Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

248 EXPERIMENT A L METHOD s F o R HE T E R o G E NE o us RE A c T I o NSIf the reverse reaction proceeds readily, a closed tube containing the materialswill probably suffice. The concentration gradient can be established by a temperaturedifference across the tube. An improved apparatus is shown in Fig. 37. Theenlargements in the tube are to reduce the linear velocity of the transporting gas.Typical examples are the transport of iridiumJo2 and rutheniumJo3 oxides.A concentration gradient can also be established by means of a pressure differential.Fig. 38 illustrates this principle. An inert gas is mixed with the substancebeing transported, lowering its partial pressure and causing it to be deposited.Fig. 38. Apparatus for the measurement of transport reactions using a pressure difference.Gaseous diffusion, with subsequent decomposition on a hot wire, has beenused for the purification of zirconiumJo4, uraniumJo5, chromium, niobium andtantalumJo6. The metal is transported as a volatile iodide, which then decomposeson the hot wire or other heated receiver. In addition to diffusion, convection isobserved in large diameter tubes (> 2 cm) at several atmospheres pressure. Thetube is placed in an inclined position with the hot end downward. Silicon in theform of its dihalides has been transported in this mannerJo6.The observed kinetics must be interpreted with some care. For many reactionsin closed tubes, gaseous diffusion is the rate determining step at pressures greaterthan 1 torr, whilst at higher pressures (> 3 at), thermal convection becomesappreciable.2.3.4 Isotope'exchangeThe work on exchange of oxygen isotopes with metal oxides has already beendiscussed in Section 2.2.4b. Hamson et have studied the exchange of J6Clin sodium chloride with gaseous chlorine. The apparatus is shown in Fig. 39.Chlorine is circulated through the apparatus by convection heating. It takes about50 minutes to traverse the long path of the geiger counter C. This enables measurementson the initial fast exchange reaction to be made before mixing has occurred.An initial baking out of the system is required to prevent formation of HCl