What do you mean when you say “2 chemical half reactions”? Do you mean that there is a chemical reaction in each terminal?
That's exactly what it means. The cell is separate into 2 sides. For the perchloric acid and iron cell I mentioned, the perchloric acid is only on the positive side, and the iron is only on the negative side.
The reaction that happens at each terminal is called a half-reaction because 2 half-reactions are needed before anything happens. The perchloric acid and iron cannot just break down on their own. The acid needs the iron before it can react and the iron needs the acid before it can react.
How do the electrons know which is the positive and negative terminals?
It's based on how spontaneous the reaction is. Look at that table again, Fe 2+ is listed twice.
Fe^{2+} + 2e^- ---> Fe Is lower on the table
Fe^{3+} + e^- ---> Fe^{2+} is higher on the table.
In one half-reaction, it's being oxidized, in the other reaction, it's being reduced. The terminals on the cell are positive or negative because one of the half-reactions is a strong oxidation reaction and one is a strong reduction half-reaction. In the perchloric acid and iron cell, the acid is able to pull the electrons off the iron. If iron was able to pull electrons away from the acid, the iron would be the positive terminal and the acid would be the negative terminal. That doesn't happen because the acid has a stronger affinity for electrons than the iron.
Also, how can the reactions be reversible? I thought that was only the case for rechargeable batteries?
The reactions can be reversed if you supply a voltage that is higher than the voltage the cell outputs. If a cell supplies 1 volt, you can reverse the reaction by supplying more than 1 volt to the cell.
To the reverse the reaction, positive on the cell should be connect to the positive on the power supply. Negative on the cell should be connected to the negative on the power supply. If you hook it up the other way, the cell will be drained of all energy very quickly.
