Lead carbide

Lead carbide is a hypothetical chemical compound of carbon and lead. Lead and elemental carbon do not normally combine, even at very high temperatures. Modern literature on lead carbide is almost non-existent. Lead carbide is a hypothetical chemical compound of carbon and lead. Lead and elemental carbon do not normally combine, even at very high temperatures. Modern literature on lead carbide is almost non-existent. J. F. Durand reported in 1923 the synthesis of lead carbide from calcium carbide CaC2 by treatment with an aqueous solution of lead(II) acetate Pb(CH3COO)2, but this result was not reproduced. A 2007 textbook repeats this claim, describing lead carbide as a green powder with formula PbC2 that is decomposed by hydrochloric acid HCl to acetylene C2H2 and lead(II) chloride PbCl2. A compound analyzed as lead carbide PbC2 has also been obtained accidentally, as a thin layer (about 10 μm thick) on the inner wall of a graphite crucible that had been used to heat a lead-bismuth eutectic alloy for 100 hours at 1073 K in a helium atmosphere.:p.27 Several reports of 'lead carbide' synthesis appeared in the early 19th century, and were widely cited and copied into textbooks during the next few decades. In 1820, for instance, a certain John claimed to have sublimated a black carbide of lead from finely divided mixture of lead and charcoal, but this claim apparently was never reproduced.:p.67 Also in 1820, Berzelius claimed that the pyrolysis (decomposition by heat) of iron-lead cyanide resulted in a double iron and lead carbide, FeC4·2PbC4. In 1823 Göbel from Jena obtained, by pyrolysis of lead tartrate in a closed vessel, a black powder that ignited spontaneously in contact with air, and believed it to be a carbide of lead. This product still provides a popular school demonstration of pyrophoricity. Shortly thereafter, Proust obtained a similar product from lead acetate and Berzelius obtained one from lead cyanide.:p.122:p.436 However, by 1870 those pyrophoric residues came to be regarded as an 'intimate mixture' of carbon and lead; and the existence of lead carbide was considered unproven.:p.67