Electrolysis of Molten Salts

The term ‘electrolysis’ comprises two terms–‘electro’ (electrons or electricity) and ‘lysis’ (breaking apart). We can define electrolysis as the process of decomposing a chemical compound into its elemental substance or other chemicals by passing an electric current through it. In this process, ions move. Salts in their solid form do not have mobile ions as they are held together by strong ionic forces. However, at high temperatures, they break down (in their molten form) into their respective ions that are mobile. So, electrolysis of molten salts means a process where electricity is passed through a salt’s liquid form (molten form) in which ions are present to decompose it into its elemental constituents.

• In electrolysis, we make a non-spontaneous reaction occur with the help of a battery that provides electric current from outside that makes the reaction feasible.

• Products are less stable than the reactants, and hence, the reaction is non-spontaneous and needs energy from an external source. The energy difference is provided by the external supply (battery). 

• Electrolysis of molten salts is important because we can have those substances in their elemental forms that are highly stable, and it is challenging to break them into their constituents.

Common Terms

• Electrolyte: Substance that contains ions (salt to be electrolysed)

• Electrode: Two types of electrodes are there- Cathode and Anode

• Electrolytic cell: Electrolysis of molten salts takes place in an electrolytic cell. An electrolytic cell converts electrical energy into chemical energy. Electric energy from the battery makes a non-spontaneous reaction (chemical energy) feasible. 

• Electrolytic solution: Mixture that contains ions that are mobile and undergo electrolysis. In other words, the molten form of salt (in case of electrolysis of molten salts) or its aqueous solution (in case of electrolysis of an aqueous solution of salts)      

Key Points

• We use a direct current power source in an electrolytic cell.

• Electrolysis takes place in the same electrolytic solution. 

• An anode is positively charged. Anions move towards the anode. Oxidation takes place at the anode.

• A cathode is negatively charged. Cations move towards the cathode. Reduction takes place at the cathode.

• A salt bridge is not required.

• Different products are obtained depending on whether the electrodes used in the process are inert or active.

• The supply of electrons is via an external battery. Electrons come in through the cathode and exit via the anode.  

Electrolysis of Molten Salts: Example of NaCl

Molten NaCl has Na+ and Cl– ions. These are mobile ions as salt in molten form. When a current is passed Na+ (positively charged) moves towards the cathode (negatively charged electrode) and Cl– (negatively charged) moves towards the anode (positively charged electrode).

• Oxidation reaction at the anode

Cl- ion gives one electron to the anode and oxidises to Cl. Then two Cl atoms combine to form chlorine gas.

 2Cl– (l)    →    2Cl (g)  + 2e–

  2Cl (g)   →     Chlorine gas                (at anode)

• Reduction reaction at the cathode

Na+ ions take the electron from the cathode and reduce it to sodium metal.

Na+ (l)   +  e–    →     Na (l)              (at cathode)

• Net reaction that takes place in the electrolysis

2Na+(l)    +   2Cl–(l)     →     2Na(l)  +   Cl2

• Products of electrolysis of molten NaCl 

Chlorine gas is released at the anode, and sodium metal is produced at the cathode.

Products of Electrolysis of Molten Salts

• Zinc chloride  

At anode: Chlorine gas

At cathode: Zinc deposition

• Copper chloride  

At anode: Chlorine gas

At cathode: Copper deposition

• HCl

At anode: Chlorine gas

At cathode: Hydrogen gas

• NaBr

At anode: Bromine gas

At cathode: Sodium deposition

Faraday’s Laws of Electrolysis

Faraday’s Laws of Electrolysis shows how the substance deposited at electrodes is related to the quantity of electric current passed. 

1. The first law states that the weight of the substance deposited at an electrode is directly proportional to the amount of electric current passed.

2. The second law states that when the same amount of electric current is passed through different electrolytic solutions, the weight of the substance deposited at an electrode is directly proportional to the chemical equivalent of the substance.  

Conclusion

Electrolysis of molten salts is a process in which electric current is passed through the molten form of salt to decompose it into its chemical constituents. It takes place in an electrolytic cell where no salt bridge is required. The anode is positively charged. The cathode is negatively charged. Products of the electrolysis of molten salts are different from the products we obtain when the aqueous solution of the same salts undergo electrolysis. It is the basis of various industrial processes where impure elements of very high reactivity are converted to a pure state. Examples are sodium, copper, etc.