Data Bases • Custom Term Papers • Free Term Papers • Free Research Papers • Free Essays • Free Book Reports • Plagiarism? Links • Top 100 Term Paper Sites • Top 25 Essay Sites • Top 50 Essay Sites • Search 97,000 Papers @ DirectEssays.com • Search 101,000 Papers @ ExampleEssays.com • Search 90,000 Papers @ MegaEssays.com Free Essays • Term Paper Sites • Chuck III's Free Essays • Free College Essays • TermPaperSites.com • Free Essays • My Term Papers • Essay World • Planet Papers	Search Lots of Essays Back to Subjects - Science Redox and examples Redox and examples Reactions that involve a change in oxidation number are called oxidation-reduction reactions. An element is oxidized if the oxidation number has become more positive in value. For example, in the equation: the oxidation number of zinc has changed from 0 to +2. The term reduction describes the opposite process, in which the oxidation number becomes more negative in value. In the same equation, for example, the hydrogen is reduced. The oxidation number has changed from +1 to 0. If everything is counted through the entire equation, oxidation and reduction are equal and balance to When electric energy is needed, batteries and fuel cells are one way to provide it. A battery chemically stores and then releases energy. A fuel cell converts energy produced by a chemical reaction directly into usable power. Batteries range in size from single-cell models smaller than coins to multi-cell units that fill large rooms. Portable radios, pocket calculators, watches, and hearing aids are typical devices powered by batteries. Very large battery installations supply standby energy for equipment such as that in telephone Alessandro Volta, an Italian professor, devised the first battery in 1800 to provide steady electric current for study and practical use. Before that time, only static electricity--a novelty with no practical value--could be produced. Batteries are either primary or secondary. A primary battery produces its energy by consuming one of the chemicals it contains. When the chemical is gone, the battery no longer produces energy and must be replaced. The carbon-zinc batteries used in flashlights and tape recorders are primary. Secondary batteries, or storage batteries, obtain energy by transforming certain kinds of chemicals to other kinds. When the change is complete, the battery no longer produces energy. It can be renewed, or recharged, however, by sending current from another source through it to restore the chemicals to their original state. An automobile battery, called a lead-acid battery, is secondary. The simplest arrangement of parts that will produce current is called a cell. A battery combines two or more cells to produce higher voltage or more current. Connecting the cells in series increases the voltage. Connecting them in parallel raises the current, or amperage. A very simple form of cell is one called a voltaic cell, in honor of Volta. It uses a strip or rod of copper, another of zinc, and sulfuric acid mixed with water. The pieces of metal are called electrodes. The solution is called the electrolyte. The copper electrode is the cathode, or positive electrode, because it has a positive electric charge. The zinc electrode is the anode, or negative electrode, because it has a negative electric charge. When the cell is not in use, the molecules of the acid in the electrolyte separate into electrically charged portions called ions. In chemical symbols, this means the sulfuric acid electrolyte (H2SO4) dissociates into two positively charged hydrogen (2H+) ions and one negatively charged sulfate ion (SO4=). Note that the sulfate ion has a double negative charge, indicated by the two minus signs. The copper electrode can start electric current flowing as soon as it is connected outside the cell to the zinc electrode. It can do this because copper attracts electrons, which make up the current, more strongly than zinc The copper electrode cannot attract electrons through the electrolyte, however, because electrons have a negative electric charge like the sulfate ions. The negative charges repel each other, and this stops the flow of electrons. Once the copper electrode starts drawing electrons through an external connection, a chemical reaction helps to keep the current going. Every zinc atom that loses electrons to the copper electrode becomes a zinc ion (Zn++) with a double positive charge. Sulfate ions promptly attract the zinc ions into the solution where they combine to form dissolved zinc sulfate (Zn++ + SO4 = Before the action starts, the sulfate and hydrogen ions cancel each other electrically in the solution. Once the hydrogen ions (2H+) are free, they seize electrons at the copper electrode, become normal hydrogen atoms (H), and form bubbles of gaseous hydrogen (H2). This allows the copper electrode to draw more electrons, which keeps the current flowing. In the process, acid and zinc are consumed. When either is used up, the battery fails. The simple voltaic cell cannot operate very long because the bubbles of hydrogen gas that collect at the copper electrode act as an insulator, stopping further electron flow. This blockage is called polarization. In 1836 John F. Daniell, an English chemist, produced a cell that was not subject to polarization. In the Daniell cell, the copper electrode forms the outer shell of the cell and contains a copper sulfate solution; the zinc electrode is immersed in a zinc sulfate or sulfuric acid solution. A porous cup keeps the two solutions apart. The cell produces current just as the voltaic does, except that copper ions from the copper sulfate seize electrons at the copper electrode. These ions are then deposited as copper atoms on the electrode. No hydrogen bubbles appear. In about 1866 the French chemist Georges Leclanche created another cell that prevented polarization. He used carbon and zinc for the positive and negative electrodes and a solution of ammonium chloride (commonly called sal ammoniac) for the electrolyte. Although this combination releases hydrogen, the gas is absorbed in a mixture of carbon grains and manganese Today's dry cell--the familiar battery used in flashlights, portable radios, and toys--employs the same materials. The ammonium chloride electrolyte is customarily a jelly absorbed in a sheet of a porous substance. The zinc is formed into a cup with the other materials inside. The top is sealed, making the cell "dry." Often a steel jacket encases the entire cell. The most common storage battery in use today is the rechargeable lead-acid type used in automobiles. The first such battery was devised in 1859 by The 12-volt battery commonly used in automobiles has six 2-volt cells connected in series. The electrolyte is dilute sulfuric acid. Each electrode is made of connected plates. Between the two sets of plates are thin separators made of wood, glass, or plastic that do not enter into the chemical reaction. The separators are porous so that electrolyte can flow around the plates. Each plate has a framework, or grid, made of a hard alloy of lead and antimony. The grid of a new positive plate is filled with lead dioxide (PbO2). The negative plate contains spongy metallic lead. Lead dioxide attracts electrons more strongly than metallic lead and starts drawing current when the external circuit is closed. This leaves lead ions (Pb++) in the negative plate. Each of these ions draws a sulfate ion (SO4=) from the solution, making lead sulfate (PbSO4) in the negative plate. This leaves free in the solution two hydrogen (2H+) ions for each sulfate ion withdrawn. Oxygen ions released by the lead dioxide join the hydrogen ions, forming molecules of water (H2O). This action keeps the positive plate clear to draw current. Since there is no polarization, the battery works as long as any acid is left to furnish sulfate ions and until each plate contains lead sulfate only. Once this condition is reached, the battery can be recharged by sending current through it in reverse. This changes lead sulfate in the plates back to lead dioxide and spongy lead and restores acid to the solution. The nickel-cadmium battery, another rechargeable type, was first used on a large scale in 1917 to help light the subway trains in Paris. Previously a natural gas system did the job, but when it exploded disastrously, costing many So-called "open" or "vented" nickel-cadmium cells have a nickel hydroxide cathode and a cadmium anode emersed in a potassium hydroxide electrolyte, all assembled in a steel container. No porous separators are used; instead, the electrodes are held in place by plastic or rubber moldings that prevent them More modern "sealed" nickel-cadmium cells contain the same substances but have a very small quantity of electrolyte, porous separators, and design features that permit gas under pressure within the cell to escape but prevent atmospheric air from drying out the electrolyte. These features make them particularly useful in portable devices. The fuel cell is a special kind of battery. Its operating principles were first discovered in 1839 by the British physicist Sir William Grove, but the device remained just a laboratory curiosity for many years. In the 1960s scientists rediscovered the fuel cell and used it to make electricity for spacecraft. The fuel cell resembles a conventional battery in that it has a positive electrode, a negative electrode, and an electrolyte. It works quite differently, however, combining oxygen and some hydrogen-containing fuel electrochemically to produce electricity. The fuel (it might be hydrogen gas, methanol, petroleum, natural gas, or propane) is passed across the fuel (positive) electrode, where it dissociates into hydrogen ions and electrons. The ions enter the electrolyte and move to the oxygen (negative) electrode. The electrons move through an external circuit, producing a current. At the negative electrode, the ions, electrons, and oxygen combine to form water. The characteristics of fuel cells are affected by their electrolyte, operating temperature, oxidant, and fuel. Although numerous combinations of these are possible, only a few are practical. Common electrolytes include phosphoric acid, potassium hydroxide, and sulfuric acid. Operating temperature can range from 120o F (50o C) to more than 1,800o F (1,000o C), depending upon the electrolyte and fuel. By the 1990s fuel cells had been built into systems incorporating a means for the storage and controlled supply of fuel and oxidant and for the removal of heat and reaction products. For possible future space flights of long duration, solar or nuclear generators are considered more suitable than fuel cells, though regenerative fuel cells may be used to supplement these devices. Hydrogen-oxygen fuel cells also have been used to power forklifts and small automotive vehicles on an experimental basis. Various exploratory attempts have been made to introduce fuel cells into commercial use, but so far none has been particularly successful. Fuel cells that operate on methanol, however, have been employed on a limited scale to power television repeater stations and navigation beacons. High-temperature solid-oxide fuel cell (SOFC) systems show great promise for the economical production of electricity and heat in a variety of commercial, residential, and industrial applications. Relying on a readily available source of heat, such as natural gas, SOFC technology is based on the ability of a stable component, such as zirconium oxide, to operate as a solid In a battery-driven circuit, the flow of electric current is produced by spontaneous chemical changes that occur at each battery terminal. In a battery, stored chemical energy is converted to electrical energy. In electrolysis the process is reversed. By forcing an electric current through some substances, it is possible to change electrical energy into stored chemical energy. The process of electrolysis causes chemical reactions that do not occur spontaneously. For example, when common table salt, or sodium chloride (NaCl), is heated to 1,486o F (808o C), the solid turns to a stable melt consisting of sodium ions (Na+) and chloride ions (Cl-). If inert electrodes are immersed in the melt and an electric current is forced through the molten salt by a sufficiently high voltage, sodium metal will be produced at one electrode and chlorine gas at the other. A similar electrolytic process is used to obtain pure aluminum from solutions of aluminum oxide. Electrolysis is important in silverplating. In this process an electric current is passed through an object that is immersed in an appropriate solution of a silver compound. If the voltage is sufficient, silver ions (Ag+) will accept electrons from the object being plated. The ions thereby change to silver atoms (Ag), which plate the surface. A similar technique is used in electroplating copper, The chemical deterioration of a material, usually a metal or metal alloy, is called corrosion. The most common causes of corrosion are contact with water and oxygen, though other substances in the earth and in the atmosphere can also cause corrosion. The material with the greatest economic importance that is most affected by corrosion is iron. The corrosion of iron is called rusting. The corrosion of metals such as aluminum, tin, copper, and zinc generally stops after a thin layer of metal oxide forms on the exposed surface of the metal. This layer serves as a barrier to further contact with oxygen. Even when iron combines with oxygen, a thin, almost invisible coating of iron oxide forms that prevents further rusting when no water molecules are present. When water is present, however, the oxide that forms is bulky and porous, allowing oxygen Other metals also corrode slightly under normal atmospheric conditions. Copper and its alloys brass and bronze are protected from continuous and penetrating corrosion by the formation of a green patina, or film, called verdigris, which is composed of copper carbonate. In many instances buildings with copper-clad roofs and trim are deliberately allowed to develop patinas because the color is considered attractive. Corrosion takes place at a much faster rate in heavily industrialized areas that have high levels of sulfur and nitrogen pollutants in the atmosphere. These compounds combine with moisture in the air to produce extremely corrosive Metals may be protected from corrosion by coating them. A variety of coating processes are used, including painting, electroplating with chromium, or plating with zinc, which is called galvanizing. Alloying steel with chromium or chromium and nickel produces stainless steel, which is resistant to rusting. Plastics, ceramics, and certain rubber compounds are also used to coat metals. Bibliography: Word Count: 2305
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