SIR VOLTA

This SIR explores the quantitative behavior of voltaic (galvanic) cells in five modes. It allows you to simulate experiments with cells of fixed (standard) and variable concentration, to explore their internal operation, and to clarify the cell convention (anode on the left.

In discover mode the screen is kept simple; you are conducting a simulated experiment. As soon as you build a cell its potential is shown on the meter.

In function mode the various directional properties are initially indicated by questions marks. You click on them to reveal their directions.

Ten electrodes are available, representing metal/metal ion (Cu(s)/Cu2+), gas/solute (Pt(s)/H2(g)/H+), two ion (Pt(s)/Fe2+,Fe3+) and insoluble salt (Ag(s)/AgCl(s)/Cl-) reactions. You can build a cell using any pair of these in any order, including two nominally identical electrodes in a concentration cell.

V: DISCOVER (Discover Standard Potentials)

This is the basic simulation. All conditions are standard (25°, one atmosphere, unit activities). You use this as a simulated laboratory experiment. You may, for example, show that the standard electrode potentials are simply the potentials of cells in which the other electrode is the standard hydrogen electrode.

V: FUNCTION

This mode effectively repeats that of SIR Polarity, but now the sign and magnitude of the cell potential, and the two electrode potentials may be revealed.

C: DISCOVER (Nernst Equation)

This works in the same way as V: Discover, except that you may change solute concentrations by clicking on them.

C: FUNCTION

This works in the same way as C: Discover, except that all of the directional properties of the cell are shown. If you change a concentration so as to move through the equilibrium point, all the indicators of polarity reverse themselves. This is quite dramatic.

CELL CONVENTION

This causes more student confusion than any other item in electrochemistry. The convention states that the anode is on the left and the cathode on the right. In the real world the anode is as often on the right as on the left.

It is best understood as a question: do all processes proceed in the direction they would if the anode were on the left? It turns out that the standard potential is defined in such a way that if it's positive all answers are "Yes", and if it's negative they are all "No".

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Updated July 17, 2000