https://doi.org/10.15255/KUI.2002.014
Published: Kem. Ind. 52 (3) (2003) 103–120
Paper reference number: KUI-14/2002
Paper type: Review
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Mechanisms of Reactions of Solid and Liquid Reactant

X. Idrizi, A. Janeković, Ž. Mrak, D. Pavišić-Strache and D. Vuina

Abstract

This review presents types of heterogeneous reactions. There are phase-boundary controlled processes reactions, reactions controlleds by nucleation, processes governed by nucleation followed by the bulk growth of nuclei, processes controlled by nucleation followed by the linear growth of nuclei and diffusion- controlled reactions. The classification of mechanisms of reactions is presented (Fig. 1). There are: Independet reactions(U): both processes begin from two different starting substances -A1 and A2. Since rate of reactions depends only on concentration of starting substance, processes are mutually independent even when they give the same final product. Competition reactions(P): both of reactions run from one starting substance, and each reduces the rate of the other. Consecutive reactions(F): reactions run consecutively one after the other. Primary formed intermediate goes to the product of reaction. The occurrence of the first reaction is condition for the begining of the second reaction. Opposite reactions(G): reactions occur one after the other, but primarily formed product again goes to the starting substance. Autocatalytic reactions(1a): presence of the final product is condition for their occurrence. For the exact treatment of kinetics measurements, autocatalytic step is a necessary separation in two processes: Mechanisms U, P, F i 1a are irreversible. Type G is balance by isothermal experiment. Under nonisothermal conditions behaviour of reaction system responds either to individual reaction or consecutive reactions with opposite symbol. Theory of interactions of liquid and polydispersive solid reactant with equations and figures is presented. In the reaction of liquid reactant and solid substance in which particles are regularly small sphere of the same diameter (monodisperse powder) three processes are possible: 1. a) interaction of solvent and solid phase, b) interaction of solid phase and substance disolved in some solvent, 2. diffusion of ions or molecules: a) from solid phase to solution, b) from solution to solid phase, 3. diffusion through the formed solid layer (wrapper of particle), which is usual of gel nature, and diffusion is possible. When (in case 1a) both phases are quiet, on boundary of the solid-liquid is limiting-concentration formed, i.e. saturated concentration which decreases with distance from boundary of phases, fig. 2. In solution, regardless of distance from the boundary layer, constant concentration is not established. In this case Fick's second law is holding. Gradient of concentration is changing continuously. At fig. 2 dependence of concentration on distance from boundary of phases after time is shown cg(II)=0. By mixing the reaction mixture, situation is fundamentally different: in narrow area around grain, gradient of concentration is formed, while in solution (matrix) concentration is the same throughout, although it is changing with time, fig. 3. It is denoted with , and is determined by kinetic experiments. When limiting concentration is attained, cg, which depends on temperature, reaction is stopped. In case 1b we study the reaction between solid phase and reactant dissolved in solvent. Initial conentration, c0, decreases with time , and after time obtaines value . In narrow layer around grain concentration decreases when one approaches the surface of the grain. At boundary phases, limiting concentration, cg, is established. Two boundary cases are possible (fig.4). If diffusion is very rapid i.e. if interaction at boundary phases is very slow, then . If reaction is very rapid, and dissolved reactant is quickly consumed, diffusion of molecules of reactant through solution is slow compared with the rate of reaction, and cg(II)=0. For cases 1a and 1b is valid: Eq. 1 shows that in a layer at the surface of solid phase the change of the amount of substance in unit of time (this may be considered as a reaction rate) is proportional to the change of concentration with distance from the phase boundary, dc/dx , area of particle, A , and constant K , which depends on process which is decisive for the rate of reaction. In the case of diffusion, constant is named diffusion coefficient and is denoted with D (Fick's first law). Next equation shows the law of linear kinetic: First factor at the left side of equation often is denoted with 1/Ks . Since Ks is constant, when law of linear kinetic is valid, interaction is not different from diffusion. Kinetic measurements give straightline which must start from starting point, fig. 5. The common rate equations in kinetic analysis of isothermal heterogeneous reactions considering the types of these reactions are also presented. There are phase-boundary controlled processes reactions (eq. 45 to 47), reactions controlled by nucleation (eq. 48,49), processes governed by nucleation followed by the bulk growth of nuclei (eq. 50 to 54), processes controlled by nucleation followed by the linear growth of nuclei (eq. 55 to 57) and diffusion- controlled reactions (eq. 58 to 62). For some reactions is competent order of reaction (eq. 63 to 65). The end of this review contains a short description of methods for determination of particle size distribution. Coulter-Counter method for determination of particle size distribution is shown on Fig. 6. Since this area of investigation is very demanding, knowledge of the types of heterogeneous reactions and their mechanisms as well as theoretical principles which explain interaction of solid-liquid at any case, faciliates understanding and possible solution for a problem of complex area just as it is the kinetic reaction.

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Keywords

heterogeneous reactions, nucleation, diffusion (Fick law), kinetic analysis, concentration gradient, centrifugal methods