Lead(II) chloride

Structure and properties

In solid PbCl₂, each lead ion is coordinated by 9 chloride ions — 6 lie at the apices of a trigonal prism and 3 lie beyond the centers of each 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. While Lead(II) chloride is abundant in many natural water reserves, it is unsafe for human consumption and must be filtered out.

Vaporized 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.

The solubility of PbCl₂ in water is low (10.8 g/L at 20 °C) and for practical purposes it is considered insoluble. Its solubility product constant (K_sp) is 5.89×10⁻⁵. It is one of only four commonly insoluble chlorides, the other three being silver chloride (AgCl) with K_sp = 1.8×10⁻¹⁰, copper(I) chloride (CuCl) with K_sp = 1.72×10⁻⁷ and mercury(I) chloride (Hg₂Cl₂) with K_sp = 1.3×10⁻¹⁸.

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

Lead(II) chloride precipitates from solution upon addition of chloride sources (HCl, NaCl, KCl) to aqueous solutions of lead(II) compounds such as Pb(NO₃)₂.

Pb(NO₃)_2(aq) + 2 NaCl_(aq) → PbCl_2(s) + 2 NaNO_3(aq)Pb(CH₃COO)_2(aq) + HCl_(aq) → PbCl_2(s) + 2 CH₃COOH_(aq)basic PbCO₃ + 2 HCl_(aq) → PbCl_2(s) + CO_2(g) + H₂OPb(NO₃)_2(aq) + 2 HCl_(aq) → PbCl_2(s) + 2 HNO_3(aq)

Treatment of lead dioxide with hydrochloric acid gives lead(II) chloride as well as chlorine gas:

PbO₂(s) + 4 HCl → PbCl_2(s) + Cl₂ + 2 H₂O

Treatment of lead oxide with hydrochloric acid gives lead(II) chloride as well as water

PbO(s) + 2 HCl → PbCl_2(s) + H₂O

PbCl_2(s) also forms by the action of chlorine gas on lead metal:

Pb + Cl₂ → PbCl₂

Reactions

Addition of chloride ions to a suspension of PbCl₂ 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_2(s).

PbCl_2(s) + Cl⁻ → [PbCl₃]⁻_(aq)PbCl_2(s) + 2 Cl⁻ → [PbCl₄]²⁻_(aq)

PbCl₂ reacts with molten NaNO₂ to give PbO:

PbCl_2(l) + 3 NaNO₂ → PbO + NaNO₃ + 2 NO + 2 NaCl

PbCl₂ is used in synthesis of lead(IV) chloride (PbCl₄): Cl₂ is bubbled through a saturated solution of PbCl₂ in aqueous NH₄Cl forming [NH₄]₂[PbCl₆]. The latter is reacted with cold concentrated sulfuric acid (H₂SO₄) forming PbCl₄ as an oil.

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

2 PbCl₂ + 4 RLi → R₄Pb + 4 LiCl + Pb2 PbCl₂ + 4 RMgBr → R₄Pb + Pb + 4 MgBrCl3 PbCl₂ + 6 RMgBr → R₃Pb-PbR₃ + Pb + 6 MgBrCl

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

PbCl₂ can be used to produce PbO₂ by treating it with sodium hypochlorite (NaClO), forming a reddish-brown precipitate of PbO₂.

Uses

Molten PbCl₂ is used in the synthesis of lead titanate (PbTiO₃) and barium lead titanate ceramics by cation replacement reactions:

xPbCl_2(l) + BaTiO_3(s) → Ba_1−xPb_xTiO₃ + xBaCl₂

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 (SnCl₂) 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 (Sb) increases corrosion resistance.

A basic chloride of lead, PbCl₂·Pb(OH)₂, 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.

PbCl₂ 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 acidic elements such as arsenic and tellurium. This is followed by the Parkes desilverization 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 lead containing compounds, exposure to PbCl₂ may cause lead poisoning.