Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 THE SOLID-GAS INTERFACE 247Much of the experimental data has been obtained by observing weight losses usingmicrobalance techniques (Section 2.1.4). This technique is almost a necessity forstudying hydrate decompositions, because of the difficulties of handling easilycondensable water vapour. It is preferable to use single crystals of known surfacearea, rather than numbers of small crystals. Reaction may also be started byrubbing the crystal surfaces with decomposition product. This technique ensuresthat the interface is formed parallel to the surfaces of the crystal and reduces theinduction period for the reaction. Britton et ~ 1.''~ have studied the decompositionof calcium and magnesium carbonates under conditions where the carbon dioxidewas continuously removed. Bircumshaw et aZ.297-299 have studied the decompositionof nickel formate, ammonium permanganate and potassium perchlorate.The other commonly used method in kinetic work is to measure the pressurerise during decomposition. Bircumshaw and Newman3 studied the decompositionof ammonium perchlorate using a McLeod gauge as the measuring device.They also studied the decomposition in a stream of nitrogen, analysing the productschemically by adsorption of C1, in potassium iodide. For measurements in a streamof inert gas, a katharometer could be used as a detection device. Other methodsof pressure measurement'' are also applicable, especially where a rapid reactiondemands that a record be obtained.2.3.3 Transport reactions301Chemical transport reactions are those in which a solid (or liquid) substance Areacts with a gas to form a vapour phase product. The reverse reaction thenoccurs in a different part of the system with the reformation of substance AA(solid) + B(gas) + C(gas) Transpor! A(so1id) + B(gas)The process appears to be one of sublimation or distillation. Substance A doesnot possess an appreciable vapour pressure however and is transported chemicallydue to the existence of a concentration gradient.B(g) 4d C(9) + B(9) *(d B(9) + C(g)Fig. 37. Apparatus for the measurement of transport reactions using a temperature difference.References pp. 270-278