STRUCTURE AND BASIC PROPERTIES

Four crystalline forms, so-called polymorphs, of ZnCl₂ are known, and in each case the Zn²⁺ ions are tetrahedrally coordinated to four chloride ligands. Rapid cooling of molten ZnCl₂ gives a glass, that is, a rigid amorphous fluid. Additionally ZnCl₂ forms hydrates and at least one mixed hydroxide, ZnClOH.

In aqueous solution, zinc chloride fully dissociated into Zn²⁺. 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)

Preparation and purification

Anhydrous ZnCl₂ can be prepared from zinc and hydrogen chloride.

Zn + 2 HCl → ZnCl₂ + H₂

Hydrated forms and aqueous solutions may be readily prepared by treating pieces of Zn metal with concentrated hydrochloric acid. Zinc oxide and zinc sulfide react with HCl:

ZnS(s) + 2 HCl(aq) → ZnCl₂(aq) + H₂S(l)

Unlike many other elements, zinc essentially exists in only one oxidation state, 2+, which simplifies purification.

Commercial samples of zinc chloride typically contain water and products from hydrolysis product. Such samples may be purified by extraction into hot dioxane, which is filtered hot and the filtrate is cooled to afford a precipitate of ZnCl₂. Anhydrous samples can be purified by sublimation in a stream of hydrogen chloride gas, followed by heating to 400 °C in a stream of dry nitrogen gas. Finally, the simplest method relies on treating the zinc chloride with thionyl chloride.