The s-Block Elements

Question 177
CBSEENCH11007155

Name the gases in which magnesium wire continues to burn and write the reactions ?

Solution

Mg wire continues to burn in CO2 and SO2 gases. The reactions are:
2 Mg space plus space CO subscript 2 space rightwards arrow space space 2 MgO space plus space straight C
3 Mg space plus space SO subscript 2 space rightwards arrow space space space 2 MgO space plus space MgS

Question 178
CBSEENCH11007156

Draw the structure of: (i) BeCl2 (vapour) (ii) BeCl2 (solid).

Solution

In the solid state, BeCl2 has polymeric chain structure. Be atom is tetrahedrally surrounded by four Cl atoms - two are bonded by covalent bonds while the other two by coordinate bonds. The polymeric structure of BeCl2 is due to its electron deficient nature. It has only four electrons in valence shell and can accept two pairs of electrons from neighbouring chlorine atoms to complete their octet.

In the vapour state, beryllium chloride exists as a dimer (Be2Cl4) which dissociates at 1200 K into monomer (BeCl2) which has a linear shape.

Question 179
CBSEENCH11007157

How does magnesium occur in nature? How is magnesium obtained by electrolysis method ?

Solution

Magnesium docs do not occur in the free state in nature. In the combined state (minerals) it occurs as,
(i) Magnesite MgCO3
(ii) Dolomite MgCO3.CaCO3
(iii) Carnallite KCl MgCl2.6H2O
(iv) Epsom salt MgSO4.7H2O
All plants and animal tissues contain small amounts of magnesium, it is contained in chlorophyll, the green colouring matter of plants. Sea water contains an appreciable amount of magnesium chloride.
Extraction of magnesium: Magnesium is usually extracted by the electrolysis of fused magnesium chloride or carnallite. It is isolated form sea water by Dow process. The various steps employed are
(i) Precipitation of magnesium hydroxide: Sea water is treated with lime water when magnesium hydroxide gets precipitated.
stack MgCl subscript 2 with Lime space water below space plus space Ca left parenthesis OH right parenthesis subscript 2 space space rightwards arrow space space stack Mg left parenthesis OH right parenthesis subscript 2 with ppt below space plus space CaCl subscript 2
(ii) Conversion of magnesium hydroxide into magnesium chloride: The precipitate of magnesium hydroxide is dissolved in hydrochloric acid to get a clear solution of magnesium chloride.
Mg left parenthesis OH right parenthesis subscript 2 space plus space 2 HCl space rightwards arrow space space space MgCl subscript 2 space plus space 2 straight H subscript 2 straight O
The solution is concentrated when MgCl2.6H2O crystallises out.
(iii) Preparation of anhydrous magnesium chloride: A current of HCl gas is passed through magnesium chloride hexahydrate when anhydrous magnesium chloride is obtained.
MgCl subscript 2. space 6 straight H subscript 2 straight O space plus space HCl space rightwards arrow space space space MgCl subscript 2 space plus space 6 straight H subscript 2 straight O space plus space HCl
(iv) Electrolysis of anhydrous magnesium chloride: Anhydrous magnesium chloride is added to a molten mixture of sodium chloride and calcium chloride (973 -1023 K). The mixture is then electrolyzed in an electrolytic cell which consists of the iron vessel which acts as a cathode. The anode consists of a graphite rod enclosed in a porcelain hood. The cell is heated externally to about 973 -1073 K. A stream of an inert gas such as coal gas is passed through the cell to check the oxidation of liberated magnesium by atmospheric oxygen. On passing electric current, the following reaction takes place:

At cathode. 
At space cathode.
Mg to the power of 2 plus end exponent space plus space 2 straight e to the power of minus space space rightwards arrow space Mg
At space anode. space
space space space space space Cl to the power of minus space rightwards arrow space space Cl space plus space straight e
space space space space space Cl space plus space Cl space rightwards arrow space space Cl subscript 2 left parenthesis straight g right parenthesis
Thus, magnesium is liberated at the cathode while chlorine gas is liberated at the anode. 

Question 180
CBSEENCH11007158

What happens when:
(i) magnesium is heated with water.
(ii) magnesium is heated in an atmosphere of carbon dioxide,
(iii) magnesium is treated with dilute sulphuric acid and
(iv) magnesium is treated with nitrogen?

Solution

(i) Magnesium reacts with boiling water to form magnesium hydroxide and hydrogen. 
Mg left parenthesis straight s right parenthesis space plus space 2 straight H subscript 2 straight O left parenthesis straight l right parenthesis space rightwards arrow from water to Boiling of space Mg left parenthesis OH right parenthesis subscript 2 left parenthesis straight s right parenthesis space plus space straight H subscript 2 left parenthesis straight g right parenthesis
(ii) Magnesium burns in the atmosphere of CO2 to form magnesium oxide. 
   2 Mg left parenthesis straight s right parenthesis plus space CO subscript 2 left parenthesis straight g right parenthesis space rightwards arrow space space 2 MgO left parenthesis straight s right parenthesis space plus space straight C left parenthesis straight s right parenthesis
(iii) Magnesium reacts with dilute sulphuric acid liberating dihydrogen gas. 
    Mg left parenthesis straight s right parenthesis space plus space straight H subscript 2 SO subscript 4 left parenthesis aq right parenthesis space rightwards arrow space space space MgSO subscript 4 left parenthesis aq right parenthesis space plus space straight H subscript 2 left parenthesis aq right parenthesis
(iv) Magnesium burns in the atmosphere of nitrogen to form magnesium nitride.
3 Mg left parenthesis straight s right parenthesis space plus space straight N subscript 2 left parenthesis straight g right parenthesis space space rightwards arrow space space Mg subscript 3 straight N subscript 2 left parenthesis straight s right parenthesis

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