Zinc Chloride - Reactions

Reactions

Molten anhydrous ZnCl₂ at 500–700 °C dissolves zinc metal, and, on rapid cooling of the melt, a yellow diamagnetic glass is formed, which Raman studies indicate contains the Zn2+
2 ion.

A number of salts containing the tetrachlorozincate anion, ZnCl2−
4, are known. "Caulton's reagent," V₂Cl₃(thf)₆Zn₂Cl₆ is an example of a salt containing Zn₂Cl2−
6. The compound Cs₃ZnCl₅ contains tetrahedral ZnCl2−
4 and Cl− anions. No compounds containing the ZnCl4−
6 ion have been characterized.

Whilst zinc chloride is very soluble in water, solutions cannot be considered to contain simply solvated Zn2+ ions and Cl− ions, ZnCl_xH₂O_(4−x) species are also present. Aqueous solutions of ZnCl₂ are acidic: a 6 M aqueous solution has a pH of 1. The acidity of aqueous ZnCl₂ solutions relative to solutions of other Zn2+ salts is due to the formation of the tetrahedral chloro aqua complexes where the reduction in coordination number from 6 to 4 further reduces the strength of the O-H bonds in the solvated water molecules.

In alkali solution in the presence of OH− ion various zinc hydroxychloride anions are present in solution, e.g.ZnOH₃Cl2−, ZnOH₂Cl2−
2, ZnOHCl2−
3, and Zn₅OH₂Cl₃·H₂O (simonkolleite) precipitates.

When ammonia is bubbled through a solution of zinc chloride the hydroxide does not precipitate, instead compounds containing complexed ammonia (ammines) are produced, Zn(NH₃)₄Cl₂ · H₂O and on concentration ZnCl₂(NH₃)₂. The former contains the Zn(NH₃)₆2+ ion and the latter is molecular with a distorted tetrahedral geometry. The species in aqueous solution have been investigated and show that Zn(NH₃)₄2+ is the main species present with Zn(NH₃)₃Cl+ also present at lower NH₃:Zn ratio.

Aqueous zinc chloride reacts with zinc oxide to form an amorphous cement that was first investigated in the 1855 by Sorel. Sorel later went on to investigate the related magnesium oxychloride cement, which bears his name.

When hydrated zinc chloride is heated, one obtains a residue of Zn(OH)Cl e.g.

ZnCl₂·2H₂O → ZnCl(OH) + HCl + H₂O

The compound ZnCl₂·½HCl·H₂O may be prepared by careful precipitation from a solution of ZnCl₂ acidified with HCl and it contains a polymeric anion (Zn₂Cl₅ −)_n with balancing monohydrated hydronium ions, H₅O₂+ ions.

The formation of highly reactive anhydrous HCl gas formed when zinc chloride hydrates are heated is the basis of qualitative inorganic spot tests.

The use of zinc chloride as a flux, sometimes in a mixture with ammonium chloride (see also Zinc ammonium chloride), involves the production of HCl and its subsequent reaction with surface oxides. Zinc chloride forms two salts with ammonium chloride, (NH₄)ZnCl₄ and (NH₄)₃ClZnCl₄, which decompose on heating liberating HCl just as zinc chloride hydrate does. The action of zinc chloride/ammonium chloride fluxes, for example, in the hot dip galvanizing process produces H₂ gas and ammonia fumes.

Cellulose dissolves in aqueous solutions of ZnCl₂ and zinc-cellulose complexes have been detected. Cellulose also dissolves in molten ZnCl₂ hydrate and carboxylation and acetylation performed on the cellulose polymer.

Thus, although many zinc salts have different formulas and different crystal structures, these salts behave very similarly in aqueous solution. For example, solutions prepared from any of the polymorphs of ZnCl₂ as well as other halides (bromide, iodide) and the sulfate can often be used interchangeably for the preparation of other zinc compounds. Illustrative is the preparation of zinc carbonate:

ZnCl₂(aq) + Na₂CO₃(aq) → ZnCO₃(s) + 2 NaCl(aq)