Descriptive chemistry

Date: 2015-10-07; view: 488.

Text 6

Various oxidized forms of lead are easily reduced to the metal. An example is heating PbO with mild organic reducing agents such as glucose. A mixture of the oxide and the sulfide heated together without any reducing agent will also form the metal.

2PbO + PbS → 3 Pb + SO₂

Metallic lead is attacked only superficially by air, forming a thin layer of oxide that protects it from further oxidation. The metal is not attacked by sulfuric or hydrochloric acids. It does, however, dissolve in nitric acid with the evolution of nitric oxide gas to form dissolved Pb(NO₃)₂.

3 Pb + 8 H⁺ + 8 NO₃^– → 3 Pb²⁺ + 6 NO₃^– + 2 NO + 4H₂O

When heated with nitrates of alkali metals, metallic lead oxidizes to form PbO (also known as litharge), leaving the corresponding alkali nitrite. PbO is representative of lead's II oxidation state. It is soluble in nitric and acetic acids, from which solutions it is possible to precipitate halide, sulfate, chromate, carbonate (PbCO₃), and basic carbonate (Pb₃(OH)₂(CO₃)₂) salts of lead. The sulfide can also be precipitated from acetate solutions. These salts are all poorly soluble in water. Among the halides, the iodide is less soluble than the bromide, which, in turn, is less soluble than the chloride.

The II oxide is also soluble in alkali hydroxide solutions to form the corresponding plumbite salt.

PbO + 2OH^– + H₂O → Pb(OH)₄^2–

Chlorination of plumbite solutions causes the formation of lead's IV oxidation state.

Pb(OH)₄^2– + Cl₂ → PbO₂ + 2 Cl^– + 2 H₂O

Lead dioxide is representative of the IV state, and is a powerful oxidizing agent. The chloride of this oxidation state is formed only with difficulty and decomposes readily into the II chloride and chlorine gas. The bromide and iodide of IV lead are not known to exist. Lead dioxide dissolves in alkali hydroxide solutions to form the corresponding plumbates.

PbO₂ + 2 OH^– + 2 H₂O → Pb(OH)₆^2–

Lead also has an oxide that is a hybrid between the II and IV oxidation states. Red lead (also called minium) is Pb₃O₄.

Lead readily forms an equimolar alloy with sodium metal that reacts with alkyl halides to form organometallic compounds of lead such as tetraethyl lead.