|MadSci Network: Chemistry|
Doug, I am interested how you predicted catalytic effects and what specific materials you chose and what your results were. This is an interesting subject. A catalyst enters into a reaction and lowers the activation energy of the reaction and is itself essentially unchanged after the reaction is at equilibrium. Of course close examination shows that the catalyst frequently does react with the material, usually in a cyclic manner, to change the reaction pathway or mechanism. The catalyst usually can be recovered virtually unchanged. If you add copperII ions to zinc metal there is a rapid reaction; copper II oxidizes the Zn forming ZnII and a copper metal plate on some of the zinc surface. This process is a single displacement reaction and is known as immersion plating. Since the copper II becomes Cu 0 the copper II is strictly not the catalyst. Neither it or the copper 0 release H2 from acid yet the reaction is accelerated so the catalyst must be the plated copper metal which was formed and which can be recovered when the zinc is dissolved. How does it work? What the copper metal does is separate in space the anode[oxidation] and cathode[reduction ]processes. Both processes were taking place on the same Zn atom, so to speak, and they were interfering with each other. As oxidation occurred zinc ion concentrations increased reducing the oxidation potential of the zinc, causing a positive charge buildup near the metal and repelling hydrogen ions and effectively slowing the reaction. This process is know as polarization and the reduction of the cell potential is known as the cell overvoltage. There may be other factors involved because the reactions are a series of elementary steps involving single electron transfers which may differ on zinc and copper surfaces so the reaction would have to be thoroughly studied to be completely figured out. The zinc metal is more electropositive than the copper metal so the zinc metal is depleted slightly of electrons while the copper is slightly enriched. The hydrogen ions are now attracted to the copper and accept an electron from the copper to form hydrogen atoms which combine to give H2 molecules. Notice there are no zinc ions to repel hydrogen ions or to lower the potential on the copper. The zinc ions are being formed on the zinc metal and are being repelled by the more positive zinc metal. In effect we have made a number of chemical cells with a zinc anode and a copper cathode and have separated the anode and cathode reactions. Indeed if a copper and zinc strip are connected by a wire and both are immersed in acid solution hydrogen gas will be evolved from the copper strip and the zinc strip will dissolve. This phenomenon is very common when dissimilar metals are in contact and are both exposed to the same electrolyte especially when the anode[more active metal] is exposed to oxygen . It also can be used to protect metal by putting a more active metal [usually magnesium protecting steel] in contact with the structure. In automobiles a variation is that the negative pole of the battery is connected to the car frame [ground]. This makes the frame electron rich and helps to slow rusting [but not completely of course]. A problem related to this is found in the DeLorean automobiles which have a stainless steel body[cathode] and a steel frame[anode]. Thes cars have a severe rusting problem at the connection bolts if they are not carefully insulated to prevent electrical contact between the frame and body. http://electrochem.cwru.e du/estir/dict.htm gives definitions
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