Question

Among the following cells, which of the following are primary cells?
(I) Leclanche cell
(II) Nickel-Cadmium cell
(III) Lead storage battery
(IV) Mercury cell

(A) I and II
(B) I and III
(C) I and IV
(D) III and IV

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 in the year 1866. 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 primary cells are Leclanche cell and mercury cell.
So, the correct answer is “Option C”.

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. The Nernst equation is given below:
$E\text{ = }{{E}^{0}}\text{ }-\text{ }\dfrac{RT}{nF}\ln \dfrac{[\text{Products }\!\!]\!\!\text{ }}{[\text{Reactants}]}$

Where,
${{E}^{0}}$ is the standard emf of the cell,
R is the universal gas constant,
T is the temperature at which the reaction takes place,
n is n-factor of the reaction,
F is the amount of charge present in 1 mole of electrons.