[No abstract available] / Science, Faculty of / Chemistry, Department of / Graduate

A kinetic survey of the reactions of the ethyl radical with conjugated cyclic defines and related compounds has revealed a great variety of reactivity with respect both to addition and metathesis. An attempt to correlate reactivity with molecular structure has been presented both in the language of polymerization kinetics and of molecular orbital theory. In particular, the correlation of the energy of activation for the addition reaction with free valence and with localization energy has been pursued with some measure of success. The results of this survey indicate that characteristic values exist for the weighted A factors of both the addition and metathetical reactions. If the measured A factor is divided by the number of equivalent most-reactive sites in the molecule, values of the quotient lie within a narrow range. A characteristic value of this quotient appears to be common to molecules of diverse structural types. The results of this survey closely parallel the results of other investigators in related fields. In particular, rate constants for the addition of the ethyl radical in the gas phase are linearly related to rate constants for the addition of the methyl and the poly-styryl radical in solution. The few deviations observed in the comparison with the polyvinyl acetate and the polyacrylonitrile radicals may be explained in terms of polar effects by reference to the general equations for radical reactivity proposed by Bamford, Jenkins and Johnston and by Schwan and Price. The significance of the ratio of rate constants for addition and metathesis of a given compound with the ethyl radical has been considered as an index for reactivity in homopolymerization. The values of the ratio accord well with the published data on the degree of polymerization of these compounds. Finally the kinetic measurements have provided further information on certain special topics including the reactions of the cyclohexadienyl radical, the electron affinity of cyclooctatetraene and the structure of cycloheptatriene. / Science, Faculty of / Chemistry, Department of / Graduate

Bibliography: pages 210-215. / A theory for the growth kinetics of planar silicide formation in single- and bi-layer metal silicon systems has been developed on the basis that the chemical potential gradient in the growing layer is the driving force for diffusion. The predictions of the theory, when applied to single layer metal-silicon systems, is in agreement with other theories and with experimental results. Planar growth of the outer silicide layer in bilayer metal-silicon systems is predicted to proceed linearly with time, both when controlled by an interfacial reaction and when limited by diffusion through the interposed silicide layer (when this layer is sufficiently thick). In the latter case it is predicted that the growth rate of the outer silicide layer is inversely proportional to the thickness of the interposed layer.

Protein Calorie Malnutrition (PCM) is a term which embraces the broad spectrum of diseases occurring where diets are poor in protein. In most cases the diseases are due to an inadequate intake of utilizable protein. A small percentage of cases are due to excessive protein loss, (e.g. burns, nephrosis·, protein loosing enteropathy) or to defects in protein· absorption (gastro-enteritis, steatorrhoea) or synthesis (liver disease). Where the caloric intake is relatively normal the disease produced, kwashiorkor, is characterised by retardation of growth and development, muscular wasting, oedema and psychic changes, usually peevishness or apathy. In addition, some cases have skin lesions and the colour and texture of the hair is often abnormal. Diarrhoea is almost constant in the recent history and is usually present on admission to hospital. The disease is most common in children who have recently been weaned.

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The gas phase reactions of atomic chlorine with hydrogen sulfide, ammonia, benzene, and ethylene are investigated using the laser flash photolysis / resonance fluorescence experimental technique. In addition, the kinetics of the reverse processes for the latter two elementary reactions are also studied experimentally. The absolute rate constants for these processes are measured over a wide range of conditions, and the results offer new accurate information about the reactivity and thermochemistry of these systems. The temperature dependences of these reactions are interpreted via the Arrhenius equation, which yields significantly negative activation energies for the reaction of the chlorine atom and hydrogen sulfide as well as for that between the phenyl radical and hydrogen chloride. Positive activation energies which are smaller than the overall endothermicity are measured for the reactions between atomic chlorine with ammonia and ethylene, which suggests that the reverse processes for these reactions also possess negative activation energies. The enthalpies of formation of the phenyl and β-chlorovinyl are assessed via the third-law method. The stability and reactivity of each reaction system is further rationalized based on potential energy surfaces, computed with high-level ab initio quantum mechanical methods and refined through the inclusion of effects which arise from the special theory of relativity. Large amounts of spin-contamination are found to result in inaccurate computed thermochemistry for the phenyl and ethyl radicals. A reformulation of the computational approach to incorporate spin-restricted reference wavefunctions yields computed thermochemistry in good accord with experiment. The computed potential energy surfaces rationalize the observed negative temperature dependences in terms of a chemical activation mechanism, and the possibility that an energized adduct may contribute to product formation is investigated via RRKM theory.

Thesis (MTech (Mechanical Engineering))--Cape Peninsula University of Technology, 2010 / Development of safe storage for reactive combustible materials to prevent possible human and environmental hazards as well as ensure and enhance industrial safety can significantly benefit from mathematical modelling of systems. In the recent past, models with varying degrees of sophistication have been developed and applied to the problem of predicting thermal criticality conditions, temperature and concentration profiles of such system. In this thesis, a model showing the temperature history of an nth order exothermic oxidation reaction in a slab of combustible material with variable pre-exponential factor, taking the consumption of the reactant into account in the presence of a convective heating and oxygen exchange at the slab surface with the ambient is presented Both transient and steady state problems are tackled The critical regime separating the regions of explosive and non-explosive paths of a one step exothermic chemical reaction is determined The governing nonlinear partial differential equations are solved numerically by method of lines (MOL), with finite difference schemes used for the discretisation of the spatial derivatives. Moreover, both fourth order Runge-Kutta numerical integration coupled with shooting methods and perturbation techniques together with a special type of Hermite-Pade series summation and improvement method were employed to tackle the steady state problem. The crucial roles played by the boundary conditions in determining the location ofthe maximum heating were demonstrated. In chapter one, the relevant applications together with previous published work on the problem were highlighted The basic mathematical theory and equations needed to tackle the problem were derived in Chapter two. In chapter three, the transient model problem was formulated, analysed and discussed. The steady state problem was formulated and solved in Chapter four. Furtherwork and concluding remarks were highlighted in Chapter five.

The recombination reaction Na + S02 + Ar was investigated at 787 16 K and at pressures from 1.7 to 80 kPa. NaI vapor was photolyzed by an excimer laser at 308 nm to create Na atoms, whose concentration was monitored by time-resolved resonance absorption at 589 nm. The rate constant at the low pressure limit is ko = (2.7 0.2) x 10-21 cm6 molecule-2 s~1. The Na-SO 2 dissociation energy E0 = 170 35 kJ mol1 was calculated with RRKM theory. The equilibrium constant gave a lower limit E0 > 172 kJ mol~ 1. By combination of these two results, E0 = 190 15 kJ mol~ 1 is obtained. The high pressure limit is k, = (1 - 3) x 10-10 cm3 molecule 1 s~1, depending on the extrapolation method used. Two versions of collision theory were employed to estimate k,.. The 'harpoon' model shows the best agreement with experiment.

<p> The growth kinetics of a plates and rods in B-brass have been calculated using a local equilibrium model. These are in good agreement with published experimental data. Along with this, quantitative microanalysis of deep etched specimens ts cited in support of the proposed diffusional mechanism for their formation. </p> <p> The model was also employed to calculated the growth rates of upper bainitic rods in the Fe-C-Nisystem at 400° C. A "phase diagram" was constructed in order to supply the effective supersaturations required by the local equilibrium model$ Lengthening rates were also calculated using a paraequilibrtum Interface condition. The experimental data were correctly predicted assuming the former case but failed at the "phase" boundary. It seems that a solute drag might well operate in this system. These results have provided some further indication of the part alloying elements play in the formation of the microstructural constituents of steel. </p> / Thesis / Master of Engineering (MEngr)