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Lead(II) chloride

Lead(II) chloride (PbCl<sub>2</sub>) is an inorganic compound which is a white solid under ambient conditions. It is poorly soluble in water. Lead(II) 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 PbCl<sub>2</sub>, 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&nbsp;pm and 2 at 370&nbsp;pm. PbCl<sub>2</sub> forms white orthorhombic needles.

In the gas phase, PbCl<sub>2</sub> molecules have a bent structure with the Cl–Pb–Cl angle being 98° and each Pb–-Cl bond distance being 2.44&nbsp;Å. Such PbCl<sub>2</sub> is emitted from internal combustion engines that use ethylene chloride-tetraethyllead additives for antiknock purposes.

PbCl<sub>2</sub> is sparingly soluble in water, solubility product K<sub>sp</sub> = at 20&nbsp;°C. It is one of only 5 commonly water-insoluble chlorides, the other 4 being thallium(I) chloride, silver chloride (AgCl) with K<sub>sp</sub> = , copper(I) chloride (CuCl) with K<sub>sp</sub> = and mercury(I) chloride (Hg<sub>2</sub>Cl<sub>2</sub>) with K<sub>sp</sub> = .

Synthesis

Solid lead(II) chloride precipitates upon addition of aqueous chloride sources (HCl, NaCl, KCl) to aqueous solutions of lead(II) compounds, such as lead(II) nitrate and lead(II) acetate:

It also forms by treatment of basic lead(II) compounds such as Lead(II) oxide and lead(II) carbonate.

Lead dioxide is reduced by chloride as follows:

It also formed by the oxidation of lead metal by copper(II) chloride:

Or most straightforwardly by the action of chlorine gas on lead metal:

Reactions

Addition of chloride ions to a suspension of PbCl<sub>2</sub> gives rise to soluble complex ions. In these reactions the additional chloride (or other ligands) break up the chloride bridges that comprise the polymeric framework of solid PbCl<sub>2(s)</sub>.

PbCl<sub>2(s)</sub> + Cl<sup>−</sup> → [PbCl<sub>3</sub>]<sup>−</sup><sub>(aq)</sub>
PbCl<sub>2(s)</sub> + 2 Cl<sup>−</sup> → [PbCl<sub>4</sub>]<sup>2−</sup><sub>(aq)</sub>

PbCl<sub>2</sub> reacts with molten NaNO<sub>2</sub> to give PbO:

PbCl<sub>2(l)</sub> + 3 NaNO<sub>2</sub> → PbO + NaNO<sub>3</sub> + 2 NO + 2 NaCl

PbCl<sub>2</sub> is used in synthesis of lead(IV) chloride (PbCl<sub>4</sub>): Cl<sub>2</sub> is bubbled through a saturated solution of PbCl<sub>2</sub> in aqueous NH<sub>4</sub>Cl forming [NH<sub>4</sub>]<sub>2</sub>[PbCl<sub>6</sub>]. The latter is reacted with cold concentrated sulfuric acid (H<sub>2</sub>SO<sub>4</sub>) forming PbCl<sub>4</sub> as an oil.

Lead(II) chloride is the main for organometallic derivatives of lead, such as plumbocenes. The usual alkylating agents are employed, including Grignard reagents and organolithium compounds:

2 PbCl<sub>2</sub> + 4 RLi → R<sub>4</sub>Pb + 4 LiCl + Pb
2 PbCl<sub>2</sub> + 4 RMgBr → R<sub>4</sub>Pb + Pb + 4 MgBrCl
3 PbCl<sub>2</sub> + 6 RMgBr → R<sub>3</sub>Pb-PbR<sub>3</sub> + Pb + 6 MgBrCl

These reactions produce derivatives that are more similar to organosilicon compounds, i.e. that Pb(II) tends to disproportionate upon alkylation.

PbCl<sub>2</sub> can be used to produce PbO<sub>2</sub> by treating it with sodium hypochlorite (NaClO), forming a reddish-brown precipitate of PbO<sub>2</sub>.

Uses

  • Molten PbCl<sub>2</sub> is used in the synthesis of lead titanate and barium lead titanate ceramics by cation replacement reactions:
  • :x&nbsp;PbCl<sub>2(l)</sub> + BaTiO<sub>3(s)</sub> → Ba<sub>1−x</sub>Pb<sub>x</sub>TiO<sub>3</sub> + x&nbsp;BaCl<sub>2</sub>
  • PbCl<sub>2</sub> is used in production of infrared transmitting glass, and ornamental glass called aurene glass. Aurene glass has an iridescent surface formed by spraying with PbCl<sub>2</sub> and reheating under controlled conditions. Stannous chloride (SnCl<sub>2</sub>) is used for the same purpose.
  • Pb is used in HCl service even though the PbCl<sub>2</sub> formed is slightly soluble in HCl. Addition of 6–25% of antimony (Sb) increases corrosion resistance.
  • A basic chloride of lead, PbCl<sub>2</sub>·Pb(OH)<sub>2</sub>, 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 (Painting) Convention, 1921.
  • PbCl<sub>2</sub> is an intermediate in refining bismuth (Bi) 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. There are now Bi, Pb, and Zn in the ore. At 500&nbsp;°C, it receives treatment from Cl<sub>2</sub> gas. First, ZnCl<sub>2</sub> forms and is excreted. Pure Bi is left behind after PbCl<sub>2</sub> forms and is eliminated. Lastly, BiCl<sub>3</sub> would form.

Toxicity

Like other soluble lead compounds, exposure to PbCl<sub>2</sub> may cause lead poisoning.

References

External links