ELECTROCHEMICAL CELLS USING SIR POLARITY
Using SIR POLARITY, introduce electrochemical cells into the circuit. The "Electrolytic Cell" option presents a chromium-plating cell as the load. Now we see how the chemical processes get involved. Their polarities too are initially covered by question marks.
A good starter is simply to deduce the
current and electron flow as before, and then to see which way the ions are going. One
click on an ion marks it, and a second click adds an arrow showing its direction of
migration. Cations flow with the "current", and anions against it; that is, the
same way as the electrons. This helps to reinforce the notion that current may equally
well be carried by positive or negative entities.
Next consider the chemical consequences. The electrons in the wires have to come from and go to somewhere in the cell. So one can deduce the electrode reactions: reductions use up electrons and oxidations produce them. The only reasonable electrode processes here involve chromium and its ion, and it’s not hard to figure out which reaction takes place at which electrode. So electron flow determines oxidation-reduction, anode-cathode, and direction of electrode reaction {Cr(s) ® Cr3+ + 3e- vs. Cr3+ + 3e- ® Cr(s)}.
There are about twelve processes in the cell, each of which has a direction. Any one can be deduced from any other. An obvious drill is to reset the cell, reveal any one, and ask the direction of another – and then ask for the chain of reasoning.
The next step is to the reversible
cell, with a salt bridge. The Cr – Cd cell used actually has a small cell potential,
but it may be driven in either direction by a sufficiently strong external source; that
is, it's reversible.
Now there are three electrolytes and a cell reaction to consider. However, the logical procedures are exactly the same as before. Just remember that you get the cell reaction by adding up the two electrode half-reactions.
In addition to providing an excellent test of logical reasoning concerning Coulomb’s law and redox reaction, this also provides a natural link to the half-reaction method of balancing redox reactions. Just note that the same current must flow everywhere, so that charge does not build up.
Click here to see how to study the workings of voltaic cells