The s-Block Elements

Question 193
CBSEENCH11007171

What happens when,
(i) magnesium is burnt in air
(ii) quick- lime is heated with silica
(iii) chlorine reacts with slaked lime
(iv) calcium nitrate is heated?

Solution
(i) Magnesium on burning in the air gives magnesium oxide.
             2 Mg left parenthesis straight s right parenthesis space plus space straight O subscript 2 left parenthesis straight g right parenthesis space rightwards arrow with Heat on top space 2 MgO left parenthesis straight s right parenthesis
(ii) Quick- lime on heating with silica (SiO2) gives calcium silicate. 
            stack CaO left parenthesis straight s right parenthesis with Quick space lime below space plus space stack SiO subscript 2 left parenthesis straight s right parenthesis with Silica below space rightwards arrow space space stack CaSiO subscript 3 left parenthesis straight s right parenthesis with Calcium space silicate below
(iii) Chlorine reacts with slaked lime to form calcium hypochlorite. 
     2 Cl subscript 2 space plus space stack 2 Ca left parenthesis OH right parenthesis subscript 2 with Slaked space lime below space space rightwards arrow space stack space CaCl subscript 2 space plus space Ca left parenthesis OCl right parenthesis subscript 2 with Bleaching space powder below space plus space 2 straight H subscript 2 straight O
(iv) Calcium nitrate on heating gives nitrogen dioxide. 
2 Ca left parenthesis NO subscript 3 right parenthesis subscript 2 space rightwards arrow with Heat on top space 2 CaO left parenthesis straight s right parenthesis space plus space 4 NO subscript 2 left parenthesis straight g right parenthesis space plus space straight O subscript 2 left parenthesis straight g right parenthesis
Question 194
CBSEENCH11007172

Discuss the composition and manufacturing details of cement.
Or
Mention the main constituents of Portland cement.

Solution

Cement is essentially a fine grind mixture of calcium aluminates and silicates which set to a hard mass when treated with water. 
Composition. The average composition of Portland cement is
CaO         50-60%          Fe2O3            1 - 2%
SiO2         20-25%         MgO               2 - 3%
Al2O3         5-9%            SO3              1 - 2%
                                     Na2O             0.5 - 1%

Manufacturing details: Raw materials. Portland cement is made by fusing together two types of materials:
(i) Calcareous (or rich in lime) such as lime- stone, chalk, alkali waste.
(ii) Argillaceous (or rich in alumina or silica) such as clay, shale, slate etc.
The main raw materials for the manufacture of cement are first crushed separately in a suitable machine. They are then mixed together in required proportions and finely ground (pulverisation). The pulverised mass (either dry or in the formed slurry with water) is introduced into a rotary kiln. The kiln consists of a sheet steel cylinder about 150m in length and 4m in diameter lined inside with fire bricks. It rotates on its axis at the rate of 30-60 revolutions per minute. Due to the rotatory motion, the charge slowly moves down the kiln. A blast of

burning coal dust is blown into the kiln from the lower end. The charge takes 2-3 hours to travel from one end to the other. During its travel, the charge passes through various temperature zones and different reactions take place.

(a) In the upper portion of the kiln (up to 1100K): The material loses its moisture.
(b) In the middle portion (1100 to 1300K). Limestone decomposes to form calcium oxide and carbon dioxide.
CaCO subscript 3 space rightwards arrow space space space CaO space plus space CO subscript 2
(c) At the lower end (1300 - 1800K). Lime and clay combine to form calcium silicate and calcium aluminate. 
          2 CaO space plus space SiO subscript 2 space space rightwards arrow space space 2 CaO. space SiO subscript 2
3 CaO space plus space SiO subscript 2 space space rightwards arrow space space 3 CaO. space SiO subscript 2
3 CaO space plus space Al subscript 2 straight O subscript 3 space rightwards arrow space space stack 3 CaO. space Al subscript 2 straight O subscript 3 with Tricalcium space silicate below
4 CaO space plus space Al subscript 2 straight O subscript 3 space plus space Fe subscript 2 straight O subscript 3 space rightwards arrow space space stack 4 CaO. space Al subscript 2 straight O subscript 3. space Fe subscript 2 straight O subscript 3 with Tetracalcium space alumino space silicate below
The hard mass thus formed is called clinker. The hot clinkers are cooled and mixed with 2 -3% gypsum and then finally powdered. The powdered material is known as cement and packed in jute or polyethene bags. Gypsum is added to retard the rate of setting of cement. Gypsum reacts with tricalcium aluminate to form calcium sulphoaluminate.
3 CaO space Al subscript 2 straight O subscript 3 space plus space 3 CaSO subscript 4 space space plus space 3 straight H subscript 2 straight O space rightwards arrow space space 3 CaO space Al subscript 2 straight O subscript 3.3 CaSO subscript 4. space 3 straight H subscript 2 straight O

Question 195
CBSEENCH11007173

Complete the following equations:
left parenthesis straight i right parenthesis space Ca space plus space straight H subscript 2 straight O space space rightwards arrow space space
left parenthesis ii right parenthesis space Ca left parenthesis OH right parenthesis subscript 2 space plus space Cl subscript 2 space rightwards arrow space
left parenthesis iii right parenthesis space BeO space plus space NaOH space rightwards arrow space
left parenthesis iv right parenthesis space BaO subscript 2 space plus space straight H subscript 2 SO subscript 4 space rightwards arrow

Solution
left parenthesis straight i right parenthesis space Ca space plus space 2 straight H subscript 2 straight O space rightwards arrow space space space Ca left parenthesis OH right parenthesis subscript 2 space plus space straight H subscript 2
left parenthesis ii right parenthesis space Ca left parenthesis OH right parenthesis subscript 2 space plus space Cl subscript 2 space space rightwards arrow stack CaOCl subscript 2 space plus space straight H subscript 2 straight O with Bleaching space powder below
left parenthesis iii right parenthesis space BeO space plus space 2 NaOH space space rightwards arrow space space stack Na subscript 2 BeO subscript 2 space plus space straight H subscript 2 straight O with Sodium space beryllate below
left parenthesis iv right parenthesis space BaO subscript 2 space plus space straight H subscript 2 SO subscript 4 space space rightwards arrow space space BaSO subscript 4 space plus space straight H subscript 2 straight O subscript 2
Question 196
CBSEENCH11007174

Contrast the action of heat on the following and explain your answer:
(i) Na2CO3 and CaCO3
(ii) MgCl2.6H2O and CaCl2.6H2O
(iii) Ca(NO3)2 and NaNO3.

Solution

(i) Na2CO3 and CaCO2:
Na2CO3 does not decompose on heating while CaCO3 decomposes on heating to produce calcium oxide and carbon dioxide.
CaCO subscript 3 space rightwards arrow space space CaO space plus space CO subscript 2 
left parenthesis ii right parenthesis space MgCl subscript 2 6 straight H subscript 2 straight O space and space CaCl subscript 2 6 straight H subscript 2 straight O
left parenthesis straight a right parenthesis space MgCl subscript 2 6 straight H subscript 2 straight O space rightwards arrow with Heat on top space MgO space plus space 2 HCl space plus space 5 straight H subscript 2 straight O
left parenthesis straight b right parenthesis space CaCl subscript 2 6 straight H subscript 2 straight O space rightwards arrow with Heat on top space space CaCl subscript 2 space plus space 6 straight H subscript 2 straight O
left parenthesis iii right parenthesis space Ca left parenthesis NO subscript 3 right parenthesis subscript 2 space and space NaNO subscript 3 colon
left parenthesis straight a right parenthesis space 2 Ca left parenthesis NO subscript 3 right parenthesis subscript 2 space space rightwards arrow with Heat on top space 2 CaO space plus space 4 NO subscript 2 space plus space straight O subscript 2
left parenthesis straight b right parenthesis 2 NaNO subscript 3 space rightwards arrow with Heat on top space 2 NaNO subscript 2 space plus space straight O subscript 2

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