Question

In which of the following cells, salt bridge is not needed?
This question has multiple correct options

A.${{Pb|PbS}}{{{O}}_{{4}}}{{(s)||}}{{{{\rm H}}}_{{2}}}{{S}}{{{O}}_{{4}}}{{|Pb}}{{{O}}_{{2}}}{{|Pb}}$
B.${{Cd|CdO(s)|{\rm K}{\rm O}{\rm H}(aq)|{\rm N}i}}{{{O}}_{{2}}}{{(s)|{\rm N}i}}$
C.\[{{Fe(s)|FeO(s)||{\rm K}{\rm O}{\rm H}(aq) - {\rm N}}}{{{i}}_{{2}}}{{{O}}_{{3}}}{{(s)|{\rm N}i}}\]
D.\[{{Zn|{\rm Z}nS}}{{{O}}_{{4}}}{{|CuS}}{{{O}}_{{4}}}{{|Cu}}\]

Answer 1

Hint:Salt Bridge is used for connecting between oxidation half cells and reduction half cells. Salt bridge is made of a strong electrolyte. It prevents the flow of solution from one of half cells to another. If there is no salt bridge used, the reaction may continue and solution in one half cell electrode will get a positive charge and other electrode would accumulate a negative charge on it. In some cases, salt bridges are not necessary. Salt bridge is used to eliminate the liquid junction potential.

Complete step by step answer:
Let us understand when a salt bridge is needed and when not.
Liquid junction potential is due to the speed of ions present in the anodic and cathodic compartments.
When 2 electrodes are immersed in single solution in the same container, salt bridge is not necessary
In galvanic cells, if both the electrolytes are mixed, current cannot be produced. So in order to prevent the mixing we use a salt bridge.
In the given question, Zn and Cu are dipped in the sulphates of the corresponding electrolyte. Therefore, a salt bridge is needed.
Therefore in \[{{Zn|{\rm Z}nS}}{{{O}}_{{4}}}{{|CuS}}{{{O}}_{{4}}}{{|Cu}}\], the electrons are moving very fast from Zn to Cu and the anion is a constant (${{S}}{{{O}}_{{4}}}^{{{2 - }}}$) Zn is more fast moving than the Cu. Therefore a salt bridge is required in this to prevent the liquid junction potential arising due to difference in speeds.
Therefore, the correct answer is option A, B and C.


Note:
If 2 electrodes are dipped in the same solution, then there is no need for a salt bridge to neutralize the charges because the ions can be easily moved through the porous barriers. We could see the use of salt bridges in galvanic cells such as the Daniel cell or Voltaic cell. The function of a salt bridge is to maintain electrical neutrality within the internal circuit.