Solid—Liquid Reactions

Solid-liquid reactions are much more complex than solid-gas reactions and include a variety of technically important processes such as corrosion and electrodeposition. When a solid reacts with a liquid, the products may form a layer on the solid surface or dissolve into the liquid phase. Where the product forms a layer covering the surface completely, the reaction is analogous to solid-gas reactions if the reaction products are partly or wholly soluble in the liquid phase, the liquid has access to the reacting solid, and chemical reaction at the interface therefore becomes important in determining the kinetics. 

The simplest solid-liquid reaction is the dissolution of a solid in a liquid. The rate at which a solid dissolves in a liquid depends on the particular crystallographic plane (face) exposed. The effect of crystallographic planes on dissolution is clear from the observation that spherical single crystals acquire polyhedral shapes while dissolving. In 

The formation of a liquid phase by melting or eutectic reactions is readily determined by DTA or DSC. Microscopic observation of a quenched sample will generally indicate the presence and amount of a liquid phase at the higher temperature. 

Many texts and monographs have been written on the subject of solution-precipitation phenomena. The formation of colloid phases and the rheological properties of suspensions are outside the scope of this chapter. The electrical properties of the solid surface and the associated layer of electrolyte or solvent phase play major roles in the transport and flocculation of the solid phase. 

Few data are available on the subject of the simultaneous reaction and comminution in solid-liquid systems. This field is occupied by ultrasound applications and synthetic methods, as summarized previously [50, 101]. Of the few applications in milling devices the reduction of liquid TiCU with Mg to Ti-powder has been 

Similar calculations for the final conditions give [with ([fi] 2)f = 3 x 10- mol/cm ] 

This reaction starts out in regime 3 but shifts to regime 1-2 at the end. In other words, a fast reaction slows down as it progresses to the end. 

In Chapter 16 we shall see how this information can be used in designing a reactor. 

A good reactor to use for laboratory experiments is a batch reactor with a qiescent interface. This is particularly useful in obtaining rate data under different controlled conditions of mass transfer. A simple way of varying the mass transfer. A simple way of varying the mass transfer effect is to float various sizes of plastic balls at the liquid-liquid interface, thus controlling the area of mass transfer. Reactors in which droplets of one liquid are introduced into the other can also be used. 

Solid-liquid reactions can be classified into three categories depending on the solubility of the solid in the liquid  

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Since the free energy of a molecule in the liquid phase is not markedly different from that of the same species volatilized, the variation in the intrinsic reactivity associated with the controlling step in a solid—liquid process is not expected to be very different from that of the solid—gas reaction. Interpretation of kinetic data for solid—liquid reactions must, however, always consider the possibility that mass transfer in the homogeneous phase of reactants to or products from, the reaction interface is rate-limiting [108,109], Kinetic aspects of solid—liquid reactions have been discussed by Taplin [110]. 

Solid-Liquid Reaction Catalyzed by a Liquid Containing Rich Phase-Transfer Catalyst — Synthesis of Hexyl Acetate... 

This work was initiated for the purpose of evaluating the feasibility of synthesizing hexyl acetate (ROAc) fi-om n-hexyl bromide (RBr) and sodium acetate (NaOAc) by a novel PTC technique. In this new technique, the solid-liquid reaction was catalyzed by a catalyst-rich liquid phase in a batch reactor. Because there a solid phase and two liquid phases coexist, it is called as a SLL-PTC system [3]. Actually, this liquid phase is the third liquid phase in the tri-liquid PTC system. It might be formed when the phase-transfer catalyst is insoluble or slightly soluble in both aqueous and organic phases. Both aqueous and organic reactants can easily transfer to this phase where the intrinsic reaction occurs [4, 5]. 

Many solid-liquid reactions are likely to benefit from the careful selection of the reactant (particle size, method of manufacture, etc.). Many reaction-crystallization systems may benefit from the use of seed crystals (see Section 5.4). 

The various types of heterogeneous reactions are shown in Table 3.3. They are broadly grouped as solid-gas, solid-liquid, solid-solid, liquid-gas, and liquid-liquid reactions. The different types included in each group are also shown in the compilation. Some representative processes have been indicated as examples. It may be pointed out that in the group of solid-liquid reactions a specific mention of what is known as autocatalytic reactions has not been made. The autocatalytic processes occur when the liquid product reacts further with the solid undergoing reaction. The dissolution of copper in dilute sulfuric acid (or aqueous ammonia) in the presence of oxygen may be cited as an example ... 

The effect of solvent on crown ether-catalysed solid-liquid reactions has not... 

Modeling the Chemical Equilibria in Solid-Liquid Reactions... 

Although there are algebraic analyses in the literature relating the progress of solid—solid reactions to diffusion constant and particle sizes, there are of little use for either prediction of even for extrapolation. Experiments frequently produce measurements which, equally badly, fit a number of models. Even solid—liquid reactions with a liquid product are likely to be difficult to model as most solids are non-homogeneous in structure. Dissolution of a solid therefore does not proceed uniformly from the outside, but rather attack occurs preferentially leading, on occasions, with larger multi-crystalline particles, to their break up. 

Catalysts synthesized from crown ether monomers 61 and 62 by copolymerization with styrene and either p-divinylbenzene or p,p -divinylbiphenyl (63) are listed in Table 14 along with their relative activities for solid/solid/liquid reactions of potassium acetate with benzyl chloride (Eq. (13)) and potassium cyanide with 1,4-dichlorobutane (Eq. (14)) in acetonitrile 183). 

Agitated vessels (liquid-solid systems) Below the off-bottom particle suspension state, the total solid-liquid interfacial area is not completely or efficiently utilized. Thus, the mass transfer coefficient strongly depends on the rotational speed below the critical rotational speed needed for complete suspension, and weakly depends on rotational speed above the critical value. With respect to solid-liquid reactions, the rate of the reaction increases only slowly for rotational speed above the critical value for two-phase systems where the sohd-liquid mass transfer controls the whole rate. When the reaction is the ratecontrolling step, the overall rate does not increase at all beyond this critical speed, i.e. when all the surface area is available to reaction. The same holds for gas-liquid-solid systems and the corresponding critical rotational speed. 

Langmuir-Hinshelwood (LH) kinetics are widely used to quantitatively delineate substrate preadsorption in both solid-gas and solid-liquid reactions. The model assumptions are stated in Table 9.2. Under these... 

Solid-Gas and Solid-Liquid Reactions in Molecular Crystals... 

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