The lead chloride is an inorganic salt whose chemical formula is PbCln, where n is the oxidation number of lead. So when lead is +2 or +4, salt is PbCltwo or PbCl4, respectively. Therefore, there are two types of chlorides for this metal.
Of the two, the PbCltwo it is the most important and stable; while PbCl4 it is unstable and less useful. The first is ionic in nature, where the cation Pbtwo+ generates electrostatic interactions with the Cl anion- to build a crystal lattice; and the second is covalent, with Pb-Cl bonds originating a lead and chlorine tetrahedron.
Another difference between the two lead chlorides is that PbCltwo It is a solid of white crystals with the shape of needles (upper image); while PbCl4 It is a yellowish oil that can crystallize at -15ºC. PbCl inputtwo it is more aesthetic than PbCl4.
In addition to what has already been mentioned, the PbCltwo it is found in nature as the mineral cotunite; while the PbCl4 no, as it is susceptible to decay. Although from PbCl4 you can get the PbOtwo, of PbCltwo derive an endless variety of organometallic compounds.
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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.
278.10 g / mol.
White colored crystals with needle shapes.
5.85 g / mL.
501ºC.
950ºC.
10.8 g / L at 20 ° C. It is poorly soluble and the water must be heated so that a considerable amount can dissolve.
2,199.
349.012 g / mol.
Yellowish oily liquid.
3.2 g / mL.
-15ºC.
50 ° C. At higher temperatures it decomposes releasing gaseous chlorine:
PbCl4(s) => PbCltwo(s) + Cltwo(g)
In fact, this reaction can become very explosive, so PbCl is stored.4 in sulfuric acid at -80ºC.
At first it was mentioned that PbCltwo is an ionic compound, so it consists of Pb ionstwo+ and Cl- 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- what cations Pbtwo+.
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.
This structure also corresponds to that of the cotunite mineral. Although the 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 these orthorhombic crystals the Pbtwo+ (grayish spheres) has nine Cl- (green spheres) surrounding him, as if he were enclosed within a triangular prism. Due to the intricacy of the structure, and the low ionic density of Pbtwo+, molecules have a hard time solvate the crystal; reason why it is poorly soluble in cold water.
When neither the crystal nor the liquid can withstand the high temperatures, the ions begin to vaporize as PbCl molecules.two discreet; 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 moleculestwo and no of ions carried by air currents.
Meanwhile, the PbCl4 it is a covalent compound. Why? Because the cation Pb4+ is smaller and also has a higher ionic charge density than Pbtwo+, which causes a greater polarization of the Cl electron cloud-. The result is that instead of an ionic-type interaction Pb4+Cl-, the Pb-Cl covalent bond is formed.
Considering this, the similarity between PbCl4 and, for example, the CCl4; both occur as single tetrahedral molecules. Thus, it is explained why this lead chloride is a yellowish oil under normal conditions; Cl atoms are poorly related to each other and "slip" when two PbCl molecules4 they are comming.
However, when the temperature drops and the molecules slow down, the probability and effects of instantaneous dipoles (PbCl4 is apolar given its symmetry); and then the oil freezes as yellow hexagonal crystals:
Note that each grayish sphere is surrounded by four green spheres. These PbCl molecules4 “Squeezed together” make up an unstable crystal that is susceptible to vigorously decomposing.
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 (PbCltwo), and lead chloride (PbCl4), respectively.
And finally, there is the systematic nomenclature, which highlights the number of each atom in the compound. Thus, the PbCltwo is lead dichloride, and PbCl4 lead tetrachloride.
There is no known practical use for PbCl4 in addition to serving for the synthesis of PbOtwo. However, the PbCltwo It 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, continuing on the subject of art, when alkalized, PbCltwoPb (OH)two It 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, BaTiO3, originates the ceramic lead barium titanate Ba1 − xPbxUncle3. If a Pbtwo+ enter the BaTiO3, a Batwo+ it must leave the crystal to allow its incorporation, and a cation exchange is then said to occur; therefore the composition of the Batwo+ is expressed as 1-x.
- And finally, from the PbCltwo several organometallic lead compounds of the general formula R are synthesized4Pb or R3Pb-PbR3.
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