Remember how I said that sometimes reactions might not occur, depending on where the elements sit on the Standard Reduction Potentials chart? (See Redox Equations if you can't remember.) Well, I lied (or at least I think I did...). It is possible to make these reactions occur, but they won't occur spontaneously. That's where electrolysis comes in- an electrical current is passed through a substance to make a reaction occur. For example, you can electrolyse sodium chloride to give you sodium and chlorine, and I'm pretty sure you can also electrolyse water to give hydrogen and oxygen.
To electrolyse something, you need some equipment: mainly a battery, some wires, and inert substances such as graphite. Oh, and you also need the solution that you want to electrolyse. The solution is known as the electrolyte while the inert substances are the electrodes.(You can have electrodes that aren't made of inert substances, but if they aren't inert then they might react in the electrolysis process. Sometimes this comes in handy, like in electroplating which I'll tell you about in a future post, but most other times you'll want to keep the electrodes inert.)
Here's a nice little diagram of how to set up the equipment:
Now, the way I think this works is that the battery provides electrons for oxidation and reduction processes to occur. Electrons flow through the battery from the positive end to the negative end, then out the negative end into the rest of the circuit, then back up the positive end. Therefore, electrons are flowing into the electrode known as the cathode, making the cathode negatively charged, and flowing out of the electrode known as the anode, making the anode positively charged. Because of this, positively- and negatively- charged ions flow to the cathode and the anode, respectively.
How will I remember all this?! you might ask. Well, you could try my Chemistry teacher's trick. The anode is positive because it starts with an A and getting an A+ is a really good thing, while the cathode is negative because it starts with a C and that's just average. Also you might find it interesting (though not really that helpful) that positive ions, also known as cations, flow to the cathode, while negative ions, also known as anions, flow to the anode. There's also another acronym that might help you remember stuff:
AN OIL RIG CAT
At the Anode, Oxidation, which Is Loss takes place while Reduction which Is Gain takes place at the Cathode.
Writing half-equations for electrolysis of molten salts is the same as writing half-equations anywhere else (see Redox Equations for more info). If you're using solutions, however, things get a bit trickier, as a solution indicates that water comes into it as well. In this case, you need to write down all of the possible oxidation reactions and all of the possible reduction reactions. FYI, here are the oxidation and reduction reactions for water:
Oxidation: 2H2O -> O2 + 4H+ + 4e-
(By the way, Wikipedia pointed out to me that if you combine these two equations you get 2H2O -> H2 + O2 : the electrolysis of water.)
Then you need to go back to the good ol' Standard Reduction Potentials table and look at those numbers on the side. Compare the standard reduction potentials of the two elements that might possibly be oxidised/ reduced. The element you need is the one with the highest standard reduction potential. (Be careful, though, as some of the reactions might be listed the other way around on your sheet. If this is the case, then you need to change the sign on the standard reduction potential value from positive to negative or vice versa. It may be easier just to look for the highest absolute value.)
(Actually I need to double check that last paragraph- this is the section where I get all confused with what's what. I'm sure it's not that confusing. One day I'll sort it out... though I can get some consolation from the fact that solutions isn't strictly required knowledge in Chemistry 2AB...)
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