Some of my thoughts on electrochemical cells

[lzxnl]

I've seen many people ask questions about these things online, so I figured I may as well write something to answer the same question once and for all.

1. What does the polarity of a cell mean?

"Positive" and "negative" are misleading ways of thinking about these cells if you don't know what they actually represent. They're not actually charged; you have the salt bridge to make sure that they are not charged. So what are these things?

The "positive" electrode means the electrode at higher electric potential. NOTE: electric potential is not electric potential energy. Electric potential is potential energy per charge. This means that a positive charge will lose potential energy and thus speed up when moving from higher potential to lower potential, whereas a negative charge will gain potential energy and thus slow down when moving from higher potential to lower potential. This agrees with the common observation that negative charges move from "negative" to "positive", i.e. lower to higher electric potential.

2. Why the heck do the polarity designations change in galvanic and electrolytic cells? AKA why are cathodes positive in a galvanic cell but negative in an electrolytic cell?

There are a few things to bear in mind here. Firstly, if we have two electrodes, electrode A and electrode B, and we assume electrode A is at higher potential, it will be at higher potential REGARDLESS if the cell is electrolytic or galvanic. What changes is the direction of the current. So you don't say "the cathode has become positive". Rather, "the positive electrode is now the cathode". As an example, suppose you had a Cu²⁺/Cu half cell and a Fe²⁺/Fe half cell. If you leave it alone, the copper half cell will attract electrons and is thus the electrode with higher electric potential. However, if you turn this into an electrolytic cell, the copper half cell will STILL have higher electric potential; it is still "positive", but the external voltage overrides these considerations and force the electrons into the iron half cell which is still at lower potential.

Now how does this external voltage work? It's actually pretty neat. Let's take the previous example of the copper and iron half cells. By design, the positive terminal of the battery is connected to the cell with higher electric potential and the negative terminal of the battery is connected to the cell with lower electric potential. You can probably see that if we reversed these terminals, the battery would support the spontaneous reaction and the extra voltage would be lost as heat.

Let us assume that initially the positive terminal of the battery and the copper half cell are at the same potential, i.e. they are connected first. Then, as the voltage drop from the positive terminal to the negative terminal of the battery must be larger than the voltage drop from the positive to negative electrode, and as the positive terminal and electrode are at the same potential, the negative terminal of the battery must be at a lower potential than the negative electrode. Think about this until you get it.

If the negative terminal of the battery is at lower potential, electrons will attempt to move from lower potential to higher potential by moving from the battery to the negative electrode. Remember; electrons move from lower to higher potential because they have a negative charge. When electrons leave the battery, an electron deficit is created at the negative terminal. Electrons then flow from the positive terminal to meet this deficit, and electrons from the positive electrode then flow to the positive terminal to meet the electron gap created. Hence, we have a circuit running!

If the negative terminal of the battery is connected to the negative electrode first so that they are at the same potential, then as the voltage rise from the negative terminal to the positive terminal is higher than that from negative electrode to positive electrode, the positive terminal is at higher potential than the positive electrode. Electrons flow from the positive electrode to the positive terminal and by similar reasoning the electrons still flow from the negative terminal to the negative electrode.

Now, as the electrons leave the positive electrode, the positive electrode is the anode, and the negative electrode is the cathode. In a conventional galvanic cell, the electrons leave the negative anode because electrons normally go from lower potential to higher. Here, the external voltage has not changed the order of potentials, but it has changed the electron flow to cause the electrons to move from the positive electrode, the new anode, and the negative electrode, the new cathode.

So...remember that "positive" and "negative" designations do not change; the "anode" and "cathode" designations do, as the external voltage reverses the current flow, meaning the electrons come from different places.

I hope this made sense.

[zvezda]

This is great stuff. Always of great help nliu

[pi]

Added to Chem Resource thread, fantastic!