Question

Write the name of the cell which is generally used in bearing aids. Write the reactions taking place at anode and the cathode of this cell.

Answer 1

Hint: Try to recall the different types of cell mentioned in the chapter Electrochemistry. The cell which can produce a current by an irreversible chemical reaction. Write the chemical reaction involved in each of the cells mentioned above to identify the reactions that are irreversible.

Complete step by step answer:
 The Leclanche cell is a battery invented by the French scientist Georges Leclanche. The battery contains an electrolytic solution of ammonium chloride, a cathode of carbon, a depolarizer of manganese dioxide (oxidizer), and an anode of zinc. In the above cell setup, zinc gets oxidized at the anode and manganese(IV)oxide gets reduced to manganese(III)oxide.
The above cell is a primary cell as the chemical reaction is irreversible. The nickel–cadmium battery (also called Ni-Cd battery or NiCad battery) is a type of rechargeable battery using nickel hydroxide as cathode and cadmium as anode. Nickel-cadmium battery is a secondary cell as the chemical reaction taking place is reversible and hence the cell can be recharged. The lead–acid battery was invented in 1859 by French physicist Gaston Plante and is the earliest type of rechargeable battery.
In lead storage batteries, solid lead is used as anode and lead oxide is used as cathode while discharging and sulfuric acid is the electrolyte. It is a secondary cell. A mercury battery (also called mercuric oxide battery) is a non-rechargeable electrochemical battery. Mercury batteries use a reaction between mercuric oxide and zinc electrodes in an alkaline electrolyte. The chemical reaction happening in mercury cells is irreversible; hence it is a primary cell. From the above explanation we can conclude that the cell which is generally used in bearing aids is the Leclanche cell.
Anode Reaction: $Zn\to Z{{n}^{2+}}+2{{e}^{-}}$
Cathode Reaction: $Mn{{O}_{2}}+NH_{4}^{+}+{{e}^{-}}\to MnO(OH)+N{{H}_{3}}$

Note: The standard emf of a cell can be found using the standard electrode potentials of the anode and cathode. However, when the concentration or activity of the compounds is not unity, we find the emf of the cell using Nernst equation.