________________
The formation of ZnSO4 on the porus pot does not affect the working of the cell, until crystals of ZnSO, are deposited along its walls.
As the Cu ions deposit on the copper vessel, it acquires positive charge. Due to building up of the positive charge on the copper vessel and negative charge on the zinc rod, the potential difference between the two poles of the cell goes on increasing. Leclanche Cell
It was invented by George Leclanche in the year 1865. Construction. It consists of a glass vessel con (NHCl) as electrolyte. An amalgamated zinc rod dipping in the NH Cl solution acts as the negative pole of the cell. The use of amalgamated zinc rod avoids the defect of local action from occurring in the cell. A porus pot containing the carbon rod is placed inside the NH Cl solution. The carbon rod acts as the positive pole of the cell. The empty space in the porus pot is filled with manganese dioxide (MnO,) and charcoal powder Manganese dioxide is used as depolariser. The use of charcoal powder makes MnO, conduction and thus, decreases the internal resistance of the cell.
Action. The electrolyte HN,Cl solution dissociates into NH,+ and Cl- ions. As zinc rod is dipping inside the NHCl solution, some of the atoms go into the solution as Zn+ ions. For each Zn** ion so produced, two electrons are left on the zinc rod and hence it becomes negative pole of the cell. The NH, ions are repelled by Zn** ions and they diffuse into the MnO, and charcoal mixture through the walls of the porus pot. Inside the glass vessel, Zn** ions combine with Cl- ions to form ZnCl2.
On diffusing into porus pot, NH+ ions extract electrons from the carbon rod making carbon rod as positively charged and producing ammonia and hydrogen gas.
Whereas the ammonia gas escapes from the cell, hydrogen gas is neutralised by MnO, producing manganese trioxide (Mn,O,) and
water.
Secondary Cells
A secondary cell is one in which chemical energy is converted into electrical energy but they do so when they are charged by passing current through them by some source.
These are also called accumulators or storage cells. These are of two types:
Jain Educationa International
248
For Personal and Private Use Only.
www.jainelibrary.org
