Electrochemical and Electrolytic Cell

Electrolytic Cell

In an electrolytic cell, electrical energy is converted into chemical energy and a non-spontaneous chemical reaction is carried out.

• The process is executed with the help of direct current.
• It can also be stated as a process in which free ion solutions are broken down.

Actual life usage of electrolytic cells: Extraction and refinement of metals, chlorine manufacturing, etc.

Electrolyte: All substances cannot conduct electricity, electrolytes are those whose atoms are closely bonded, but when dissolved in water, the molecules split into free positively charged ions and negatively charged ions called cation and anion, respectively.

• Electrolytes can be an aqueous solution of acid, alkali, or molten metal solutions.
• These can be classified into three categories: weak, strong and nonelectrolytes.

Electrodes: Rods made up of inert conducting materials that allow the passage of electricity through an electrolytic solution are called electrodes; generally, metal rods are used in cells.

• Cathode refers to the rod connected to the negative terminal of the electric source. 
• Anode refers to the rod connected to the positive terminal of the electric source.

In an electrolytic cell, upon partially immersing two electrodes and applying an electric potential difference, In contrast, the free negative ions move towards the anode.

The positive ions get electrons from the cathode and are reduced, whereas the negative ions give electrons to the anode and are oxidised.

Electrolysis of Molten NaCl

Electrolyte: Molten NaCl, conductor of electricity.

• NaCl in solid form is inactive, and the atoms are closely linked. 
• Alternatively, molten NaCl consists of Na+ and Cl- atoms.

Electrode: A set of inert electrodes

When an electric current is applied to molten NaCl, the Na+ ions move to the cathode and Cl- ions get attracted to the anode.

Liquid Na is produced at the cathode (-)    2Na+ + 2e- 🡪 2Na

Gaseous Cl2 is produced at the anode (+) 2Cl-  🡪 Cl2 + 2e-

2Na+ + 2Cl-  🡪 2Na + Cl2

Na(l)  🡪 Na(s) 

This reaction is non-spontaneous below 801 degrees Celsius.

Electrochemical Cell 

Those cells in which the conversion of chemical energy to electrical energy takes place via the aid of the energy generated by a redox reaction are called electrochemical Cells.

These are also called galvanic cells. In these cells, both the electrodes are dipped in different salt solutions, the transfer of electrons occurs through the wire connecting the electrodes.

Both the containers are connected using a salt bridge.

At anode: Electrons are liberated

At cathode: Electrons are received

Half-cell: The cell is divided into two halves; each half consists of two different electrolytes

It consists of two electrodes at the anode of the oxidation part of the reaction, whereas at the cathode, the reduction part of the reaction occurs.

A conducting fluid separates the two electrodes. 

1. Electrochemical cells are used in various vital batteries, such as Lead-Acid batteries.
2. Fuel cells are also a type of electrochemical cell that can produce clean energy.

Example of electrochemical cells:

Copper Cu(s) in a silver nitrate solution AgNO3(s) is an example of an electrochemical cell.

In this reaction, AgNO3 splits into Ag+ and NO3- ions. 

Upon the introduction of the copper electrode into the solution that contains silver ions Ag+ (aq), Cu(s) oxidises to Cu2+(aq), and Ag(aq) is reduced to Ag(s). 

When electrons are gained at an electrode or reduction takes place, that electrode is called the cathode, whereas when electrons are lost, or oxidation takes place, that electrode is called an anode.

In the above example, according to the reactions, the cathode is the copper electrode, and the anode is the silver electrode.

Difference between Electrolytic and Electrochemical Cell 

1. Basic Principle: Electric energy is converted to chemical energy in electrolytic cells. In electrochemical cells, the opposite occurs, and chemical energy gets converted to electrical energy.
2. Reaction Type: In electrolytic cells, a non-spontaneous reaction occurs, while the basis for electrochemical cells is a spontaneous redox reaction.
3. Driving Force: In an electrolytic cell, an external source of electricity is required to cause the reaction, while no external source is needed to drive electrochemical reactions.
4. Oxidation Reaction: Oxidation occurs at the cathode in electrolytic cells, while oxidation occurs at the anode in electrochemical cells.
5. Reduction Reaction: Reduction occurs at the anode in an electrolytic cell, while reduction occurs at the cathode in an electrochemical cell.
6. The cathode is negative, and the anode is positive in electrolytic cells, while the reverse is true for electrochemical cells.
7. Electrolyte: In an electrolytic cell, there is only one electrolyte for both the electrodes, while in an electrochemical cell, there are two types of electrolytes, different for both the electrodes.
8. Electrodes: In electrolytic cells, there is only one electrode type, but in electrochemical cells, there are two different types of electrodes.
9. Salt Bridge: In electrochemical cells, the two halves are connected by a salt bridge, while there is no such bridge in electrolytic cells.
10. Electron generation: In an electrochemical cell, electrons are generated due to the reaction occurring in the cell. In electrolytic cells, electrons are generally required to generate electrons.

Conclusion

Electrochemical cells can operate like electrolytic cells if the external reverse potential is applied to the galvanic cell. The reaction is not suppressed until the reverse potential reaches 1.1 V. In this case, no current flows through the cell. When the external potential rises further, the reaction acts in the opposite direction.