Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

254 EXPERIMENTAL MET HODS FOR HETEROGENEOUS RE ACTIONSdue to the formation of hydrofluorosilicic acid. Bradley3 30 has studied the reactionof copper with a solution of sulphur in carbon tetrachloride. Reaction was followedby removing solution at intervals, separating off the copper sulphide and weighingthe sulphur present after evaporation of the solution.Exchange of silver with radioactive silver nitrate solution has been studied by Tingleye? al. 31, The l1 'Ag isotope has a half life of 270 days. Various surface treatmentswere given to silver foils, which were then cemented into a wax block and suspendedin the 0.1-0.005 N silver nitrate solution. After reaction, specimens were rinsed toremove solution and the activity determined with an end-window P., counter.Fluctuations of the counter efficiency were corrected by standardisation againsta known "'Ag source. The method of foil pretreatment was found to have agreat effect on the kinetics of the exchange process.Calorimetry has been used by Smith332 to measure the rate of acetylation ofcellulose. The apparatus is shown in Fig. 41. A coating of ice is formed inside thedouble wall of the Dewar vessel, the rest of the space and the burette containingwater. As the reaction proceeds, the heat evolved causes some of the ice to meltand alter the volume occupied by the water and ice. The change is noted on theburette. A correction is necessary for the heat leak into the calorimeter in theabsence of reaction, but this can be obtained by observations prior to the experiment.The 'rate of heat transfer must be large compared with the rate of heatevolution, which suggests that the Dewar vessel should be made of metal and thereaction mixture well stirred. A disadvantage of the method is that the rate ofreaction can only be measured at one temperature. Experiments at other temperatureswould require the use of other materials with a suitable melting point andvolume change on liquefaction.4. The solid-solid interface4.1 INTRODUCTIONThere are certain essential differences between solid state reactions and reactionsinvolving gaseous or liquid phases. In the latter case, the kinetic motion of thereactant molecules ensures that they are available to one another for reactionunder conditions which can be defined by statistical laws. Solid state reactionsoccur between apparently regular crystal lattices, in which the kinetic motion isvery restricted and depends on the presence of lattice defects. Interaction can onlyoccur at points of contact between the reacting phases and is therefore dependenton particle size and particle size distribution. The factors which govern the rateof a solid state reaction are (i) the rate of the boundary phase processes which leadto the consumption of the original lattices, and (ii) the rate of particle transferthrough the product layer.