The s-Block Elements

  • Question 181
    CBSEENCH11007159

    Describe briefly the preparation and uses of magnesium chloride (MgCl2.6H2O).

    Solution

    Preparation:
    (i) In the laboratory, magnesium chloride is prepared by the action of hydrochloric acid on magnesium oxide or carbonate. 
             MgO space plus space 2 HCl space rightwards arrow space space MgCl subscript 2 space plus space straight H subscript 2 straight O
MgCO subscript 3 space plus space 2 HCl space rightwards arrow space space MgCl subscript 2 space plus space CO subscript 2 space plus space straight H subscript 2 straight O
    The reaction mixture is cooled and concentrated when crystals of MgCl2.6H2O separate out.
    (ii) From carnallite: The mineral carnallite (KCl.MgCl2.6H2O) is powdered and then boiled with water. On cooling KCl is crystallised out while magnesium chloride is left in the mother liquor. The mother liquor is separated, concentrated by evaporation and cooled when crystals of MgCl2.6H2O separate out.
    (iii) From sea water: Sea water is concentrated and then treated with lime when magnesium hydroxide gets precipitated. 
          stack MgCl subscript 2 with Sea space water below space plus space Ca left parenthesis OH right parenthesis subscript 2 space space rightwards arrow space space space stack Mg left parenthesis OH right parenthesis subscript 2 with ppt. below space plus space CaCl subscript 2
    The precipitate of Mg(OH)2 is dissolved in HCl when a solution of magnesium chloride is obtained. The solution of magnesium chloride on concentration and cooling yields crystals of MgCl2.6H2O.
    Mg left parenthesis OH right parenthesis subscript 2 space plus space 2 HCl space rightwards arrow space space MgCl subscript 2 space plus space 2 straight H subscript 2 straight O
     It is used:
    (i) in the preparation of magnesia cement.
    (ii) in the extraction of magnesium metal. 

    Question 182
    CBSEENCH11007160

    How is slaked lime prepared? What are its properties and uses?

    Solution

    Slaked lime is prepared by the following methods:
    (i) From quick lime: By treating quicklime with water, slaked lime is formed. 
        CaO space plus space straight H subscript 2 straight O space rightwards arrow space space Ca left parenthesis OH right parenthesis subscript 2 space plus space Heat
    When water is added to quicklime, a huge amount of heat is produced along with the hissing sound. 
    (ii) From calcium chloride: By treating calcium chloride with caustic soda, slaked lime is formed.
    CaCl subscript 2 space plus space 2 NaOH space rightwards arrow space space Ca left parenthesis OH right parenthesis subscript 2 space plus space 2 NaCl
    Properties:
    (i) Slaked lime is a white amorphous powder.
    (ii) A suspension of slaked lime in water is called milk of lime.
    (iii) The aqueous layer which is decanted from the precipitated calcium hydroxide is called lime water.
    (iv) On passing carbon dioxide through lime water, the lime water turns milky due  to the formation of insoluble calcium carbonate. 
    Ca left parenthesis OH right parenthesis subscript 2 left parenthesis aq right parenthesis space plus space CO subscript 2 left parenthesis straight g right parenthesis space rightwards arrow space space CaCO subscript 3 left parenthesis straight s right parenthesis space plus space straight H subscript 2 straight O left parenthesis straight l right parenthesis
    If carbon dioxide is passed in excess, a clear solution is again obtained. This is because the insoluble calcium carbonate changes into soluble calcium bicarbonate.
    stack CaCO subscript 3 left parenthesis straight s right parenthesis with Insoluble below space plus space straight H subscript 2 straight O left parenthesis straight l right parenthesis space plus space CO subscript 2 left parenthesis straight g right parenthesis space rightwards arrow space space space stack Ca left parenthesis HCO subscript 3 right parenthesis subscript 2 with Soluble below left parenthesis aq right parenthesis
    If the clear solution is heated, it again turns milky due to the decomposition of calcium bicarbonate into calcium carbonate. 
    stack Ca left parenthesis HCO subscript 3 right parenthesis subscript 2 with left parenthesis Soluble right parenthesis below space rightwards arrow with Heat on top space CaCO subscript 3 left parenthesis straight s right parenthesis space plus space CO subscript 2 left parenthesis straight g right parenthesis space plus space straight H subscript 2 straight O left parenthesis straight l right parenthesis
    (v) Slaked lime reacts with chlorine to form bleaching powder.
     
    Ca left parenthesis OH right parenthesis subscript 2 left parenthesis straight s right parenthesis space plus space Cl subscript 2 left parenthesis straight s right parenthesis space rightwards arrow stack CaOCl subscript 2 left parenthesis straight s right parenthesis with Bleaching space powder below

    Uses. It is used:
    (i) for the preparation of ammonia
    (ii) for white washing
    (iii) in the purification of sugar
    (iv) in the softening of hard water.

    Question 183
    CBSEENCH11007161

    What is lime stone? How is it prepared? Give its properties and uses. 

    Solution

    Limestone or marble is calcium carbonate (CaCO3).
    Preparation: In the laboratory, it is prepared by the action of sodium carbonate on calcium chloride when a white precipitate of calcium carbonate is obtained.
    CaCl subscript 2 left parenthesis aq right parenthesis space plus space Na subscript 2 CO subscript 3 left parenthesis aq right parenthesis space rightwards arrow space space CaCO subscript 3 left parenthesis straight s right parenthesis space plus space 2 NaCl left parenthesis aq right parenthesis
    Properties: (i) It is a white powder, sparingly soluble in water. 
    (ii) On heating to about 1200K, it decomposes to form calcium oxide. 
    CaCO subscript 3 left parenthesis straight s right parenthesis space rightwards arrow with 1200 space straight K on top space CaO left parenthesis straight s right parenthesis space plus space CO subscript 2 left parenthesis straight s right parenthesis
    Uses: It is used
    (i) in the form of marble for making floors. 
    (ii) in toothpaste.
    (iii) in the form of fine powder in face powders and 
    (iv) in the form of limestone for making lime, cement and glass.

    Question 184
    CBSEENCH11007162

    How is quick lime prepared on a commercial scale? How is it converted into slaked lime?

    Solution
    Commercial preparation: Quicklime is anhydrous calcium oxide (CaO) and is formed by heating limestone (CaCO3) in a specially designed kiln.
    CaCO subscript 3 space rightwards harpoon over leftwards harpoon with Heat on top space CaO space plus space CO subscript 2 semicolon space space space increment straight H space equals space plus 179.9 space kJ
    Formation of slaked lime: Slaked lime is formed when quicklime is added to water. The reaction is highly exothermic in nature and called slaking of lime.
    CaO space plus space straight H subscript 2 straight O space rightwards harpoon over leftwards harpoon space space space Ca left parenthesis OH right parenthesis subscript 2 space plus space Heat

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