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Alum

Alum

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Alum

It is double sulphate having molecular formula M’2SO4 . R”’2(SO4)3 . 24H2O 

Or

M’R”’ (SO4)2 . 12H2O

 

Example: K2SO4 . Al2 (SO4)3 . 24H2O(Potash alum)

K2SO4 . Cr2(SO4)3 . 24H2O (Chrome alum)

Potash alum  (K2SO4 . Al2(SO4)3 . 24H20 :

Methods of preparation:

1) By potassium sulphate or Aluminium sulphate :

When equimolar amount of potassium sulphate or aluminium sulphate are dissolved in water then solution on concentration gives crystals of alum.

K2SO4 + 3Al2(SO4)3 + 24H2O ———> K2SO4 . Al2(SO4)3 . 24H2O

2) From Alunite : 

K2SO4 . Al2(SO4)3 . 4Al(OH)3 + 6H2SO4 ———>K2SO4 + 3Al2(SO4)3 + 12H2O

K2SO4 + 3Al2(SO4)3 + 24H2O ———> K2SO4 . Al2(SO4)3 . 24H2O

Properties :

i) It is colourless crystalline solid, soluble in water. Its aqueous solution contains  ions, therefore its aqueous solution  gives test of .

ii) Hydrolysis :

Aluminium sulphate of potashalum is hydrolysed to give strong acid H2SO4. Therefore its aqueous solution  is acidic.  (K2SO4 is not hydrolysed because it is a salt of strong acid and strong base.)

Al2(SO4)3 + 6 H2O ———>2 Al(OH)3 + 3H2SO4

iii) Effect of heat :

It melts at 90 degree, at 200 degree water of crystallisation  is given out and a porous substance is formed which is called burnt alum. On redness aluminium sulphate decomposes.

K2SO4 . Al2 (SO4)3  . 24H2O ———> K2SO4 . Al2(SO4)3 + 24H2O

K2SO4 . Al2(SO4)3 ———-> K2SO4 +Al2O3 + 3 SO3

Uses:

i) It is used as mordant in dye.

ii) In the purification of water.

iii) In coagulation of blood .

iv) It is also used as an antiseptic agent.

Note: CuSO4 . Fe2(SO4)3 . 24H2O is double sulphate but it is pseudo alum because copper is divalent .

Conversions :

i) Al2O3 to anhydrous Aluminium chloride

Al2O3 + 3C + 3Cl2 ——–> 2AlCl3 + 3CO

ii) Al to anhydrous AlCl3

2Al + 6HCl ——–> 2AlCl3 + 3H2

iii) Potassium sulphate to potash alum

K2SO4 + Al2(SO4)3 + 24H2O ——–> K2SO4 . Al2(SO4)3 . 24H2O

iv) Aluminium chloride to Aluminium bromide :

AlCl3 + 3NH4OH ——–> Al(OH)3 + 3NH4Cl

Al(OH)3 + 3HBr ———> AlBr3 + 3H2O

v) Alumina or Bauxite to  PotashAlum :

Al2O3 . 2H2O + 3H2SO4 ——–> Al2(SO4)3 + 5H2O

K2SO4 + Al2(SO4)3 + 24H2O ———> K2SO4 . Al2(SO4)3 . 24H2O

vi) Aluminium to potashalum :

Al + H2SO4 ———> Al2(SO4)3 + SO2 + H2O

K2SO4 + Al2(SO4)3 + 24H2O ———-> K2SO4 . Al2(SO4)3 . 24H2O

vii) Aluminium chloride to aluminium sulphate :

AlCl3 + 3NH4OH ———-> Al(OH)3 +3NH4Cl

2Al(OH)3 +3 H2SO4 ———-> Al2(SO4)3 + 6H2O

viii) Aluminium sulphate to anhydrous AlCl3 :

Al2(SO4)3 + 6NH4OH ———-> 2Al(OH)3 + 3(NH4)2 SO4

Al2O3 + 3C + 3Cl2 ———-> 2AlCl3 + 3CO

ix) Potash Alum to Al2(SO4)3 :

K2SO4 . Al2(SO4)3 . 24H2O +6 NH4OH ———>2Al(OH)3 +3 (NH4)2SO4 + K2SO4 + 24 H2O

2Al(OH)3 + 3H2SO4 ———-> Al2(SO4)3 + 6H2O

 

 

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Saroj Bhatia: Dr. Saroj Bhatia is an Ph.D in chemistry who has been teaching chemistry for over a decade. Currently she is a respected principal of a renowned college in her hometown. She took this medium for online users. Her proudest achievement is helping people learn chemistry.
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