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THE BEST REVIEW FOR EXAM 4 – HOLCOMBE SPRING 2010 BY: PAUL ALEXANDER 1 A battery is a galvanic cell or, more commonly, a group of galvanic cells connected in series , where the potentials of the individual cells add to give the total battery potential. Some metals , such as copper, gold, silver, and platinum, are relatively difficult to oxidize . They are often called noble metals. Metals corrode because they oxidize easily. However, most metals develop a thin oxide coating that tends to protect their internal atoms against further oxidation. Lead Acid storage: Reduction occurs at the cathode; the Pb in PbO2 is reduced to Pb in PbSO4. The primary means of protection is the application of a coating, most commonly paint or metal plating, to protect the metal from oxygen and moisture . Chromium and tin are often used to plate steel (Section 11.8) because they react with oxygen to form a durable, effective oxide coating. Zinc, also used to coat steel in a process called galvanizing , does not form an oxide coating. Cathodic protection is a method most often used to protect steel in buried fuel tanks and pipelines. An active metal , such as magnesium, is connected by a wire to the pipeline or tank to be protected. Because magnesium is a better reducing agent than iron , electrons are furnished by the magnesium, keeping the iron from being oxidized. As oxidation occurs, the magnesium anode dissolves, and so it must be replaced periodically. Ex. Exam 4 Magnesium metal is placed in contact with an underground steel storage tank to inhibit the corrosion of the tank because magnesium metal is a better reducing agent than iron. Just shows how much this question/material comes up, so know it!! A galvanic cell produces current when an oxidation-reduction
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THE BEST REVIEW FOR EXAM 4 – HOLCOMBE SPRING 2010 BY: PAUL ALEXANDER 2 reaction proceeds spontaneously. A similar apparatus, an electrolytic cell , uses electrical energy to produce chemical change. The process of electrolysis involves forcing a current through a cell to produce a chemical change for which the cell potential is negative ; that is, electrical work causes an otherwise nonspontaneous chemical reaction to occur . Electrolysis has great practical importance; for example, charging a battery, producing aluminum metal, and chrome plating an object are all done electrolytically. An electrolytic cell uses electrical energy to produce a chemical change that would otherwise not occur spontaneously. The electrolysis of water produces hydrogen gas at the cathode (on the left) and oxygen gas at the anode (on the right). Because metals are typically very good reducing agents , most are found in nature in ores , mixtures of ionic compounds often containing oxide, sulfide, and silicate anions. Electroplating: Purification of metals is another important application of electrolysis. For example, impure copper from the chemical reduction of copper ore is cast into large slabs that serve as the anodes for electrolytic cells. Aqueous copper sulfate is the
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