Zinc chloride (ZnCl<sub>2</sub>)

What is Zinc Chloride?

Zinc chloride is a chemical compound with the formula ZnCl2. There exist nine different crystalline forms of zinc chloride that are currently known. These hydrates of ZnCl2 are either white in colour or colourless. All of them are highly soluble in water. Zinc chloride exhibits hygroscopic qualities, i.e. it attracts and captures the water molecules in its environment.

Zinc Chloride

Zinc Chloride

The five known hydrates of zinc chloride have the general formula ZnCl2(H2O)n where the values of n can be 1, 1.5, 2.5, 3, and 4. Solid ZnCl2 exhibits polymorphism and can take on one of the following crystal structures: tetragonal, monoclinic, and orthorhombic.

Structure

Zinc chloride is an inorganic binary salt. A molecule of zinc chloride features ionic bonding between the zinc cation (Zn2+) and the chloride anions (Cl). The structure of a ZnCl2 molecule is illustrated below.

Zinc Chloride Structure

Structure of Zinc Chloride Molecules

It is important to note that the zinc-chlorine bond in ZnCl2 possesses some covalent characteristics, which accounts for its low melting point and its solubility in ethereal solvents.

Preparation

The reaction between metallic zinc and hydrogen chloride gas yields the anhydrous form of zinc chloride. The chemical equation for this reaction is given by:

Zn + 2HCl → ZnCl2 + H2

In order to obtain a hydrated form of the compound, hydrochloric acid can be used to treat zinc instead of hydrogen chloride. Hydrochloric acid also reacts with zinc sulfide to form zinc chloride and hydrogen sulfide. The chemical equation for this reaction is given by:

ZnS + 2HCl → ZnCl2 + H2S

Since the only oxidation state exhibited by zinc is +2, the purification of the product is relatively simple.

Properties of ZnCl2

Chemical Data

Zinc Chloride ZnCl2
Molar Mass 136.315 grams per mole
Density 2.907 grams per cubic centimetre
Melting Point 563 K (290oC)
Boiling Point 1005 K (732oC)

Physical Properties

  • Zinc chloride is solid at room temperature and has a white crystalline appearance. It is odourless.
  • The solubility of this compound in water corresponds to 432g/100g. It is also soluble in acetone, ethanol, and glycerol.
  • The four polymorphs of ZnCl2 feature a tetrahedral coordinate geometry between the Zn2+ ions and the Cl
  • Molten zinc chloride is highly viscous and has a relatively low electrical conductivity value.

Chemical Properties

  • When ZnCl2 is dissolved in water, the resulting solution is acidic in nature. The pH of an aqueous solution of zinc chloride with a concentration of 6M is 1.
  • This compound reacts with ammonia to form complexes. Examples include Zn(NH3)4Cl2 and ZnCl2(NH3)2.
  • When heated, the hydrated form of zinc chloride loses water and small quantities of ZnCl(OH) are obtained.
  • The chemical equation for this reaction is given by: ZnCl2.2H2O → ZnCl(OH) + H2O + HCl

What are the Uses of Zinc Chloride?

Since it reacts with metal oxides to yield compounds with the general formula MZnOCl2 (Where M is the metal), zinc chloride is used as a flux/cleaning agent for soldering purposes. These fluxes have the ability to dissolve the layer of oxides on the surface of the metal. Some other uses of this compound are listed below.

  • In its molten state, this compound acts as a catalyst for some aromatization reactions. For example, hexamethyl benzene can be obtained from methanol with the help of a molten ZnCl2
  • Since it acts as a moderately strong Lewis acid, this compound can also serve as a catalyst for the Fischer indole synthesis reaction and some Friedel-Crafts acylation reactions.
  • The Lucas reagent is a solution of anhydrous zinc chloride and concentrated hydrochloric acid. This reagent is very useful in the preparation of alkyl chlorides.
  • A mixture of zinc oxide and hexachloroethane is used in some smoke grenades. Upon ignition, these compounds react to form a smoke of zinc chloride, which serves as a smokescreen.
  • ZnCl2 is also useful in fingerprint detection since it forms an easily detectable complex with Ruhemann’s purple.
  • The aqueous solutions of this compound, when diluted, can be used for disinfecting purposes. It is a constituent of several antiseptic mouthwash products.

To learn more about zinc chloride and other chemical compounds containing zinc, such as zinc sulfate, register with BYJU’S and download the mobile application on your smartphone.

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