|MadSci Network: Physics|
Hi, Chris. What an interesting question. I like it. It takes something youíve learned about (electrolysis) and adds a twist from a totally different topic (alternating current). Itís thought processes like this that really add to your understanding. Good job! Answering your question turns out to be the subject of a whole area of physical chemistry Ė chemical kinetics. While a lot of chemistry is concerned with steady-state, or equilibrium, conditions, what youíre after in this question is how reactions change as their conditions change. To clarify, you want to know what happens to the normal electrolysis reactions when the voltage on the electrodes oscillates periodically from plus to minus at some frequency. As you guessed, it is a real question whether water will be able to grab electrons from the electrodes fast enough to convert to gas before the voltage flips. Because Iím not a chemist, Iím not going to be able to give you a quantitative answer. Iíll sketch what could happen, give you my best guess as to what might happen, and then give references that you can go to for more details. [For a primer on electrolysis, see the excellent demo site at: http://www.chem.uiuc.e du/clcwebsite/elec.html ] The simplest thing that could happen is the reaction occurs just as you would expect from the voltages on the electrodes, at each instant of time. Thus, half of the time oxygen is produced at one electrode while the other half of the time itís produced at the other electrode. Meanwhile, hydrogen is being produced at the opposite electrode. The gas formed at the top of each tube would have equal volumes, comprised of two parts hydrogen to one part oxygen. This is what would happen if the reactions all occurred much faster than the AC time period. The converse situation is that your oscillating voltage flips sign so fast that the reaction occurs at a rate too slow to observe. Another thing that may occur is both reactions are fast enough for your oscillation, but you deplete the water faster than it can be replenished. This is a more-complex situation than the previous case Ė here weíre limited by chemical _transport_ rather than the actual rate of reaction. For example, during one half of the voltage cycle, water is used up to produce hydrogen gas and OH ions next to one of the electrodes. In the next half of the cycle, water is used up to produce oxygen gas and H ions. Eventually, youíre going to need more water to get near the electrode to continue the electrolysis. If the necessary water is prevented from approaching the electrodes due to all the reaction products in the way, then the reaction will grind to a halt. So, among all these things, what might actually happen? Iím guessing that the reaction is plenty fast, so unless your AC frequency is really high you wonít be limited by rates of reaction. Even better, the gases produced during the reaction will cause a lot of turbulence near the electrodes as they bubble up to the top. This means that there will be efficient mixing of water and reactants nearby the electrodes, and a natural means of getting the reaction products out of the way (not to mention the fact that the H ions and OH ions will quickly re-form new water). Therefore, I believe that electrolysis will proceed normally, but because of the alternating voltages youíll have oxygen and hydrogen being produced on both electrodes. For further reference, you can see that this idea forms the basis for measuring reaction kinetics as well as transport properties. Thereís an excellent overall reference for chemical kinetics and electrochemistry at: http://www.cheng.cam.ac.uk/research/groups/electroch em/JAVA/electro chemistry/ELEC/contents.html Within this site, they outline a method for examining chemical reactions based exactly on the kind of AC oxidation/reduction reaction you asked about. The method is called _cyclic_voltammetry_, and is treated in their chapter at: http://www.cheng.cam.ac.uk/research/groups/e lectrochem/JAVA/electro chemistry/ELEC/l4html/cv.html#cyclic A less-detailed, slide presentation on different types of voltammetry (including cyclic voltammetry) can be found at: http://ull.c hemistry.uakron.edu/analytical/Voltammetry/ For basic electrolysis questions, including the sticky point of what kinds of electrodes you need to avoid the complication of competing reactions, see: http: //www.madsci.org/posts/archives/may98/893445065.Ch.r.html http://antoine.frostburg.edu/chem/senese/101/redox/faq/safe- water-electrolysis.shtml
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