Preparation of Inorganic Compounds

Objectives

Our objectives are to prepare:

  • A pure sample of ferrous ammonium sulphate (Mohr’s salt) [FeSO4. (NH4)2SO4. 6H2O]
  • A pure sample of potash alum (Fitkari) [K2SO4. Al2(SO4). 24H2O]
  • A pure sample of the complex potassium trioxalatoferrate (III), K3[Fe(C2O4)3]. 3H2O

The Theory

What is an inorganic compound?

Inorganic compounds are substances that do not come from living things. They are formed by non-living natural processes or by laboratory preparation methods. The branch of chemistry that deals with the behaviour and properties of inorganic compounds is called Inorganic Chemistry. Inorganic compounds are found in nature in the form of minerals. 

What are the different types of inorganic compounds?

The two important classes of inorganic compounds are Coordination Compounds and Double Salts.

Coordination compounds (complex compounds)

Complex compounds are formed by a large number of transition metals in which the metal atom is bound to neutral molecules or to negatively charged species called ligands. The elements of group 3-12 are called transition metals.

These compounds are also called coordination compounds. The ligands donate electrons to the metal atoms and the metal atoms accept these electrons to form a ligand-metal coordinate bond. The number of ligands directly bonded to the central metal atom is called the coordination number of a complex. The structure of coordination compounds was first proposed by Alfred Werner.  He proposed the concept of a primary valence and secondary valence for a metal ion. Primary valences are satisfied by the central ions and secondary valences are satisfied by the ligands. Secondary valence is equal to the coordination number.

On example is Potassium trioxalatoferrate (III) { K3[Fe(C2O4)3].3H2O}

Potassium trioxalatoferrate (III) is a coordination compound. In this complex, iron is the central metal ion and oxalate [(C2O4)2-] is the ligand. Oxalate is a bidentate ligand in which two oxygen atoms donate electrons to the central iron atom. It is an octahedral transition metal complex in which iron is in the +3 oxidation state. So we can say that in complex potassium trioxalatoferrate (III), the central Fe3+ ion is octahedrally surrounded by bidentate oxalate ligands.  Potassium acts as the counter ion, and the Fe3+ ion, along with the ligand, constitute the coordination sphere.  The structure is shown below.

 

Other examples are  [Co(NH3)6]3+, [PtCl4]2-, Ni(CO)4.

In coordination compounds, the central metal ion and the ligands attached to it are enclosed in a square bracket and are collectively termed as the coordination sphere. The ionisible group is written outside the bracket and is called counter ion. For example, in the complex, K2[PdCl4], the coordination sphere is [PdCl4]2- and the counter ion is K+.

Double Salts

Double salts are compounds that contain more than one cation or anion. Double salts are obtained by the combination of two different salts. The two salts crystallise together to form a single substance, but it ionises as two distinct salts when dissolved in water. The properties of double salts are different from its component single salts.

One example is Ferrous ammonium sulphate (Mohr’s salt) [FeSO4.(NH4)2SO4.6H2O]

Ferrous ammonium sulphate is a double salt of ferrous sulphate and ammonium sulphate (Mohr’s salt). It has the formula FeSO4.(NH4)2SO4.6H2O.  It contains two different cations Fe2+ and NH4+. It is used as a primary standard in volumetric analysis. The crystals are light green coloured and are monoclinic in shape. 

  

Another example is Alum.  Alums are the most common examples of double salts. These are double sulphates having general formula X2SO4.M2(SO4)3.24H2O

Where X = monovalent cation such as K+, Na+, NH4+ etc.

M = trivalent cation such as Al3+, Fe3+,Cr3+ etc.

If you take Potash alum (Fitkari) [K2SO4.Al2(SO4)3.24H2O], it is a double salt of potassium sulphate and aluminium sulphate. The crystals of potash alum are octahedral in shape and are highly soluble in water. It is commonly used in the purification of water.

 

Other examples are  Soda alum (Na2SO4.Al2(SO4)3.24H2O), Ferric alum (NH4)2SO4. Fe2(SO4)3. 24H2O) etc.                        

These are crystalline solids and are soluble in water. Due to hydrolysis, their aqueous solutions have acidic character.

Learning Outcomes

  • Students understand the terms ‘inorganic compounds, double salts and coordination compounds.
  • Students will acquire the skills to prepare  Mohr’s salt , Potash alum  and  Potassium trioxalatoferrate
  • Students will identify the chemicals and apparatus required for the preparation of Mohr’s salt, potash alum and potassium trioxalatoferrate.
  • Once students understand the concept of the experiment and the different steps, they can perform the experiment in the real lab more accurately and quickly.

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