Galvanic Cells - Practice Questions & MCQ

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Standard eletrode potential for Sn⁴⁺/ Sn²⁺couple is +0.15 V and that for the Cr³⁺/ Cr couple is -0.74 V. These two couples in their standard state are connected to make a cell. The cell potential will be:

+1.19 V

+0.89 V

+0.18 V

+1.83 V

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Concepts Covered - 2

Galvanic Cells

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.

Feature	Cathode	Anode
Sign	Positive due to consumption of electrons	Negative due to release of electrons
Reaction	Reduction	Oxidation
Movement of electrons	Into the cell	Out of cell

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
$\\\mathrm{Zn}(\mathrm{s})\left|\mathrm{ZnSO}_{4}(\mathrm{aq})\right|\left|\mathrm{CuSO}_{4}(\mathrm{aq})\right| \mathrm{Cu}(\mathrm{s})\\\mathrm{ \quad \quad \quad \quad \quad \: \: \: \: Saltbridge}$
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.

Electrochemical Cell	Electrolytic Cell
It is a combination of two half cells, containing the same or different electrodes in the same or different electrolytes.	It is a single cell containing the same electrodes present in the same electrolyte.
The anode is negative, the cathode is positive	The anode is positive, the cathode is negative
Electrons move from anode to cathode in the external circuit.	Electrons enter through cathode and leave through the anode.
It converts chemical energy into electrical energy, produced as a result of a redox reaction.	It converts electrical energy into chemical energy. Energy is supplied to the electrolytic solution to bring about the redox reaction.
The cell reaction is spontaneous.	Cell reaction is non-spontaneous.
Salt bridge is required.	No salt bridge is required.

Problem in Galvanic Cells and Salt Bridge

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, large amount of Zn²⁺ ions deposit on the negatively charged electrode and thus prevent the flow of the elctrons.
Similarly, on the cathode side, Cu²⁺ ions get the electrons and convert to Cu. Due to this, after some time, large amount of SO^2-₄ ions deposit on the positively charged electrode. Since SO^2-₄ ions are negatively charged thus they repel the incoming electrons towards the positively charged electrode.

Thus because of this prevention of electrons movement, the current will stop to flow and the cell will not work anymore.

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