Primary cells are not rechargeable and can be used only once. They have to be disposed off once they are exhausted or fully discharged. Examples of primary cells are dry cells, alkaline cells and mercury cells. Secondary cells are rechargeable when they are exhausted and can be used again and again. Secondary cells can be recharged by passing an electric current through them in the opposite direction to the current flow during discharged or used. Examples of secondary cells include lead-acid accumulator, nickel-cadmium cell and lithium-ion cells.
A dry cell consists of a carbon rod(cathode) and a metal container made of zinc(anode). The electrolyte is ammonium chloride in the from of a paste. The carbon rod is surrounded by a mixture of carbon powder and manganese (iv) oxide, which is again surrounded by ammonium and zinc chloride paste. Carbon powder is a good electric conductor and it reduces the resistance in the cell. Carbon in powder form also increases the surface area of the carbon electrode.
When the dry cell is in use, the zinc metal releases electrons and is ionised to form Zn2+ ions. Thus, zinc acts as the reducing agent. At the anode:
Zn(s) --> Zn2+(aq) + 2e-
Electrons flow from the zinc metal container through the external circuit to the carbon rod where NH4+ ions receive electrons to produce ammonia gas and hydrogen gas. Thus, ammonium ions act as the oxidising agent. At the cathode:
2NH4+(aq)+ 2e- --> 2NH3(aq)+ H2(g)
The hydrogen produced surrounds the carbon rod and acts as an insulator, reducing the effiency of the cell. This effect is called polarization. The function of manganese(iv) oxide, MnO2 is to reduce cell polarisation. Manganese(iv) oxide oxidises the hydrogen gas and minimises the formation of gas bubbles at the carbon rod when the cell is in use.
The hydrogen produced surrounds the carbon rod and acts as an insulator, reducing the effiency of the cell. This effect is called polarization. The function of manganese(iv) oxide, MnO2 is to reduce cell polarisation. Manganese(iv) oxide oxidises the hydrogen gas and minimises the formation of gas bubbles at the carbon rod when the cell is in use.
2MnO2 + H2 --> MnO3 + H2O
Therefore, the overall reaction is:
When the cell produces electric current, zinc metal dissolves. When the zinc metal container is perforated and the electrolyte leaks out, the dry cell cannot be used anymore.
Therefore, the overall reaction is:
Anode(-) : Zn(s) --> Zn2+(aq) + 2e-
Cathode(+) : 2NH4+(aq)+ 2e- --> 2NH3(aq)+ H2(g)
Overall : Zn(s) + 2NH4+(aq) --> Zn2+(aq) + 2NH3(aq)+ H2(g)
Cathode(+) : 2NH4+(aq)+ 2e- --> 2NH3(aq)+ H2(g)
Overall : Zn(s) + 2NH4+(aq) --> Zn2+(aq) + 2NH3(aq)+ H2(g)
When the cell produces electric current, zinc metal dissolves. When the zinc metal container is perforated and the electrolyte leaks out, the dry cell cannot be used anymore.
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