Simple Cells:
Two rods of different metals (called electrodes) dipped in a chemical solution (called electrolyte) constitute a simple cell. These are called the primary cells in which the electrodes are gradually consumed and the action of a cell is stopped after some time. It cannot regain back its original state and becomes useless.
In the simplest primary cell, a glass vessel consisting of dilute sulfuric acid has two electrodes of zinc and copper metals. Zinc atoms, released in the solution leave two electrons and become doubly ionized as-
| Zn ———-> Zn++ + 2 electrons ……….(i) |
In turn, the two electrons released by the Zn atom cling to the Zn rod and make it negatively charged. Now, if the electrodes are externally connected by a wire, the two H+ ions released by H2SO4 molecule travel to the copper rod and give the positive charges to it. Thus, the copper rod becomes positively charged. The electrons accumulated on the Zn rod travel through the external wire to the copper rod in order to equalize the difference of potential between the two electrodes. But fresh Zn atoms go into the solution to give more electrons to the Zn rod; thus, the potential difference is maintained. Thus, a steady flow of electrons from Zn to Cu is maintained while Zn atoms are continuously consumed and the quantity of ZnSO4 salt keeps on increasing in the solution. Electrons reaching copper rod neutralize the positive charges given by hydrogen ions. This can be written in reaction form as-
| 2H+ + 2 electrons ———-> H2 ……….(iv) |
Thus, free hydrogen molecules are formed which go out into the atmosphere.
As the negative charges (electrons) drift from the negative terminal (zinc) to the positive one (copper), the conventional current I is said to be flowing from the positive terminal of the cell to the negative one through the external wire (or external circuit). As long as the current keeps on flowing, Zn atoms from the zinc rod keep on going into the solution. Thus the zinc rod is being consumed continuously as long as the current is drawn out in the external circuit.
The copper rod is at a positive potential of 0.46 volt with respect to the electrolyte while the zinc rod is at a negative potential of 0.62 volt with respect to the electrolyte. Thus, the potential difference between the two electrodes is maintained at 0.46 – (-0.62) = 1.08 volt. As the current keeps on flowing in the external circuit, the chemical reaction mentioned above is continued to maintain this 1.08 volt potential difference. When the external circuit is removed, still the chemical reaction inside the cell takes place till the potential difference of 1.08 volts is reached. The more we use the cell by drawing current in an external circuit, the more zin is consumed and ultimately a stage comes when no more zinc remains available for the reactions to be maintained, thus the cell stops functioning. Such a cell has, therefore, a definite life. The reactions are not reversible and thus the life of the cell may not be increased. Such a cell is called a primary cell.
Examples of Simple Cell:
We will describe some of the simple cells normally used in laboratories.
(1) Daniel Cell- It consists of a copper vessel that serves as the positive electrode. This vessel contains copper sulfate solution which acts as a depolarizer. A porus pot (P) containing dilute sulfuric acid is immersed in the copper sulfate solution as shown in the figure. An amalgamated zinc rod dipped in dilute H2SO4 serves as the negative electrode. A perforated shelf (S) at the top of the copper vessel is filled with CuSO4 crystals so as to keep the concentration of CuSO4 solution in the copper vessel constant.
(2) Leclanche Cell- It consists of a glass vessel (G) containing a concentrated solution of ammonium chloride as an electrolyte. An amalgamated zinc rod is dipped in NH4Cl solution and this serves as the negative electrode. A porus pot P is also dipped in NH4Cl solution that contains a carbon rod (the positive electrode) with manganese dioxide and charcoal powder filled around. MnO2 acts as a depolarizer and charcoal powder just helps to make manganese dioxide electrically conducting.
As usual, Zn atoms go into the solution of NH4Cl in ionized from as-
| Zn ———-> Zn++ + 2 electrons ……….(vi) |
Simultaneously, NH4Cl molecules in solution release H+ and Cl– ions along with ammonia gas as-
| NH4Cl ———-> NH3 + H+ + Cl– ……….(vii) |
The Zn++ ions and Cl– ions remain in the solution with low mobility as parts of the ZnCl2 molecule.
| Zn++ + 2Cl– ———-> ZnCl2 ……….(vii) |
While the hydrogen ions drift to the carbon rod to make it the positive electrode. The emf of this cell has a final value of 1.45 volts. The layers of hydrogen ions over the carbon rod causing polarization are oxidized by the depolarizer as-
| 2H+ + 2MnO2 ———-> Mn2O3 + H2O + 2 units of positive charge ……….(ix) |
The depolarizing action is comparatively slow due to the usage of a solid depolarizer. Thus, if the cell is used for a long time, the accumulation of hydrogen will increase, and polarization will cause a reduction of emf. If the external circuit is disconnected for some time, the accumulated hydrogen layer gets time to be disposed of and thus the emf gets back its original value of 1.45 volts. Thus, the Leclanche cell is better used where an intermittent supply of current is required, like in electric bells, telephones, electric balancing experiments, etc.
(3) Dry Cell- It is a modified and miniature form of the Leclanche Cell. It is easily portable and a convenient source of direct current.
Construction- It consists of a cylindrical zinc vessel containing a moist paste of Plaster of Paris, ammonium chloride, and zinc chloride. The purpose of adding zinc chloride is to maintain the moistness of the paste because zinc chloride is highly hygroscopic. The zinc vessel is the negative electrode (cathode) and ammonium chloride is the exciting fluid i.e., electrolyte.
The anode (positive electrode) of the cell is a carbon road placed in the middle of the zinc vessel. The carbon rod is surrounded by a mixture of manganese dioxide and powdered carbon. While manganese dioxide acts as the depolarizer, the carbon powder reduces the internal resistance of the cell. The carbon rod is fitted with a brass cap. This brass cap is treated as the positive terminal of the cell. The carbon rod is effectively insulated from the bottom of the zinc vessel by means of a tar paper washer.
The top of the cell has a layer of sawdust which acts as a bed for a sealing layer of bitumen. The gases formed by chemical action escape via the vents in this layer.
Action-
At Cathode: MnO2 + NH4+ + e– ———-> MnO(OH) + NH3
{Oxidation state of Mn decreases from +4 to +3}
At Anode: Zn (s) ———-> Zn+2 + 2e–
The Zn+2 ions combine with NH3 liberated at the cathode to form a diammine Zn (II) cation.
Zn+2 + 2NH3 ———> [Zn(NH3)]+2
Dry cells do not have a long life as acidic NH4Cl corrodes the Zn container even if the cell is not in use. The cell potential of dry cells lies in the range of 1.25-1.5 V.
Uses- They are used in torch lights, transistor sets, electric bells, and many electronic circuits.
(4) Standard Cell (Weston Type)- The emf of a cell usually decreases with continuous use. However, special cells have been designed in which the emf practically remains constant for several years, provided they are used with care. Such cells are known as standard cells, they serve as the standard for the comparison of emf’s. It should be noted that such cells are not meant for supplying currents, hence current should not be drawn from a standard cell.
One of the standard cells is known as the Weston Cadmium cell. It is commonly used in potentiometric circuits. Its emf at 20°C is 1.0183 V. It consists of an H-shaped sealed glass vessel with a horizontal tube serving as the connecting link. In this cell, a cadmium amalgam forms the negative electrode whilst mercury is the positive electrode. The electrolyte is a saturated solution of cadmium sulfate and mercurous sulfate is the depolariser.
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