Chem Study Guide: Electrolysis

IB Chemistry III Study Guide: Electrolysis Quiz Wednesday, 18 March 2009

Electrolysis is the forced redox of ions, either molten or in aqueous solution. Remember: The difference from voltaic cells is the fact that here, the anode is positive (+) and the cathode is negative (-). Oxidation and reduction are the same as in voltaic cells: Anode  Oxidation : an ox Cathode Reduction: red cat That mnemonic only works both ways with that setup. You can say “red ox,” but not “an cat.” So if you use the trick, try both ways to double-check. Think of e- flow: Electrons leave the anode during oxidation, making it (+), and go to the cathode for reduction, making it (-). For this to occur, the oxidized species (The anion! Or maybe water in solution electrolysis) travels to the anode to give up its electrons, and the reduced species (The cation! Or maybe water in solution electrolysis) travels to the cathode to take the electrons.

Basically, electrolysis is socialism: The oxidized species (the anion) is a cold-hearted philanthropist. He goes to the anode to give up all his negative, unwanted possessions (electrons). In the process, he makes the anode positive because all the electrons are LEAVING it. The reduced species (the cation) is a poor hobo. He goes to the cathode to take the negative possessions (electrons) because—hey, they’re better than nothing. In the process, he makes the cathode negative because all the electrons are ARRIVING. Oh, but it’s forced socialism, because the hobo doesn’t really want the electrons. Thus a battery! Continued…

TWO TYPES OF SOCIALISM/ELECTROLYSIS: Diagrams

Notice the consistencies between them: •

• Electrons flow from anode (+) to cathode (-) • Oxidation occurs at anode (+), Reduction at cathode (-) Both are connected by a battery and inert electrodes (usually platinum, but we used graphite in our puddle lab, remember?). The battery is required!

The only difference:

Because molten electrolysis is occurring in pure ions, only the ions can be oxidized or reduced. But in solution electrolysis, because it deals with aqueous solutions, either the cation or anion or both could be replaced by water in redox. So you have to have a competition with solution electrolysis.

…Who will get the negative gift? The battery is the initiator of electrolysis. It is what forces the redox to take place. The idea of forced redox means that the overall voltage of the reaction (the potential for the reaction to take place) is negative— which in turn means that the reaction would not occur naturally. The battery has to give the little jump start (literally) to keep things going. However, with aqueous solution, the species that will undergo oxidation or reduction, like we said, could be either water or the ion. The competition is to see which one will be easier to reduce/oxidize.

Whichever has a higher potential is the one reacting. This comes from the reduction potentials chart: for reduction, just take the number given. For oxidation, take the opposite (so make positives negative and negatives positive). Example: Reduction – K+ or Water? K+ + e-  K (s) = -2.93 V H2O + e-  ½H2 (g) + OH= -0.83 V They’re both negative values, but -0.83 is a much greater number than -2.93. So in this electrolyis, H2O, not K+, will be reduced.

NOTE This is not an absolute judgment. It’s not whatever has a larger absolute value. It’s based on the number line:

Since -0.83 is more positive/greater, it is reached sooner (don’t think of Western left to right reading: right side is positive and greater here) You can think of it as the battery starting at a really high number (like +3) and moving downward (to -3) until it hits either an ion (like K+) or water. Whichever it hits first gets to react. Here, it was water.

More Extended Metaphor:

The battery is like a lazy person with a stick. It’s going to poke the philanthropist (the species being oxidized) until he gives up some electrons, but once that happens, the battery will give up. It doesn’t care who gives electrons first (water or the ion): whoever gives in sooner is oxidized. Then the battery starts poking hobos. Like before, it’s lazy, and it’ll only poke until one hobo gives in and takes the electrons. If the hobo turns out to be water, so be it; if he’s a cation, great—he’ll be reduced.

Another Note Nitrates and Sulfates almost NEVER oxidize. It’s so rare that they’re not even listed on the reduction potentials chart, not even in the hugely negative section. So if you ever have them in solution electrolysis, you know that the water will be oxidized instead. Don’t even bother looking for the potentials—they’re not there. (…“But there’s a sulfate one!” you cry? The sulfate one is for reduction and not the same thing. Look at its products).

Qualities of Solution Electrolysis with Water as a Participant When water is one of the species being either oxidized or reduced (or both—it can happen), the water changes the qualities of solution.

Know where to find these two equations on the reduction potentials chart: RED: H2O (l) + e-  ½H2(g) + OH-(aq) OX: H2O (l)  ½O2(g) + H+(aq)+ e-

-0.83 V -1.23 V

H2O Reduction leads to OH- (basic sol’n) and hydrogen gas at cathode H2O Oxidation leads to H+ (acidic sol’n) and oxygen gas at anode Good luck, and watch those hobos! -Autumn