What is a primary cell ? Describe the construction and working of dry

1.	What is a primary cell ? Describe the construction and working of dry cell,
Answer» Solution :Primary CELLS are those in which the electrode reactions cannot be reversed by an external electric energy source. In these cells, the reaction occurs only once and after use they BECOME dead. Therefore, they are not chargeable. (a) Dry cell. The dry cell was invented by G. Leclanche. It consists of a zinc cylinder. This is filled with a paste of `NH_(4)Cl` and little of `ZnCl_(2)` in the form of paste in water. The zinc cylinder ACTS as anode. The cathode is a carbon rod. The carbon rod is surrounded by a black paste of manganese dioxide `(MnO_(2))` and carbon powder. When the cell is operating, zinc loses electrons and `Zn2^(+)` ions dissolve in the electrolyte. The electrons pass. around the external circuit and are taken up at cathode. This causes DISCHARGE of `NH_(4)^(+)` ions from the electrolyte. The reactions occurring at the electrodes are given below : At anode `ZntoZn^(2+)+2e^(-)` At cathode `2NH_(4)^(+)(aq)+2MnO_(2)(s)+2e^(-)toMn_(2)O_(3)(s)+NH_(3)+H_(2)O` It gives a voltage of about 1.5 V. (ii) Uses. The dry cells are used in torches, toys, flash lighters, calculators, tape-recorders and in many other devices.

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