Physical states reaction rate, influence

Chemical kinetics deals with reaction rates and the stepwise molecular events by which a reaction occurs. Under a given set of conditions, each reaction has its own rate. Concentration affects rate by influencing the frequency of collisions between reactant molecules. Physical state affects rate by determining the surface area per unit volume of reactant(s). Temperature affects rate by influencing the frequency and, even more importantly, the energy of the reactant collisions. 

Experimental Materials. All the data to be presented for these illustrations was obtained from a series of polyurethane foam samples. It is not relevant for this presentation to go into too much detail regarding the exact nature of the samples. It is merely sufficient to state they were from six different formulations, prepared and physically tested for us at an industrial laboratory. After which, our laboratory compiled extensive morphological datu on these materials. The major variable in the composition of this series of foam saaqples is the aaK>unt of water added to the stoichiometric mixture. The reaction of the isocyanate with water is critical in determining the final physical properties of the bulk sample) properties that correlate with the characteristic cellular morphology. The concentration of the tin catalyst was an additional variable in the formulation, the effect of which was to influence the polymerization reaction rate. Representative data from portions of this study will illustrate our experiences of incorporating a computer with the operation of the optical microscope. 

A characteristic of aldehyde polymerization is the precipitation, often with crystallization, of the polymer during polymerization. Depending on the solvent used, polymerization rate, state of agitation, and other reaction conditions, the polymerization can slow down or even stop because of occlusion of the propagating centers in the precipitated polymer. The physical state and surface area of the precipitated polymer influence polymerization by their effect on the availability of propagating centers and the diffusion of monomer to those centers. 

Influence of Interpolymer Properties. As stated earlier, the physical and chemical properties of interpolymers markedly influence the reaction rate after the induction period. If the monomer present yields a polymer comparable in viscosity with the initial mixture the rate of scission will not accelebrate. For example, the polymerization rate of chloroprene on mastication with natural rubber does not increase as markedly with conversion (69), see Fig. 19, as with methyl methacrylate and styrene. The reason is the chloroprene-rubber system remained elastic and softer than the original rubber. 

The organic solvent is the most important variable as it controls partition and diffusion of the reactants between the two immiscible phases, the reaction rate, solubility, and swelling of permeability of the growing polymer. The solvent should be of such composition so as to prevent precipitation of the polymer before a high molecular weight has been attained. The final polymer should not dissolve in the solvent. The type of solvent will influence the characteristics of the physical state of the final polymer. Solvents such as chlorinated or aromatic hydrocarbons make useful solvents in this system. 

It must be repeated that this argument depends upon the assumption that there is only one way in which the molecules of formic acid can be attached to the surface of the catalyst. There is, however, some evidence against this assumption. Constable finds that the two simultaneous reactions undergone by allyl alcohol when passed over heated copper are differently influenced by the physical state of the catalyst. This points to the conclusion that there are two independent centres of activity on the catalyst surface with two different modes of adsorption, or, at any rate, centres where the energy of adsorption is so different that different reactions are facilitated. Hoover and Rideal f find that the two alternative decompositions of ethyl alcohol by thoria show a different behaviour towards poisons, which points to the same conclusion. 

The chemistry of lipid decomposition in foods at elevated temperatures is complicated. Multiple reactions and interactions can occur rapidly and competitively. The rates and pathways of these reactions are influenced significantly by temperature, reaction time, other constituents in the surrounding environment, physical state, and molecular organization. 

Polanyi s scientific work lay most squarely within a physical chemistry that encompassed thermodynamics, X-ray crystallography, the study of reaction rates, and the application of quantum mechanics to the study of molecular forces and transition states. In two particular areas, the investigation of solid-surface adsorption phenomena and X-ray diffraction studies of the properties of solids, Polanyi helped establish new scientific specialities, at the boundaries of physics and chemistry, for studying the solid state. He also turned his research experiences in these fields into a basis for the formulation of a new philosophy of science centered on scientific practice, rather than scientific ideas. [1] It is these themes that I would like to explore, with remarks in my conclusion on Polanyi s influence in solid-state science. 

The influence and impact of these semi-empirical calculations and absolute reaction rate theory on the thinking of physical organic chemists was profound. It makes clear, for example, the electronic basis for some of Ingold s broad generalizations, e.g. In bimolecular eliminations, E2, in systems H—Cp—Ca—X, where X may be neutral or charged, the ]8-CH electrons, independently of the electrostatic situation, enter the Ca octet on the side remote from X, because repulsive energy between electron-pairs in the transition state can thus be minimized the result is anti-elimination, independently of the structural details of the system (Ingold, 1953). 

Rate of Solubility—The rath of solubility of small particles depends on a great number of variables. Eq (12-2) takes into account free surface energy (a) and particle surface (1 /d). These are purely surface considerations, and are scarcely complete in themselves. The shape of the surface and its physical state must also be specified, that is, its relative freedom from contamination which might influence the speed of reaction. The effect of packing density and the extent of agitation imparted to the particles are also important, particularly with regard to exposure of fresh surfaces and formation of possible gas pockets. The liquid and liquid-solid phases jointly are additional important considerations. The volume of the liquid, its temperature, and the amount of dissolved solid already in solution must enter into all calculations. Nor can we ignore the chemical nature of the substances involved in the... 

Another factor that can influence the rate of a reaction is the physical state of the reactants. Ground-up chalk will dissolve much more quickly in vinegar than a solid stick of chalk. The phase of the reactants can also influence the rate of the reaction liquid gasoline bums gasoline vapor explodes. Foreign surfaces are always present in a reaction, even if it is just the surface of the container, so they have to be considered, too. Sometimes these surfaces increase the rate of reaction by holding one reactant... 

Factors that influence polarization include (I) electrode size, shape, and composition (2) composition of the electrolyte solution (3) temperature and stirring rate (4) current level and (5) physical state of species involved in the cell reaction. 

The ultimate purpose of these types of tests is to evaluate two similar (in results) but different occurrences. These are runaway chemical reactions and exothermic chemical decompositions. The first may actually just be a desired reaction out of control while the second is an undesired reaction out of control. Among the purposes which analytical tests serve are the determination of the "onset" of exothermic (endothermic) decomposition. While frequently a specific temperature is cited for such "onsets," one must remember that this temperature is highly dependent on instrument sensitivity, degree of adiabaticity and time-temperature history. It should be stated that tests results are accurate only for the exact conditions under which they were run. Physical factors such as density and geometry can also influence test data. In theory, reaction rates are not a step function but are continuous. 

Experimental conditions influencing the reaction rate include the structure of the reacting species, the concentration of reactants, the temperature of reactants, the physical state of reactants, and the presence or absence of a catalyst. 

When a chemical system is submitted to certain physical conditions, its composition can be determined by thermodynamic means if each transformation is performed as a succession of equilibrium states. In fact some competitive reactions may occim and the evolution of the system is determined by the fastest reactions. If consecutive reactions take place, it is the slowest ones which govern the evolution and the system can stay in a metastable state during an undetermin time. Thus, as they do not take account of time, the thermodynamic laws are often used to provide the real composition of such a chemical system. But in the general case one have recomse to kinetic models which tend to predict the influence of physical conditions on the reaction rates. 

In dilute solutions it is possible to relate the activity coefficients of ionic species to the composition of the solution, its dielectric properties, the temperature, and certain fundamental constants. Theoretical approaches to the development of such relations trace their origins to classic papers by Debye and Huckel (6-8). For detailed treatments of this subject, refer to standard physical chemistry texts or to treatises on electrolyte solutions [e.g., that by Hamed and Owen (9)]. The Debye-Hiickel theory is useless for quantitative calculations in most of the reaction systems encountered in industrial practice because such systems normally employ concentrated solutions. However, it may be used together with transition state theory to predict the qualitative influence of ionic strength on reaction rate constants. 

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