Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

264 EXPERIMENTAL METHODS FOR HETEROGENEOUS REACTIONS(2) For a chemical reaction occurring at a constant film pressure where theproduct is solubleA = A, exp (- kt)where A is the surface area.(3) At a constant film pressure where the products are insoluble and remain inmonolayer form(A-A,)/(A,-A,)= exp (-kt)It is assumed that the reactant molecule areas are additive and the substrateconcentration remains essentially constant.(4) A similar equation can be derived for the change in film pressure at totalconstant area(K-Z,)/(X~-~,)= exp (-kt)The above equation assumes that the film pressure is a linear function of composition.(5) Measurement of surface potentials leads to similar equations. At constantarea(AV-AV,)/(AV,,-AV,)= exp (-kt)If the total area varies during reaction(AAV -A, AV,)/(A, AVO -A, A V,) = exp ( - kt)The assumption is that the effective dipole moment, and hence the orientation,remain constant. This is most likely to be true at constant film pressure.5.2 MEASUREMENT OF FILM PRESSURE-FILM AREA CHARACTERISTICS(a) Wilhelmy slideThis is the simplest experimental method for the determination of film pressure,which is defined as the difference in surface tension between the pure solvent andthe film covered surface. The equipment consists of a thin glass, platinum or micaslide suspended from a balance and dipping into the film covered surface. Anychange in surface tension causes the slide to rise or fall until buoyancy compensationis reached.