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The Velocity of Reduction of the Oxides of Lead, Cadmium, and Bismuth by Carbon Monoxide, A.Nd the Existence of the Suboxides of These Metals

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The Velocity of Reduction of the Oxides of Lead, Cadmium, and Bismuth by Carbon Monoxide, A.Nd the Existence of the Suboxides of These Metals View Article Online / Journal Homepage / Table of Contents for this issue 154 BRISLEE : THE VELOCITY OF REDUCTION OF THE OXIDES OF XVI.-The Velocity of Reduction of the Oxides of Lead, Cadmium, and Bismuth by Carbon Monoxide, a.nd the Existence of the Suboxides of these Metals. By FRANCISJOSEPH BRISLEE, D. Sc. THEfollowing research was undertaken with the view of determining the conditions under which suboxides are produced by the reduction of higher oxides, by means of carbon monoxide, the velocity of the reduction, and the equilibria existing between the various oxides, carbon monoxide, and carbon dioxide. Most suboxides, when treated with ordinary chemical reagents, such as acids or alkalis, behave as though they were a mixture of metal and Published on 01 January 1908. Downloaded by Universitat Politècnica de València 29/10/2014 09:23:26. oxide. Lead suboxide, for example, when acted on by an acid, behaves as if it were a mixture of lead and lead oxide. Moist lead suboxide is oxidised to lead hydroxide by the action of air, whereas a mixture of lead and lead oxide remains unchanged under the same conditions. This was first observed by Pelouze and confirmed by later observations. The conditions for instability and stability of the intermediate sub- oxides are as follows. If the free energy of the lead suboxide is greater than the sum of the free energy of the lead and the free energy of the lead oxide, then the lead suboxide will tend to decompose into lead and lead oxide. The lead suboxide would, under these conditions, be an unstable compound, and only capable of exist- ence in a metastable state. On the other hand, if the free energy of the lead suboxide is less than the sum of the free energy of the lead View Article Online LEAD, CADMIUM, AND BISMUTH BY CARBON MONOXIDE. 155 and the free energy of the lead oxide, then the suboxide is a stable compound. These conditions may be reversed by a change of temper- ature, such a condensed ” reaction as the above possessing usually a definite transition-point at a definite temperature. That the so-called suboxide is not a mixture of lead and lead oxide was proved by Tanatar (Zeitsch. ccnorg. Chem., 1901, 27, 304), who determined the heat of solution of the suboxide in acetic acid. He showed that when a mixture of lead and lead oxide is treated with dilute acetic acid, the heat of the reaction is greater than when the suboxide of lend is subjected to the same treatment. From these experiments, Tanatar found that the decornposi tion of lead suboxide into lead and lead oxide is attended with an absorption of 5.452 Cal. He further showed that the density of the suboxide at 18’ is betwoen 8.347 and 8.342, whilst the density of a mixture of lead and lead oxide in the corresponding proportion is 9.973 to 9.9’79. That the greenish-grey product, obtained by igniting lead oxalate to a higher temperature than that at which the suboxide is formed, is different from the suboxide, although analytically identical, was shown by determining the heat of solution in N/2 acetic acid, which was found to be 15.500 Cal., that is, as much as for 1 gram-molecule of lead oxide. Moreover, the density of this product was found to be 9.973 to 9.979, or the same as that of a mixture of lead and lead oxide. If it be assumed that in a condensed system the total energy change represents very nearly the change of free energy, then it follows that the lead suboxide would be stable with respect to lead and lead oxide. Preliminary experiments were made on the production of lead sub- oxide by reducing lead oxide with carbon monoxide at 300O. Carbon monoxide was circulated by means of an apparatus similar to that introduced by Collie (Trans,, 1889, 55, 133), modified by Bone and Published on 01 January 1908. Downloaded by Universitat Politècnica de València 29/10/2014 09:23:26. Wheeler (Trans., 1903, 81, 535), and used by them for reaction- velocity experiments, over a known weight of pure lead oxide contained in a porcelain boat. The circulation was continued for twenty-four hours, and the temperature was maintained constant at 300’. After cooling, the boat was again weighed ; the loss amounted to 3.40 per cent. The product mas a velvety, greyish-black powder, which, on treatment with dilute nitric or acetic acid, left a residue of spongy lead. The residue of lead was weighed after digesting the product with dilute acetic acid, and it constituted 48.54 per cent, of the whole When the substance was moistened with water and exposed to air, it was speedily oxidised to lead hydroxide, The reduction was repeated, using a rather larger quantity of lead oxide, and again the product was identical in its behaviour with that obtained by Tanatar. On still further reduction, the suboxide was gradually reduced to metallic View Article Online 156 BRISLEE : THE VELOCITY OF REDUCTION OF THE OXIDES OF lead. The products of these subsequent reductions were carefully tested for carbonate, but in every case this was found to be absent, This non-formation of carbonate is due to the fact that the partial pressure of the carbon dioxide was always below the dissociation- pressure of lead carbonate. Colson (Compt. rend., 1905, 140, 865) found that at 280° the dissociation-pressure of lead carbonate is 548 mm., whilst under the conditions of the experiments the partial pressure of the carbon dioxide never exceeded 200 mm. Further, at the temperature employed, namely, 300°, neither the lead suboxide nor the metallic lead were capable of reacting with the carbon dioxide formed, and experiments made with a view to determining the equilibrium : Pb,O + CO, 2PbO + GO or 2Pb + CO, Pb,O + CO gave negative results. Discontinuities in the Rats of Reduction.-Glaser (Zeitsch. ayaory. Chem., 1903, 36, 1) has shown that by the reduction of lead oxide with hydrogen at 211O and 235O, lead suboxide can be obtained, and points out that the reaction : Pb,0 + H, = 2Pb + H,O proceeds much more slowly than the reaction : 2Pb0 + H, = Pb,O + H,O. The time-reduction curves of lead oxides should show breaks at, or near, the points at which the reduction of one oxide is complete and the reduction of the other commences. For example, if the reduction : 2Pb20+ GO = Pb,O + CO, proceeds at a much faster rate than the reduction : Published on 01 January 1908. Downloaded by Universitat Politècnica de València 29/10/2014 09:23:26. Pb,O + CO = 2Pb + CO,, then the time-reduction curve should show a break at the point where the lead oxide is completely reduced to lead suboxide. On the other hand, in measuring the rate of reaction in such a system, the rate measured may be the sum of two or more simultaneous reductions ; for example, if at first lead oxide is reduced to lead suboxide and a small proportion of the suboxide is also reduced to lead at the same time, but with a different velocity, then the break in the time- reduction curve will occur at the commencement of the second phase of the reaction : Pb,O -+- Pb, and hence mill not exactly coincide with the point at which the higher oxide is completely reduced to the intermediate suboxide. In the following experiments, the point at which the break in the View Article Online LEAD, CADMIUM, AND BISMUTH BY CARBON MONOXIDE. 157 time-reduction curve occurs is very near the point at which the reduction of the higher oxide ends and that of the lower oxide begins. EXPERIMENTAL. The apparatus employed in determining the velocity of reduction is shown in Fig. 1. The tubes A and B are about 70 cm. long and 2 cm. internal diameter. These tubes were enclosed in a double-jacketed tube furnace, the position of which is denoted by the lines K and L. The furnace was protected from draughts by sheets of asbestos board, and the gas pressure kept constant by a Stott regulator. Under these conditions, the temperature was maintained at 300°, with a maximum variation of & 2O. The temperature was measured by a thermometer FIG. 1. 128 Published on 01 January 1908. Downloaded by Universitat Politècnica de València 29/10/2014 09:23:26. filled with nitrogen, so placed that its bulb was just between the tubes A and B, in the neighbourhood of the position of the boat. The thermometer was compared from time to time with an accurate thermo- electric pyrometer. The tubes A and B were connected with the circulator D,which was similar to that introduced by Collie (Zoc. cit.). The tube connecting B with the circulator carried a tap, C, used for exhausting the apparatus and also for introducing the gas, and a three-way tap connected with a manometer. The tube connecting A with the circulator carried a gas sampler, G, and the bulbs, E, contained a small quantity of mercury by means of which the speed of circulation of the gases could be judged. The whole of the apparatus was joined together by glass connexions, except A, which was fitted with a rubber stopper, and there was a joint of stout rubber tube at 121, wired on, to allow of ready insertion and withdrawal of the boat. View Article Online 158 BRISLEE : THE VELOCITY OF REDUCTION OF THE OXIDES OF The experiments for determining the rate of reduction of the oxides of lead, cadmium, and bismuth were made in the following way.
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