Neutralisation, rate coefficient

Other results also confirm the important role of internal diffusion. Experimental activation energies (67—75 kJ mol"1) of the sucrose inversion catalysed by ion exchangers [506—509] were considerably lower than those of a homogeneously catalysed reaction (105—121 kJ mol"1) [505, 506,508] and were close to the arithmetic average of the activation energy for the chemical reaction and for the diffusion in pores. The dependence of the rate coefficient on the concentration in the resin of functional groups in the H+-form was found to be of an order lower than unity. A theoretical analysis based on the Wheeler—Thiele model for a reaction coupled with intraparticle diffusion in a spherical bead revealed [510,511] that the dependence of the experimental rate coefficient on acid group concentration should be close to those found experimentally (orders, 0.65 and 0.53 for neutralisation with Na+ and K+ ions respectively [511] or 0.5 with Na+ ions [510]). 

Just as in the gas-phase, thermodynamics tells only part of the story in respect of reactions in solution kinetics also plays its part. An important additional consideration is that, in solution, if a bimolecular reaction is intrinsically fast as, for example, in acid-base neutralisation, the rate-determining process can be the diffusion of the reactants through the solvent before they encounter one another. If the reaction occurs every time the reactants (say, A and B) meet and they are assumed to be spheres with radii Ta and rg, it can be shown that the rate coefficient ( d) for the diffusion-controlled reaction is given by ... 

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