Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

100 EXPERIMENTAL METHODS FOR SLOW REACTIONSing gas that was present. Moseley and R ~bb~~' have made use of the pressurechange occurring in a gas resulting from the adiabatic temperature rise in the initialstages of a photochemical reaction The technique was used to determine therate constant for methyl radical combination3 31. The small pressure changes weremeasured by means of a very sensitive diaphragm manometer as a change in capacitybetween the diaphragm and a small brass probe A (Fig. 69).An important method (already mentioned, p. 11) for determining the extentof a heterogeneous contribution to a homogeneous reaction is that of differential~alorimetry~~. Two fine-gauge wire thermocouples enclosed in thin Pyrex or silicacapillaries are mounted at the centre and close to the wall of the RV (Fig. 70).L YFig. 70. Differential calorimetry. From ref. 47a.The difference in readings of these thermocouples gives a measure of the temperaturerise of the gas at the centre of the RV relative to the walls. For an entirelyhomogeneous reaction, the initial temperature rise for a cylindrical RV is given bywhere qg is the heat liberated per second and h is the coefficient of thermal conductivityof the gas. For an entirely heterogeneous reaction, the initial temperaturerise ATw is given byAT, = q,r In (R/r)/2nh(R+r)(R')where qw is the heat liberated at the walls per second, R is the radius of the RVand r is the radius of the capillary. For both heterogeneous and homogeneouscontributions we have2r In (R/r)AT = qg+qw * 14n.hR+rIf the rates of the homogeneous and heterogeneous reactions are We and W,respectively, the overall rate W = W.+ W,.