...s reaction causes a current to flo through a conductor that connects the positive and negative terminals. In a dry cell the zinc casing serves as the anode and is consumed in the anodic electrode reaction Zns -- Zn2 2e- the zinc ion dissolves in the moist ZnCl2-NH4Cl electrolyte. A carbon rod serves as the cathode, but it is chemically inert. The cathode electrode reaction, hich consumes MnO2, is best ritten as iMn4 2O2-s H2O e- -- iMn3 O2- OH-s OH- here the square brackets indicate the species present in the solid phase at the cathode. The cathode reaction actually occurs ithin the solid structure the carbon rod serves only to transfer electrons from the external circuit. The dry cell has a potential difference of about 1.25 V the zinc electrode is negative. It is a good source of electrical poer and the materials of construction are relatively cheap. The cell voltage during discharge falls off rather badly and the dry cell is not a good source of poer hen a constant voltage is needed. They are called dry cell because they electrolyte is a paste rather than a liquid. Types of dry cells 1. Primary cells - They are not rechargeable. The cell ill not function once their chemicals are used up and the cellshave to be thron aay. Examples are zinc-carbon cell, alkaline manganese cell and silver oxide cell. 2. Secondary cells - They are rechargeable can be recharged and can be used again. Example nickel-cadmiumcell. Note The lead-acid accumulator car battery used in cars is also a secondary cell but it is not considered as a type of dry cell. Ho to make a dry cellDry cells are one of the most commonly used household objects. e use dry cells in atches, torches, transistors, alkmans and even the remote controls of our TVs. Dry cells provide the necessary electricity required to poer these devices. A normal dry cell is cylindrical in shape made of zinc. A carbon rod passes through its center and a paste of manganese dioxide and ammonium chloride surrounds this rod. hen the both ends of the cell are z connected to a bulb through a ire, the bulb glos due to the flo of current. The voltage of such a cell is about 1.5 volt. Let us no make a dry cell at home.Material Required A small carbon plate A small zinc plate Manganese dioxide Starch poder Ammonium chloride Cotton ool Copper ire To metallic clips One 1.5 volt bulb and one bulb holder INCLUDEPICTURE http.education.eth.netimages16dry20cell.jpg t MERGEFORMATINET Procedure Make some starch paste by mixing some starch and ater and then boiling it. Add sufficient quantity ofmanganese dioxide to the starch paste,making a very thick paste of manganese dioxide. Spread this manganese dioxide paste evenly on the zinc plate. No take some cotton ool and flatten it to fit the shape of the zinc plate. Soak this cotton ool in ammonium chloride solution. No add another layer of manganese dioxide paste over the cotton ool. No put the carbon plate over this layer of manganese dioxide and your dry cell is ready to use. To see the dry cell in action connectires to the to ends of the bulb holder and connect the other ends of the ires to the carbon and zinc plates using metallic clips. The bulb begins to glo.The forms of dry cells Dry cell - acid form This is the source of poer for an ordinary flashlight. Most flashlight batteries produce 1.5 volts. The case of the cell is zinc metal acting as the anode. At the center of the cell is a stick of graphite for the cathode. The graphite stick is surrounded by a paste of MnO2 and NH4Cl. INCLUDEPICTURE httppc65.frontier.osrhe.eduhssciencedrycell.gif t MERGEFORMATINET Half-reactions for this cell are Anode - Zn s INCLUDEPICTURE httppc65.frontier.osrhe.eduhssciencearro.jpg t MERGEFORMATINET Zn2 2e - Cathode - 2NH4 aq 2MnO2 s 2e - INCLUDEPICTURE httppc65.frontier.osrhe.eduhssciencearro.jpg t MERGEFORMATINET Mn2O3 s 2NH3 aq H2O l Remember that the term battery refers to to or more cells connected together. The 9-volt transistor battery is a true battery. It contains six individual 1.5-volt cells connected in series to produce 9 volts. Dry cell - alkaline form The NH4Cl in the acid form cell is replaced by KOH and the zinc is in poder form rather than a solid piece of metal. The graphite cathode is eliminated and acid corrosion of the container does not occur. The alkaline cell is more efficient and can be miniaturized to fit more varied applications. In the alkaline version the half-cell reactions are Zn 2 OH- ------- ZnO H2O 2e- 2 MnO2 2e- H2O ------- Mn2O3 2 OH- The alkaline dry cell lasts much longer as the zinc anode corrodes less rapidly under basic conditions than under acidic conditions. HYPERLINK http.interstatebatteries.comestoredefault.asp Lead storage battery - acid form The normal car battery consists of six 2-volt cells connected in series to produce 12 volts. Each cell uses a plate of lead for the anode. The cathode is PbO2 poder formed into a conducting grid. The electrodes are immersed in dilute H2SO4. Half-reactions for each cell are Anode - Pb s SO4 -2 aq INCLUDEPICTURE httppc65.frontier.osrhe.eduhssciencearro.jpg t MERGEFORMATINET PbSO4 s 2e - Cathode - PbO2 s SO4 -2 aq 4H aq 2e - INCLUDEPICTURE httppc65.frontier.osrhe.eduhssciencearro.jpg t MERGEFORMATINET PbSO4 s 2H2O l Pb s and PbO2 s are converted to PbSO4 s hile the battery is being used. hen the battery is recharged, the overall reaction proceeds in the reverse direction, restoring the reactants. This allos the battery to continue to be used. Most car batteries become useless after 3 to 5 years because side reactions occur that produce a sludge that interferes ith the batterys operation. The amount of charge in this type of battery can be determined by measuring the density of the electrolyte. Sulfuric acid has a density greater than ater. As the battery operates, sulfuric acid is consumed, loering the density of the electrolyte. If the density falls belo 1.2 gcm3, the battery is ready for recharging. Other types of dry cell batteries are the silver battery in hich silver metal serves as an inert cathode to support the reduction of silver oxide Ag2O and the oxidation of zinc anode in a basic medium. The type of battery commonly used for calculators is the mercury cell. In this type of battery, HgO serves as the oxidizing agent cathode in a basic medium, hile zinc metal serves as the anode. Another type of battery is the nickelcadmium battery, in hich cadmium metal serves as the anode and nickel oxide serves as the cathode in an alkaline medium. Unlike the other types of dry cells described above, the nickelcadmium cell can be recharged like the lead-acid battery. EMBED ord.Picture.8 1OSTia.012IoIIaHaHCJaJBOJPJQJaJphOJPJQJaJajaUajCJ
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