Daniell Cell
Electrochemistry is the study of the production of electricity from energy released during spontaneous chemical reactions and the use of electrical energy to bring about non-spontaneous chemical transformations. The cell below represents Daniell cell which converts the chemical energy liberated during the redox reaction to electrical energy.
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
The reaction is a combination of two half-cell reactions, one called the oxidation half-cell and the other called the reduction half-cell. Each half-cell consists of a metallic electrode dipped into an electrolyte. The two half-cells are connected through a metallic wire and switch externally. The electrolyte of the two half cells is connected internally through a salt bridge. At each electrode-electrolyte interface there is a tendency for metal ions from the solution to deposit on the metal electrode, trying to make it positively charged. At the same time, metal atoms of the electrode have a tendency to go into the solution as ions and leave behind the electrons at the electrode, trying to make it negatively charged. A potential difference develops between the electrode and the electrolyte, which is called electrode potential. When the concentrations of all the species involved in a half-cell is unity, then the electrode potential is known as the standard electrode potential.
For a general half-cell; Mn+(aq) + ne− → M(s), the Nernst equation is written as
\( E_{(M^{n+}/M)}=E^{o}_{(M^{n+}/M)}-\frac{RT}{nF}\ln\frac{1}{[M^{n+}]} \)
where \( E^{o}_{(M^{n+}/M)} \) is the standard reduction potential of the half-cell, R is gas constant (8.314 J K−1 mol−1), F is Faraday constant (96487 C mol−1), T is temperature in kelvin and [Mn+] is the concentration of the species, Mn+.
Why is a salt bridge used in Daniell cell?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → To maintain electrical neutrality
A salt bridge is an inverted U-tube filled with a gel (agar-agar) containing a concentrated inert electrolyte such as KCl or \( \ce{KNO3} \). It connects the two half-cells of the Daniell cell.
As the cell works, \( \ce{Zn^{2+}} \) ions enter the solution at the anode, which would build up a positive charge there. \( \ce{Cu^{2+}} \) ions leave the solution at the cathode, leaving an excess of \( \ce{SO4^{2-}} \) ions and a negative charge. Such charges would soon stop the flow of electrons.
The salt bridge prevents this: its anions move into the anode compartment and its cations move into the cathode compartment. In this way it maintains electrical neutrality of both solutions and completes the internal circuit, so the current can keep flowing.
Why the other options are incorrect
Option (1) To prevent oxidation of zinc — Incorrect. Oxidation of zinc at the anode is the very reaction that produces the current; the salt bridge allows it to continue.
Option (3) To prevent reduction of copper — Incorrect. Reduction of \( \ce{Cu^{2+}} \) at the cathode is the other half of the cell reaction and must take place.
Option (4) To allow easy flow of electrolyte from anode to cathode — Incorrect. The salt bridge keeps the two solutions from mixing; only ions from its own electrolyte move, and electrons flow through the external wire.