RedoxandElectrochemistry
2011-09-20 16:43:00   来源：liuxuepaper.Com    双击单词自动翻译

　　As you can see, the anode is the electrode at which oxidation occurs; you can remember this if you remember the phrase “an ox”—“oxidation occurs at the anode.” Reduction takes place at the cathode, and you can remember this with the phrase “red cat”—“reduction occurs at the cathode.” An important component of the voltaic cell is the salt bridge, which is a device used to maintain electrical neutrality; it may be filled with agar, which contains a neutral salt, or be replaced with a porous cup. Remember that electron flow always occurs from anode to cathode, through the wire that connects the two half-cells, and a voltmeter is used to measure the cell potential in volts.Batteries are cells that are connected in series; the potentials add to give a total voltage. One common example is the lead storage battery (car battery), which has a Pb anode, a PbO2 cathode, and H2SO4 electrolyte is their salt bridge. 　　Standard Reduction Potentials　　The potential of a voltaic cell as a whole will depend on the half-cells that are involved. Each half-cell has a known potential, called its standard reduction potential . The cell potential is a measure of the difference between the two electrode potentials, and the potential at each electrode is calculated as the potential for reduction at the electrode. That’s why they’re standard reduction potentials, not standard oxidation potentials. Here is the chart:

　　On this reduction potential chart, the elements that have the most positive reduction potentials are easily reduced and would be good oxidizing agents (in general, the nonmetals), while the elements that have the least positive reduction potentials are easily oxidized and would be good reducing agents (in general, metals). Let’s try a quick problem. 　　Example　　Which of the following elements would be most easily oxidized: Ca, Cu, Fe, Li, or Au?　　Explanation　　Use the reduction potential chart: nonmetals are at the top and are most easily reduced. Metals are at the bottom and are most easily oxidized. Lithium is at the bottom of the chart—it’s the most easily oxidized of all. So the order, from most easily oxidized to least easily oxidized, is Au, Fe, Cu, Ca, Li.　　Example　　Which one of the following would be the best oxidizing agent: Ba, Na, Cl, F, or Br? 　　Explanation　　Using the reduction potential chart and the fact that oxidizing agents are the elements that are most easily reduced, we determine fluorine is the best oxidizing agent.　　Electrolytic Cells　　While voltaic cells harness the energy from redox reactions, electrolytic cells can be used to drive nonspontaneous redox reactions, which are also called electrolysis reactions. Electrolytic cells are used to produce pure forms of an element; for example, they’re used to separate ores, in electroplating metals (such as applying gold to a less expensive metal), and to charge batteries (such as car batteries). These types of cells rely on a battery or any DC source—in other words, whereas the voltaic cell is a battery, the electrolytic cell needs a battery. Also unlike voltaic cells, which are made up of two containers, electrolytic cells have just one container. However, like in voltaic cells, in electrolytic cells electrons still flow from the anode to the cathode. An electrolytic cell is shown below.