Double Salts

Double salts are ionic substances made up of equimolar amounts of two simple salts. Carnallite- KCl.MgCl2.6H2O and Mohr’s salt- FeSO4.(NH)2SO4.6H2O) are examples of solid salts that do not exist in aqueous states since they would entirely dissociate into ions in aqueous states. The study of double salts and the coordination complex makes for an interesting one. 

What is Double Salt?

A double salt is a crystalline salt made up of two simple salts but with a crystal structure that isn’t the same as either of them. Because anions and cations are present in all salts, a double salt has more than one cation or anion. Both salt components crystallise in the same ionic lattice in the double salt. This is an ionic lattice, which is a regular arrangement of ions. The double salt alum is a common example. Aluminium cations and sulphate anions are found in them.

Complexes should not be confused with double salts. Double salts are found exclusively in solids. When dissolved in water, a double salt acts like a mixture of the two individual salts: it entirely dissociates into simple ions, whereas a hexaaqua complex does not; the complex ion remains unchanged. Similarly, K4[YbI6] is a complex salt that contains the discrete [YbI6]4 ion, which is stable in water. In many circumstances, square brackets “[]” signify the complex ion. Mixed-crystal systems, in which two salts co-crystallize, are distinct from double salts; the former includes a chemical combination with a fixed composition, whereas the latter is a mixture. In general, the properties of the double salt will differ from those of its single constituent salts.

What is the Coordination Complex?

• Coordination complexes are complex structures of a central metal ion surrounded by ligands, which are molecules or ions. Coordinate covalent bonds attach these ligands to the central metal ion
• A positive charge is always present in the central metal ion. Lone electron pairs are abundant in the ligands. The metal ion receives these electrons to lower its positive charge. A coordinate bond refers to this sort of bonding. The type of metal ion and the number of ligands present in the coordination complex determine the structure of the complex
• The coordination number of a complex chemical determines its structure. The number of ligands attached to a metal ion is expressed as a coordination number. A coordination compound’s coordination number usually ranges from 2 to 9
• Coordination compounds contain the following structures (also known as geometries): The oxidation state of the central metal ion determines the colour of almost every coordination complex. The number of electrons taken from a metal atom is known as its oxidation state. The electronic changes between atomic orbitals of the metal atom due to light absorption give rise to these colours. Almost all copper compounds, for example, are blue or blue-green in colour, and ferric chloride is also blue or blue-green in colour

Properties of Double Salts

• The attachment of two or more simple salts results in a molecular or higher-order complex called a double salt
• Only in a solid state can double salt exist
• In an aqueous solution or other solvents, they become fully ionised
• In water, double salts dissolve
• There are no isomerism qualities in them
• There are one or more crystal water molecules in each double salt molecule

Similarities between Double Salt and Coordination Complex

1. Coordination compound and double salt are both complex compounds.
2. Many ions are present in double salt and coordination compounds (anions and cations).

Dissimilarities between Double Salt and Coordination Complex

Coordination compounds are complex structures of a central metal ion surrounded by molecules or ions. In contrast, double salts are crystalline salts with the composition of a mixture of two simple salts but with a different crystal structure than either.

1. A double salt comprises two different salt compounds crystallised in the same ionic lattice. In contrast, a coordination complex includes a metal ion surrounded by ligands bound to it by coordinate covalent bonds.
2. Coordinate compounds have covalent connections between the metal ion and the ligands, whereas double salts have ionic interactions between cations and anions.
3. Double salts disintegrate into ionic species when dissolved in water, whereas coordination compounds are soluble compounds that do not dissociate into ionic species when dissolved in water.

Conclusion

When dissolved in water, a double salt crystallises as one salt compound yet ionises as two separate salt compounds. Potassium sodium tartrate, aluminium sulfoacetate (which has two different anions), and other double salts are examples. When a double salt is dissolved in water, it forms aqueous ionic species. When K+, Ce3+, and F– ions are dissolved in water, KCeF4, a double salt, gives out K+, Ce3+, and F– ions. Individual salt compounds from which the double salt is produced have properties that differ from double salt.

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