Lead chloride is an inorganic compound which is a white solid under ambient conditions. It is poorly soluble in water. Lead chloride is one of the most important lead-based reagents. It also occurs naturally in the form of the mineral cotunnite.
Structure and properties
In solid PbCl2, each lead ion is coordinated by nine chloride ions in a tricapped triangular prism formation — six lie at the vertices of a triangular prism and three lie beyond the centers of each rectangular prism face. The 9 chloride ions are not equidistant from the central lead atom, 7 lie at 280–309 pm and 2 at 370 pm. PbCl2 forms white orthorhombic needles. In the gas phase, PbCl2 molecules have a bent structure with the Cl–Pb–Cl angle being 98° and each Pb–-Cl bond distance being 2.44 Å. Such PbCl2 is emitted from internal combustion engines that use ethylene chloride-tetraethyllead additives for antiknock purposes. PbCl2 is sparingly soluble in water, solubility productKsp = at 20 °C. It is one of only 5 commonly water-insoluble chlorides, the other 4 being thallium chloride, silver chloride with Ksp =, copper chloride with Ksp = and mercury chloride with Ksp =.
Occurrence
PbCl2 occurs naturally in the form of the mineral cotunnite. It is colorless, white, yellow, or green with a density of 5.3–5.8 g/cm3. The hardness on the Mohs scale is 1.5–2. The crystal structure is orthorhombic dipyramidal and the point group is 2/m 2/m 2/m. Each Pb has a coordination number of 9. Cotunnite occurs near volcanoes: Vesuvius, Italy; Tarapacá, Chile; and Tolbachik, Russia.
Synthesis
Double displacement/Metathesis
Lead chloride precipitates upon addition of aqueous chloride sources to lead compounds
PbCl2 can be formed by the reduction of copper chloride by lead metal:
:Pb + CuCl2 → PbCl2 + Cu
Direct chlorination
PbCl2 also forms by the action of chlorine gas on lead metal:
Reactions
Addition of chloride ions to a suspension of PbCl2 gives rise to soluble complex ions. In these reactions the additional chloride break up the chloride bridges that comprise the polymeric framework of solid PbCl2. PbCl2 reacts with molten NaNO2 to give PbO: PbCl2 is used in synthesis of lead chloride : Cl2 is bubbled through a saturated solution of PbCl2 in aqueous NH4Cl forming 2. The latter is reacted with cold concentrated sulfuric acid forming PbCl4 as an oil. Lead chloride is the main for organometallic derivatives of lead, such as plumbocenes. The usual alkylating agents are employed, including Grignard reagents and organolithium compounds: These reactions produce derivatives that are more similar to organosilicon compounds, i.e. that Pb tends to disproportionate upon alkylation. PbCl2 can be used to produce PbO2 by treating it with sodium hypochlorite, forming a reddish-brown precipitate of PbO2.
Uses
Molten PbCl2 is used in the synthesis of lead titanate and barium lead titanate ceramics by cation replacement reactions:
:x PbCl2 + BaTiO3 → Ba1−xPbxTiO3 + x BaCl2
PbCl2 is used in production of infrared transmitting glass, and ornamental glass called aurene glass. Aurene glass has an iridescent surface formed by spraying with PbCl2 and reheating under controlled conditions. Stannous chloride is used for the same purpose.
Pb is used in HCl service even though the PbCl2 formed is slightly soluble in HCl. Addition of 6–25% of antimony increases corrosion resistance.
A basic chloride of lead, PbCl2·Pb2, is known as Pattinson's white lead and is used as pigment in white paint. Lead paint is now banned as a health hazard in many countries by the White Lead Convention, 1921.
PbCl2 is an intermediate in refining bismuth ore. The ore containing Bi, Pb, and Zn is first treated with molten caustic soda to remove traces of arsenic and tellurium. This is followed by the Parkes process to remove any silver and gold present. The ore now contains Bi, Pb, and Zn. It is treated with Cl2 gas at 500 °C. ZnCl2 forms first and is removed. Then PbCl2 forms and is removed leaving pure Bi. BiCl3 would form last.
Toxicity
Like other soluble lead compounds, exposure to PbCl2 may cause lead poisoning.
