Galvanic Cells - Practice Questions & MCQ

It is the device in which the decrease of free energy during the indirect redox reaction is made to convert chemical energy into electrical energy. Luigi Galvani and Alessandro Volta developed such devices therefore these cells are also known as Galvanic cells or Voltaic cell or Redox cells.

Species having a greater tendency of getting oxidised (greater oxidation potential value ) is selected as the anode while species having a greater tendency of getting reduced (greater reduction potential value) is selected as cathode. 

It is, however, conventional to deal in terms of the reduction potentials by default so, species with greater reduction potential is selected as Cathode and the one with lesser reduction potential is selected as cathode.

Galvanic Cell

• 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's cell, electrons flow from zinc electrode to copper electrode through the external circuit while metal ions flow form one-half cell to the other through a salt bridge.
• Here current flows from the copper electrode to zinc electrode that is, cathode to anode in an external circuit.
• Daniel cell is a reversible cell while a voltaic cell may be reversible or irreversible depending upon the e.g. if one of the products is gaseous and escapes, then the cell is not reversible.
   

• On anode side in galvanic cell, the electrons move towards the cathode side and Zn²⁺ ions go to the electrolytic solution. Due to this after some time, the large amount of Zn²⁺ ions accumulates near the negatively charged electrode and thus prevents the flow of the electrons.
• Similarly, on the cathode side, Cu²⁺ ions get the electrons and convert to Cu. Due to this, after some time, a large amount of $\mathrm{SO}_4^{2-}$ ions accumulate near the positively charged electrode. Since $\mathrm{SO}_4^{2-}$ ions are negatively charged, thus they repel the incoming electrons towards the positively charged electrode.

Thus because of this hindrance to the movement of electrons, the current will stop to flow and the cell will not work anymore. Thus to remove this problem in a galvanic cells, we use the salt bridge.

Salt Bridge
Salt bridge is an inverted U-Tube glass vessel. In salt bridge, there is an inert electrolyte like KCl, K₂SO₄, KNO₃, NH₄Cl, etc. This inert electrolyte is mixed with agar-agar solution and gel-like substance is prepared to make the movement of ions easier. 
Now this inverted U-Tube vessel in place in the cell and then Cl^- starts to move towards the anode side or positively charged solution and neutralize it. Similarly, K⁺ ions start to move towards the cathode side or negatively charged solution and neutralize it. Thus after the neutralization of both solutions, electrons will flow and the cell will start working.