Dry cell
Dry cell (plural dry
cells) noun
part of a battery:
a current-generating electric cell that cannot be regenerated and contains an
electrolyte in the form of a paste or within a porous material to keep it from
spilling.
The
most common form of primary cell is the Leclanché cell, invented by the French
chemist Georges Leclanché in the 1860s. It is popularly called a dry cell or
flashlight battery. The Leclanché cell in use today is very similar to the
original invention. The electrolyte consists of a mixture of ammonium chloride
and zinc chloride made into a paste. The negative electrode is made of zinc, as
is the outside shell of the cell, and the positive electrode is a carbon rod
surrounded by a mixture of carbon and manganese dioxide. The Leclanché cell
produces about 1.5 V.
Dry Cell
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The functional elements of a dry cell battery are the negative terminal (a zinc can which encloses the battery materials), the positive terminal (the carbon rod and carbon and manganese dioxide mixture that surrounds the rod), and an electrolyte paste between the two terminals. The electrolytic paste facilitates a chemical reaction involving the constituents of both terminals; this reaction causes a current to flow 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 Zn(s) --> 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, which consumes MnO2, is best written as:
[Mn4+ + 2O2-] + H2O + e- -->
[Mn3+ + O2- +
Where the square brackets indicate the species present in the solid phase at the cathode. The cathode reaction actually occurs within 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 power 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 power when 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 will not function once their chemicals are used up and the cells have to be thrown away. 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-cadmium cell.
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.
How to make a dry cell
Dry cells are one of the most commonly used household objects. We use dry cells in watches, torches, transistors, walkmans and even the remote controls of our TVs. Dry cells provide the necessary electricity required to power 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. When the both ends of the cell are z connected to a bulb through a wire, the bulb glows due to the flow of current. The voltage of such a cell is about 1.5 volt. Let us now make a dry cell at home.
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Material Required
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Procedure Make some starch paste by mixing some starch and water and then boiling it. Add sufficient quantity of manganese dioxide to the starch paste, making a very thick paste of manganese dioxide. Spread this manganese dioxide paste
evenly on the zinc plate. Now take some cotton wool and flatten it to fit the
shape of the zinc plate. Soak this cotton wool in ammonium chloride solution.
Now add another layer of manganese dioxide paste over the cotton wool. Now put the
carbon plate over this layer of manganese dioxide and your dry cell is ready
to use. The forms of dry cells Dry cell - acid form:
Half-reactions for this cell are: Anode - Zn (s) Cathode - 2NH4+ (aq) + 2MnO2
(s) + 2e - Remember that the term 'battery' refers to two 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 powder 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 The alkaline dry cell lasts much longer as the zinc anode corrodes less rapidly under basic conditions than under acidic conditions.
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 powder formed into a conducting grid. The electrodes are immersed in dilute H2SO4. Half-reactions for each cell are: Anode - Pb (s) + SO4 -2
(aq) Cathode - PbO2 (s) + SO4 -2
(aq) + 4H + (aq) + 2e - Pb (s) and PbO2 (s) are converted to PbSO4 (s) while the battery is being used. When the battery is recharged, the overall reaction proceeds in the reverse direction, restoring the reactants. This allows 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 with the battery's 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 water. As the battery operates, sulfuric acid is consumed, lowering the density of the electrolyte. If the density falls below 1.2 g/cm3, the battery is ready for recharging. Other types of dry cell batteries are the silver battery in which 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, while zinc metal serves as the anode. Another type of battery is the nickel/cadmium battery, in which 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 nickel/cadmium cell can be recharged like the lead-acid battery. |
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Zn+2
+ 2e - 
Lead
storage battery - acid form: 
