LEAD CHLORIDE: PROPERTIES, STRUCTURE, USES - CHEMISTRY - 2025

The lead chloride is an inorganic salt having the chemical formula PbCl _n, where n is the oxidation number of lead. Thus, when lead is +2 or +4, the salt is PbCl ₂ or PbCl ₄, respectively. Therefore, there are two types of chlorides for this metal.

Of the two, PbCl ₂ is the most important and stable; while PbCl ₄ is unstable and less useful. The first is ionic in nature, where the Pb ²⁺ cation generates electrostatic interactions with the Cl anion ^- to build a crystal lattice; and the second is covalent, with Pb-Cl bonds creating a lead and chlorine tetrahedron.

Precipitated PbCl2 needles. Source: Rrausch1974

Another difference between the two lead chlorides is that PbCl ₂ is a solid with white, needle-shaped crystals (top image); while PbCl ₄ is a yellowish oil that can crystallize at -15ºC. From the outset, PbCl ₂ is more aesthetic than PbCl ₄.

In addition to what has already been mentioned, PbCl ₂ is found in nature as the mineral cotunite; while PbCl ₄ does not, since it is susceptible to decomposition. Although PbCl ₄ can be used to obtain PbO ₂, an endless variety of organometallic compounds are derived from PbCl ₂.

Properties

The properties of lead chloride are essentially dependent on the oxidation number of lead; since chlorine does not change, but the way it interacts with lead does. Therefore, both compounds must be addressed separately; lead (II) chloride on the one hand, and lead (IV) chloride on the other.

-Lead (II) chloride

Molar mass

278.10 g / mol.

Physical appearance

White colored crystals with needle shapes.

Density

5.85 g / mL.

Melting point

501 ° C.

Boiling point

950 ° C.

Water solubility

10.8 g / L at 20 ° C. It is poorly soluble and the water must be heated so that a considerable amount can dissolve.

Refractive index

2,199.

Lead (IV) chloride

Molar mass

349.012 g / mol.

Physical appearance

Yellowish oily liquid.

Density

3.2 g / mL.

Melting point

-15 ° C.

Boiling point

50 ° C. At higher temperatures it decomposes releasing chlorine gas:

PbCl ₄ (s) => PbCl ₂ (s) + Cl ₂ (g)

In fact, this reaction can become very explosive, so PbCl ₄ is stored in sulfuric acid at -80ºC.

Structure

-Lead (II) chloride

At the beginning it was mentioned that PbCl ₂ is an ionic compound, so that it consists of Pb ²⁺ and Cl ^- ions that build a crystal in which a Pb: Cl ratio equal to 1: 2 is established; that is, there are twice as many Cl ^- anions as there are Pb ²⁺ cations.

The result is that orthorhombic crystals are formed whose ions can be represented with a model of spheres and bars as in the image below.

Structure of the cotunite. Source: Benjah-bmm27.

This structure also corresponds to that of the cotunite mineral. Although bars are used to indicate a directionality of the ionic bond, it should not be confused with a covalent bond (or at least, purely covalent).

In such orthorhombic crystals, Pb ²⁺ (grayish spheres) has nine Cl ^- (green spheres) surrounding it, as if it were enclosed within a triangular prism. Due to the intricacy of the structure, and the low ionic density of Pb ²⁺, it is difficult for the molecules to solvate the crystal; which is why it is poorly soluble in cold water.

Gas phase molecule

When neither the crystal nor the liquid can withstand the high temperatures, the ions begin to vaporize as discrete PbCl ₂ molecules; that is, with Cl-Pb-Cl covalent bonds and an angle of 98º, as if it were a boomerang. The gas phase is then said to consist of these PbCl ₂ molecules and not of ions carried by air currents.

Lead (IV) chloride

Meanwhile, PbCl ₄ is a covalent compound. Why? Because the Pb ⁴⁺ cation is smaller and also has a higher ionic charge density than Pb ²⁺, which causes a greater polarization of the Cl ^- electron cloud. The result is that instead of an ionic type Pb ⁴⁺ Cl ^- interaction, the covalent Pb-Cl bond is formed.

Considering this, the similarity between PbCl ₄ and, for example, CCl ₄ is understood; both occur as single tetrahedral molecules. Thus, it is explained why this lead chloride is a yellowish oil under normal conditions; Cl atoms are loosely related to each other and "slip" when two PbCl ₄ molecules approach.

However, when the temperature drops and the molecules become slower, the probability and effects of instantaneous dipoles increase (PbCl ₄ is apolar given its symmetry); and then the oil freezes as yellow hexagonal crystals:

Crystal structure of PbCl4. Source: Benjah-bmm27

Note that each grayish sphere is surrounded by four green spheres. These "packed" PbCl ₄ molecules make up an unstable crystal that is susceptible to vigorous decomposition.

Nomenclature

The names: lead (II) chloride and lead (IV) chloride correspond to those assigned according to the Stock nomenclature. Since the oxidation number +2 is the lowest for lead, and +4 the highest, both chlorides can be named according to traditional nomenclature as plumbose chloride (PbCl ₂), and lead chloride (PbCl ₄), respectively.

And finally there is the systematic nomenclature, which highlights the number of each atom in the compound. Thus, PbCl ₂ is lead dichloride, and PbCl _{4 is} lead tetrachloride.

Applications

There is no known practical use for PbCl ₄ other than serving for the synthesis of PbO ₂. However, PbCl ₂ is more useful and that is why only some uses for this specific lead chloride will be listed below:

- Due to its highly luminescent nature, it is intended for photographic, acoustic, optical and radiation detector devices.

- As it does not absorb in the region of the infrared spectrum, it is used for the manufacture of glasses that transmit this type of radiation.

- It has been part of what is called golden glass, an attractive material with iridescent bluish colorations used for ornamental purposes.

- Also, following the subject of art, when alkalized, PbCl ₂ · Pb (OH) ₂ acquires intense whitish tones, being used as the white lead pigment. However, its use has been discouraged due to its high toxicity.

- Melted and mixed with barium titanate, BaTiO ₃, gives rise to the ceramic barium titanate and lead Ba _{1 − x} Pb _x TiO ₃. If a Pb ²⁺ enters BaTiO ₃, a Ba ²⁺ must leave the crystal to allow its incorporation, and a cation exchange is then said to occur; therefore the composition of Ba ²⁺ is expressed as 1-x.

- And finally, from PbCl ₂, various organometallic lead compounds of the general formula R ₄ Pb or R ₃ Pb-PbR _{3 are} synthesized.

References

1. Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
2. Wikipedia. (2019). Lead (II) chloride. Recovered from: en.wikipedia.org
3. Chemical Formulation. (2019). Lead (IV) chloride. Recovered from: formulacionquimica.com
4. Clark Jim. (2015). The chlorides of carbon, silicon, and lead. Recovered from: chemguide.co.uk
5. Spectral and Optical Nonlinear studies on Lead Chloride (PbCl ₂) crystals.. Recovered from: shodhganga.inflibnet.ac.in
6. National Center for Biotechnology Information. (2019). Lead chloride. PubChem Database; CID = 24459. Recovered from: pubchem.ncbi.nlm.nih.gov