Question

Anhydrous $MgC{{l}_{2}}$ can be prepared by heating $MgC{{l}_{2}}.6{{H}_{2}}O$ :
A.In a current of dry $HCL$ gas
B.With carbon
C.Until it fuses
D.With lime

Answer 1

Anhydrous $MgC{{l}_{2}}$ can only be prepared when there will be a suppression of hydrolysis in $MgC{{l}_{2}}.6{{H}_{2}}O$ as it contains six molecules as a water of crystallization.

Complete step by step answer:
-$MgC{{l}_{2}}.6{{H}_{2}}O$ has $M{{g}^{2+}}$ cation which has high hydration energy. Hydration energy is defined as the amount of energy released when one mole of ion undergoes hydration. A molecule having high hydration energy means it will undergo a hydrolysis process.
-When $MgC{{l}_{2}}.6{{H}_{2}}O$ reacts with dry $HCl$ gas, then it gives anhydrous $MgC{{l}_{2}}$ with water. Here, dry $HCl$ gas suppresses the hydrolysis reaction. In the hydrolysis process the water molecule is added , this addition process sometimes splits the substance into two parts. We cannot use concentrated $HCl$ because it is a solution which contains water molecules. So the hydrolysis process will take place and there will be no formation of $MgC{{l}_{2}}$ .
-Now let us see the reaction:
$MgC{{l}_{2}}.6{{H}_{2}}O+dryHCl\to MgC{{l}_{2}}+{{H}_{2}}O$
On heating with carbon and lime, anhydrous $MgC{{l}_{2}}$ is not formed.
So the correct option is $\left( A \right)$ which is in a current of dry $HCl$ gas.

Additional Information:
When only $MgC{{l}_{2}}$ is heated, it gives $Mg\left( OH \right)Cl$ .
In the Dow process, when magnesium hydroxide is treated with hydrochloric acid then it gives magnesium chloride as a product.
Magnesium chloride is used as a catalyst in the Ziegler natta catalyst which produces polyolefins. It is also used in dicing of parking lots, highways. It is also used in pharmaceutical preparation.
Anhydrous magnesium chloride can be prepared from ammonium magnesium chloride when the temperature range is $50-{{400}^{\circ }}C$

Note:
When magnesium chloride is heated , it undergoes partial hydrolysis and therefore magnesium chloride formed is not completely anhydrous . Heating is necessary for production of $MgC{{l}_{2}}$ otherwise it will not suppress the hydrolysis process.