The s-Block Elements

  • Question 145
    CBSEENCH11007123

    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.

    Question 146
    CBSEENCH11007124

    Name a few important uses of sodium hydroxide. 

    Solution

    Sodium hydroxide is used:
    (i) in the manufacture of metallic sodium, artificial silk and dyes.
    (ii) in the refining of petroleum and as a reagent in the laboratory. 
    (iii) in soap, textile and paper industries.
    (iv) in the preparation of soda lime (NaOH + CaO).
    (v) for preparing bleaching agents like sodium hypochlorite and for the purification of bauxite.

    Question 147
    CBSEENCH11007125

    Complete the following reactions:
    left parenthesis straight i right parenthesis space straight H subscript 2 left parenthesis straight g right parenthesis space plus space straight M subscript straight m straight O subscript straight o left parenthesis straight s right parenthesis space space rightwards arrow with increment on top
left parenthesis ii right parenthesis space CO left parenthesis straight g right parenthesis space plus space straight H subscript 2 left parenthesis straight g right parenthesis space space rightwards arrow with increment on top
left parenthesis iii right parenthesis space straight C subscript 3 straight H subscript 8 left parenthesis straight g right parenthesis space plus space 3 straight H subscript 2 straight O left parenthesis straight g right parenthesis space rightwards arrow from Catalyst to increment of
left parenthesis iv right parenthesis space Zn left parenthesis straight s right parenthesis space plus space NaOH left parenthesis aq right parenthesis space rightwards arrow with heat on top

    Solution
    left parenthesis straight i right parenthesis space OH subscript 2 left parenthesis straight g right parenthesis space plus space space straight M subscript straight m straight O subscript straight o left parenthesis straight s right parenthesis space space rightwards arrow space space space mM left parenthesis straight s right parenthesis space plus space oH subscript 2 straight O left parenthesis straight l right parenthesis
left parenthesis ii right parenthesis space CO left parenthesis straight g right parenthesis space plus space 2 straight H subscript 2 left parenthesis straight g right parenthesis space rightwards arrow from Catalyst to increment of space stack CH subscript 3 OH left parenthesis straight l right parenthesis with Methanol below
left parenthesis iii right parenthesis space straight C subscript 3 straight H subscript 8 left parenthesis straight g right parenthesis space plus space 3 straight H subscript 2 straight O space left parenthesis straight g right parenthesis space rightwards arrow from 1270 space straight K to Ni of space 3 CO left parenthesis straight g right parenthesis space plus space 7 straight H subscript 2 left parenthesis straight g right parenthesis
left parenthesis iv right parenthesis space Zn left parenthesis straight s right parenthesis space plus space 2 NaOH left parenthesis aq right parenthesis space rightwards arrow with heat on top space Na subscript 2 ZnO subscript 2 left parenthesis aq right parenthesis space plus straight H subscript 2 left parenthesis straight g right parenthesis
    Question 148
    CBSEENCH11007126

    Explain what happens when:
    (i) Sodium hydrogen carbonate is heated
    (ii) Sodium amalgam reacts with water
    (iii) Fused sodium metal reacts with ammonia? 

    Solution

    (i) When sodium hydrogen carbonate is heated, sodium carbonate is formed.
       2 NaHCO subscript 3 space rightwards arrow with Heat on top space space Na subscript 2 CO subscript 3 space plus space CO subscript 2 space plus space straight H subscript 2 straight O
    (ii)  Sodium amalgam reacts with water liberating hydrogen gas. 
    (iii) When ammonium is passed through molten sodium, it yields sodamide evolving Hgas.
    2 Na plus 2 NH subscript 3 space rightwards arrow with 475 space straight K on top space 2 NaNH subscript 2 space plus space straight H subscript 2

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