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Concept
Dioxyde de plomb
Résumé
Lead(IV) oxide, commonly known as lead dioxide, is the inorganic compound with the formula PbO2. It is an oxide where lead is in an oxidation state of +4. It is a dark-brown solid which is insoluble in water. It exists in two crystalline forms. It has several important applications in electrochemistry, in particular as the positive plate of lead acid batteries.
Lead dioxide has two major polymorphs, alpha and beta, which occur naturally as rare minerals scrutinyite and plattnerite, respectively. Whereas the beta form had been identified in 1845, α-PbO2 was first identified in 1946 and found as a naturally occurring mineral 1988.
The alpha form has orthorhombic symmetry, space group Pbcn (No. 60), Pearson symbol oP12, lattice constants a = 0.497 nm, b = 0.596 nm, c = 0.544 nm, Z = 4 (four formula units per unit cell). The lead atoms are six-coordinate.
The symmetry of the beta form is tetragonal, space group P42/mnm (No. 136), Pearson symbol tP6, lattice constants a = 0.491 nm, c = 0.3385 nm, Z = 2 and related to the rutile structure and can be envisaged as containing columns of octahedra sharing opposite edges and joined to other chains by corners. This contrasts with the alpha form where the octahedra are linked by adjacent edges to give zigzag chains.
Lead dioxide decomposes upon heating in air as follows:
24 PbO2 → 2 Pb12O19 + 5 O2
Pb12O19 → Pb12O17 + O2
2 Pb12O17 → 8 Pb3O4 + O2
2 Pb3O4 → 6 PbO + O2
The stoichiometry of the end product can be controlled by changing the temperature – for example, in the above reaction, the first step occurs at 290 °C, second at 350 °C, third at 375 °C and fourth at 600 °C. In addition, Pb2O3 can be obtained by decomposing PbO2 at 580–620 °C under an oxygen pressure of . Therefore, thermal decomposition of lead dioxide is a common way of producing various lead oxides.
Lead dioxide is an amphoteric compound with prevalent acidic properties. It dissolves in strong bases to form the hydroxyplumbate ion, [Pb(OH)6]2−:
PbO2 + 2 NaOH + 2 H2O → Na2[Pb(OH)6]
It also reacts with basic oxides in the melt, yielding orthoplumbates M4[PbO4].
Source officielle
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