Effect of reaction temperature

Since product distributions depend on the relative rates of competing reactions, effects of temperature on products depend on differences in activation energies, AE. For each pair of competing reactions of the tert-Bu02 radicals, the following AE values (in kilocalories per mole) and qualitative effects of increasing temperature are estimated. Some of these values were considered under Liquid-Phase Oxidations. ... 

The Glow of Phosphorus—Effect of Pressure upon Oxidation of Phosphorus— Velocity of the Reaction—Effect of Temperature—Production of Ozone— Inhibition of the Glow—Nature of the Chemiluminescence—Ionisation by the Glow—The Emission Spectrum. 

Butkovskaya, N.I., Kukui, A., Le Bras, G. Branching fractions for H2O forming channels of the reaction of OH radicals with acetaldehyde. J. Phys. Chem. A 108, 1160-1168 (2004) Butkovskaya, N.I., Kukui, A., Pouvesle, N., Le Bras, G. Foimatimi of nitric acid in the gas-phase H02-hNO reaction Effects of temperature and water vapor. J. Phys. Chem. A109,6509-6520 (2005)... 

Inherent Diastereoselectivity Effect ofHydrogen Bonding on Diastereoselectivity of Patemo Btlchi Reaction Induced Diastereoselectivity Effect of Concentration on the Stereoselectivity of the Patemo Btlchi Reaction Effect of Temperature and Solvent Viscosity on the Stereoselectivity of the Paternb- Buchi Reaction... 

Reactor temperature and pressure. If there is a significant difierence between the effect of temperature or pressure on primary and byproduct reactions, then temperature and pressure should be manipulated to improve selectivity and minimize the waste generated by byproduct formation. d. Catalysts. Catalysts cam have a major influence on selectivity. Changing the catalyst can change the relative influence on the primary and byproduct reactions. 

The effect of temperature on the non-catalysed reaction was difficult to disentangle, for at lower temperatures the autocatalytic reaction intervened. However, from a limited range of results, the reaction appeared to have an experimental activation energy of c. +71 kj moh. ... 

The catalysed reaction was considered to arise from the heterolysis of dinitrogen pentoxide induced by aggregates of molecules of nitric acid, to yield nitronium ions and nitrate ions. The reaction is autocatalytic because water produced in the nitration reacts with the pentoxide to form nitric acid. This explanation of the mechanism is supported by the fact that carbon tetrachloride is not a polar solvent, and in it molecules of nitric acid may form clusters rather than be solvated by the solvent ( 2.2). The observation that increasing the temperature, which will tend to break up the clusters, diminishes the importance of the catalysed reaction relative to that of the uncatalysed one is also consistent with this explanation. The effect of temperature is reminiscent of the corresponding effect on nitration in solutions of nitric acid in carbon tetrachloride ( 3.2) in which, for the same reason, an increase in the temperature decreases the rate. 

Adsorption is invariably an exothermic process, so that, provided equilibrium has been established, the amount adsorbed at a given relative pressure must diminish as the temperature increases. It not infrequently happens, however, that the isotherm at a given temperature Tj actually lies above the isotherm for a lower temperature Ti. Anomalous behaviour of this kind is characteristic of a system which is not in equilibrium, and represents the combined effects of temperature on the rate of approach to equilibrium and on the position of equilibrium itself. It points to a process which is activated in the reaction-kinetic sense and which therefore occurs more rapidly as temperature is increased. 

The following data for a 2 factorial design were collected during a study of the effect of temperature, pressure, and residence time on the %yield of a reaction. " ... 

The coordinates of thermodynamics do not include time, ie, thermodynamics does not predict rates at which processes take place. It is concerned with equihbrium states and with the effects of temperature, pressure, and composition changes on such states. For example, the equiUbrium yield of a chemical reaction can be calculated for given T and P, but not the time required to approach the equihbrium state. It is however tme that the rate at which a system approaches equihbrium depends directly on its displacement from equihbrium. One can therefore imagine a limiting kind of process that occurs at an infinitesimal rate by virtue of never being displaced more than differentially from its equihbrium state. Such a process may be reversed in direction at any time by an infinitesimal change in external conditions, and is therefore said to be reversible. A system undergoing a reversible process traverses equihbrium states characterized by the thermodynamic coordinates. 

These pioneers understood the interplay between chemical equiUbrium and reaction kinetics indeed, Haber s research, motivated by the development of a commercial process, helped to spur the development of the principles of physical chemistry that account for the effects of temperature and pressure on chemical equiUbrium and kinetics. The ammonia synthesis reaction is strongly equiUbrium limited. The equiUbrium conversion to ammonia is favored by high pressure and low temperature. Haber therefore recognized that the key to a successful process for making ammonia from hydrogen and nitrogen was a catalyst with a high activity to allow operation at low temperatures where the equiUbrium is relatively favorable. 

CP esters are generally prepared as the ammonium salt [9038-38-4] by the reaction of cellulose with phosphoric acid and urea at elevated temperatures (130—150°C). The effects of temperature and urea/H PO /cellulose composition on product analysis have been investigated (33). One of the first commercially feasible dameproofing procedures for cotton fabric, the Ban-Flame process (34,35), was based on this chemistry. It consists of mixing cellulose with a mixture of 50% urea, 18% H PO, and 32% water. It is then pressed to remove excess solution, heated to 150—175°C for 5—30 minutes, and thoroughly washed (36). 

The electrical characteristics of ceramic materials vary gteady, since the atomic processes ate different for the various conduction modes. The transport of current may be because of the motion of electrons, electron holes, or ions. Electrical ceramics ate commonly used in special situations where reftactoriness or chemical resistance ate needed, or where other environmental effects ate severe (see Refractories). Thus it is also important to understand the effects of temperature, chemical additives, gas-phase equilibration, and interfacial reactions. 

Inert gas pressure, temperature, and conversion were selected as these are the critical variables that disclose the nature of the basic rate controlling process. The effect of temperature gives an estimate for the energy of activation. For a catalytic process, this is expected to be about 90 to 100 kJ/mol or 20-25 kcal/mol. It is higher for higher temperature processes, so a better estimate is that of the Arrhenius number, y = E/RT which is about 20. If it is more, a homogeneous reaction can interfere. If it is significantly less, pore diffusion can interact. 

Sample stability becomes increasingly important as the time between sampling and analysis increases. Effects of temperature, trace contaminants, and chemical reactions can cause the collected species to be lost from the collection medium or to undergo a transformation that will prevent its recovery. Nearly 100% recovery is also required because a variable recovery rate will prevent quantification of the analysis. Interference should be minimal and, if present, well understood. 

Finally, there is an interesting article" that shows how to use Taylor s series to generate shortcut methods from established theory. Examples are given for developing a criterion for replacing log mean temperature differences with average differences and for estimating the effect of temperature on reaction rate. 

The effect of temperature on different types of reaction is shown in Figure 7.5. 

Dimethyl sulfoxide (DMSO) has been used to effect the elimination of sulfonates at elevated temperatures (see, for example, ref. 237). Benzene-sulfonates are recommended. The elimination of a variety of sulfonates proceeds readily in this medium in the presence of potassium /-butoxide. A -Compounds have been formed at 100°, but heating is not necessary. The effects of temperature change, orientation of the hydroxy group and changes in the sulfonate employed have been examined. The principal side reaction appears to be formation of the original alcohol (uninverted), particularly with equatorial mesylates at low temperatures it is minimized with axial tosylates. 

Table 1. Effect of Temperature of Decomposition of the Product of Reaction of Pentafluorophenyllithium with Dimethyl Oxalate on Product Distribution [JJ]...

FIGURE 14.12 The effect of temperature on enzyme activity. The relative activity of an enzymatic reaction as a fnncdon of tempera-tnre. The decrease in the activity above 50 C is dne to thermal denatnradon. 

The molecular mechanics calculations discussed so far have been concerned with predictions of the possible equilibrium geometries of molecules in vacuo and at OK. Because of the classical treatment, there is no zero-point energy (which is a pure quantum-mechanical effect), and so the molecules are completely at rest at 0 K. There are therefore two problems that I have carefully avoided. First of all, I have not treated dynamical processes. Neither have I mentioned the effect of temperature, and for that matter, how do molecules know the temperature Secondly, very few scientists are interested in isolated molecules in the gas phase. Chemical reactions usually take place in solution and so we should ask how to tackle the solvent. We will pick up these problems in future chapters. 

In 1927 Putochin studied the effect of temperature on the nature of the products formed when the formylation reaction was carried out in benzene and observed that 1-formyl derivatives were the major products obtained at low temperatures, whereas the 3-formyl derivatives predominated at higher temperatures. Britton et al. in 1947 claimed that the formation of the 3 -formylindole derivative is probably favored, relative to the alternate 1-formylation process, by elevated temperatures and pressures.However, it was apparently not possible to suppress completely the formation of the 1-formyl derivatives and yields of the order of 40% of both products were usually obtained. 

The effect of temperature on the kinetics of the direct radiation method is quite complex. Increase in temperature increases the monomer diffusion rate but also increases transfer and termination reaction rates of the growing chains and reduces the importance of the gel effect. Solubility and radical mobility may also change as the temperature is varied [88,89]. 

This is also an endothermic reaction, and the equilibrium production of aromatics is favored at higher temperatures and lower pressures. However, the relative rate of this reaction is much lower than the dehydrogenation of cyclohexanes. Table 3-6 shows the effect of temperature on the selectivity to benzene when reforming n-hexane using a platinum catalyst. 

For diffusion controlled corrosion reactions e.g. dissolved oxygen reduction, and the effect of temperature which increases diffusion rates, then by substituting viscosity and the diffusion coefficients at appropriate temperatures into the Reynolds No. and Schmidt No., changes in corrosion rate can be calculated. 

Bearing in mind the importance of the rate determining process and because of the complex situation in corrosion reactions of having two electrode processes, the effect of temperature is best illustrated by reference to specific situations. 

Dissolved oxygen reduction process Corrosion processes governed by this cathode reaction might be expected to be wholly controlled by concentration polarisation because of the low solubility of oxygen, especially in concentrated salt solution. The effect of temperature increase is complex in that the diffusivity of oxygen molecules increases, but solubility decreases. Data are scarce for these effects but the net mass transport of oxygen should increase with temperature until a maximum is reached (estimated at about 80°C) when the concentration falls as the boiling point is approached. Thus the corrosion rate should attain a maximum at 80°C and then decrease with further increase in temperature. 

To understand the effect of temperature on product distribution, let s briefly review what we said in Section 5.7 about rates and equilibria. Imagine a reaction that can give either or both of two products, B and C. 

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