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The S-Block Elements

Question
CBSEENCH11007123
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How is sodium hydroxide manufactured? Discuss in brief the details of the process.
Or
With the help of a diagram, show the reactions at the cathode and anode in the manufacture of sodium hydroxide by the Castner - Kellner process.

Solution
In the Castner-Kellner process, sodium hydroxide is manufactured by electrolysis of an aqueous solution of sodium chloride (brine).
NaCl space space rightwards arrow space space space Na to the power of plus space plus space Cl to the power of minus space left parenthesis ionisation right parenthesis At space cathode colon space Na to the power of plus space plus space straight e to the power of minus space rightwards arrow space space Na At space anode colon space space space space space space space Cl to the power of minus space space rightwards arrow space space space Cl space plus straight e to the power of minus space space space Cl space plus space Cl space rightwards arrow space space Cl subscript 2

Castner - Kellner cell consists of a rectangular iron tank. It is divided into three compartments by two non-porous state partitions. The two partitions suspended from the top almost reach the bottom of the cell, without touching it. A layer of mercury is placed at the bottom of the cell. The mercury in one compartment can flow into another compartment but the solutions in the compartment cannot intermix with one another. The cell is provided with an eccentric wheel at its bottom. The bottom layer of mercury can be put in motion with the help of this wheel.

The mercury at the bottom of the cell acts as an intermediate electrode by induction. It serves as the anode in the middle compartment and as a cathode in the outer compartments. These outer compartments are provided with graphite anodes. The saturated brine solution is put in them. The middle compartment contains dilute caustic soda. A series of iron rods fitted in this compartment act as a cathode. On passing electric current, the following reactions occur:

1. In the outer compartments. Sodium chloride solution is electrolyzed. Chlorine is liberated at the anodes. Sodium ions are discharged at the mercury cathode and metallic sodium forms. This combines with mercury forming sodium amalgam (Na-Hg). The sodium amalgam formed is transferred to the central compartment by giving a slight rocking motion to the cell.
                  NaCl space space rightwards arrow space space space Na to the power of plus space plus space space Cl to the power of minus
    At anode:
                Cl to the power of minus space space rightwards arrow space space space Cl space plus space straight e to the power of minus Cl space plus space Cl space rightwards arrow space space Cl subscript 2
At cathode:
               Na to the power of plus space plus space straight e to the power of minus space space rightwards arrow space space Na Na space plus space Hg space rightwards arrow space space space stack Na divided by Mg with Sodium space amalgam below space
2. In the central compartment. Here sodium-amalgam acts as an anode, while the iron rods act as a cathode. The sodium amalgam reacts with water to form sodium hydroxide. Here, following reactions take place:    
At Na-Hg anode. 
           2 Na minus Hg space space rightwards arrow space space 2 Na to the power of plus space plus space 2 Hg space plus space 2 straight e to the power of minus
At iron cathode.
      2 straight H subscript 2 straight O space plus space 2 straight e to the power of minus space space rightwards arrow space space 2 OH to the power of minus space plus space straight H subscript 2 2 Na to the power of plus space plus space 2 OH to the power of minus space rightwards arrow space space 2 NaOH Net space reaction. space space 2 Na divided by Hg space plus space 2 straight H subscript 2 straight O space space rightwards arrow space space space 2 NaOH space plus space straight H subscript 2 plus 2 Hg
Hydrogen escapes out through an outlet at the top. The strength of sodium hydroxide in the central compartment gradually increases, when it reaches a concentration of about 20%, it is removed and evaporated to get solid sodium hydroxide.

Some More Questions From The s-Block Elements Chapter

What is the nature of lithium compounds?

Group 1 elements are called alkali metals. Why?

Why caesium can be used in photoelectric cell while lithium can not be?

In aqueous solution, Li+ ion has lowest mobility. Why?

Lithium has highest ionisation energy in group 1 elements, yet it is strongest reducing agent. Comment.

The softness of group 1 elements increases down the group with increasing atomic number. Give reason. 

Which is the most reactive alkali metal and why?

Why alkali metals do not form M2+ ions?

Name the radio active element in group 1. How does it resemble with the remaining elements of the group?

Why alkali metals are normally kept in kerosene oil?

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