work without any net chemical reaction occurring. The number of Cu2+ ions and the amount of copper metal in the system does not change; it is the distribution of these substances in the cell that provides the driving force.

From the point of view of the second law of thermodynamics, having two solutions of different concentrations in the same container is a highly non-random situation, and the system will attempt to remedy this by diffusing the solutions into each other to form a uniform concentration throughout. Putting copper electrodes into the solutions offers an alternative method of achieving the same end. The upper electrode can release a Cu2+ ion into the dilute solution; the electrons so produced then travel through the wires to the other electrode, and a Cu2+ ion is removed from the concentrated solution, reduced to a copper atom and plated out on the electrode. (Of course the events of oxidation and reduction really go on simultaneously and the electrons produced by oxidation are not exactly the same ones used up in reduction, but the narrative approach may be a little clearer to students.) The system wants to achieve randomness strongly enough that it will give the electrons sufficient push (the cell potential) that they may be used to do electrical work (but not much: the output is in the microwatt range).

The Nernst equation (below) can be used to predict the voltage of this cell. For most

The interface between the two solutions is stable for several hours, so the demonstration can be repeated without using any more copper solutions. A certain amount of diffusion of the solutions into each other will not change the cell potential so long as the diffusion does not reach the area of the electrodes.

At the end of the demonstration, the solutions can be mixed to show that the cell potential now disappears.