Salt Bridge - Practice Questions & MCQ

It maintains electrical neutrality in two compartments by allowing movement of anions towards anodic compartment and cations towards cathodic compartment.

• It is a glass tube having KCl, KNO₃, ammonium nitrate in a gelatin gel or agar-agar paste.
• The gelatin gel allows ionic movement through it but prevents any kind of mixing.
• In the case of KCl or ammonium nitrate, the ionic mobility of cation and anion are same.

Function of a Salt Bridge

• A salt bridge acts as an electrical contact between the two half-cells.
• It prevents mechanical flow of solution but it provides free path for the migration of ions to maintain an electric current through the electrolyte solution. It prevents the accumulation of excess charges.
• A salt bridge helps in maintaining the charge balance in the two half cells.
• A salt bridge minimizes/eliminates the liquid junction potential.
  Liquid Junction Potential: The unequal rates of migration of the cations and anions across a liquid-liquid junction give rise to a potential difference across the junction. This potential difference across the liquid-liquid junction is called liquid junction
  potential.

NOTE: If salt bridge is removed the emf of the cell drops to zero.

• Sometimes, both the electrodes are dipped into the some electrolytic solution. in such a case, there is no requirement of salt bridge.

• The Daniel cell is a typical galvanic cell. It is designed to make use of the spontaneous redox reaction between zinc and cupric ion to produce an electric current.
• The Daniel cell can be conventionally represented as

$\begin{gathered}\mathrm{Zn}(\mathrm{s})\left|\mathrm{ZnSO}_4(\mathrm{aq})\right|\left|\mathrm{CuSO}_4(\mathrm{aq})\right| \mathrm{Cu}(\mathrm{s}) \\ \text { Saltbridge }\end{gathered}$

• The Daniel cell reaction is represented as
  $\mathrm{Zn}(\mathrm{s})+\mathrm{Cu}^{2+}(\mathrm{aq}) \rightarrow \mathrm{Zn}^{2+}(\mathrm{aq})+\mathrm{Cu}(\mathrm{s})$
• In Daniel cell, electrons flow from zinc electrode to copper electrode through external circuit while metal ions flow form one half cell to the other through salt bridge.
• Here current flows from copper electrode to zinc electrode that is, cathode to anode in external circuit.
• Daniel cell is a reversible cell while a voltaic cell may be reversible or irreversible.
  A voltaic cell is reversible only when it satisfies following conditions:
• The emf of the external source is more than that of a voltaic cell so that current may flow from the external source into the voltaic cell and cell reaction can be reversed.
• If the emf of the voltaic cell is more than that of the external source current flows from the voltaic cell into the external source.