Temperature activation energy

As defect clusters tend to disassociate at high temperatures, the aggregation enthalpy, Af/agg, would tend to zero at high temperatures. The high-temperature activation energy would then simply correspond to the migration enethalpy ... 

A plot of the dissolution rate against driving force, AC, is shown in Figure 13. This shows a linear dependence on undersaturation and a slight dependence on temperature (activation energy of 10 kJ/mole). This indicates that the dissolution is mass transfer controlled. The results can be correlated by... 

Reaction Catalyst Start temperature Activation energy... 

In developing mathematical expressions for selectivities, knowledge of the rate equations are required. This is because the instantaneous selectivity is defined in terms of the rate ratios. The parameters that affect the instantaneous and the overall selectivities are exactly the same as those influencing the reaction rates, namely, the concentration, temperature, activation energy, time of reaction (residence time in flow reactors), catalysts, and the fluid mechanics. 

Fig. 3.28. Low temperature activation energy versus square root of electric field extrapolated to zero field. The zero field activation energy was found to be Aq = 0.05 eV [54].

Kinetics of Yielding. Tensile yield stresses were measured at several Instron rates for block polymer B. These results and those of Bauwens-Crowet et ah (9) for BPA polycarbonate were analyzed in the framework of the Rhee-Eyring stress bias activation theory for comparison purposes. Yield stress of the block polymer is roughly half that of the homopolymer at a given temperature. The apparent activation volume for the block polymer is double that of the homopolymer at each temperature. Activation energies at zero stress are essentially the same (60 kcal/mole for the block polymer vs. 70 kcal/mole for BPA polycarbonate above —50°C). 

It has been known since the early studies of Kearney (192) that succinate dehydrogenase undergoes reversible activation by substrates, competitive inhibitors, and phosphate. The activation of succinate dehydrogenase was shown to be a characteristic of both the soluble and particle-bound enzyme and a slow process requiring many minutes of incubation with the activator at ambient or higher temperatures (activation energy = 31-33 kcal/mole). It has been suggested that the enzyme exists in a free equilibrium between the unactivated and the activated forms, and that the activator interacts with the latter and establishes a new equilibrium in favor of the activated state of the enzyme (23, 25, 193 see also 194 for an expanded mechanism). 

Absolute temperature Activation energy Shear activation volume Die exit velocity Distance from die cone apex Geometric factor Die semi-angle Intrinsic thermal expansivity... 

Table 28.1 Peak temperatures, activation energies and postulated modes of molecular motion for the mechanical relaxations in polystyrene...

Studies of the effect of temperature on V and have been made. From the variation of reaction velocity with temperature, activation energies have been found (see Table XVIII, p. 350). The activation energy of the removal of pyridoxal 5-phosphate from rabbit-muscle phosphorylase b has been calculated as 11.7 Kcal./mole, and the activation energy of its recombination with the enzyme was 22.3 Kcal./mole. ... 

Table 5-0 Rheological Properties of Alcohols and Wines, and Effect of Temperature (activation energy, E )... 

Where n varies with temperature, the apparent activation energy of dissolution will vary with pH of the system. As a consequence, in the literature, two kinds of are discussed apH-dependent and a pH-independent activation energy (Chen and Brantley, 1998). The pH-dependent (apparent) activation energy, E , is reported by investigators who plot In (rate) versus l/T, and is valid only at the pH of measurement. The pH-independent is determined from aplot of In(feH) versus 1/T. Where n is independent of temperature, = E. For phases where n increases with temperature, activation energies reported in Table 6 are larger... 

The thermal Z —> E isomerization of azobenzene has been widely used to determine free volume in polymers at room and temperatures as low as 4 K.90b9i Jhe thermal reaction is also important in the context of photo-response, as an information written or a signal or state produced by switching E to Z is slowly fading. However, the Z-lifetime is strongly modified by strain in the molecule Z-azobenzene in solution at room temperature has a half life of about 2 days the Z,E E,E isomerization in the [3.3] 4,4 )azo-benzenophane 9 has a half life of ca. 4 min. the [2.2] 4,4 )azobenzenophane 7 has a half life of ca. 15 seconds and in dibenzo[2.2][4.4 )-azobenzeno-phane 8 the life of the E,Z-isomer drops to 1 s. On the other hand, the Z,Z Z,E isomerization in these phanes is slowed down enormously Z,Z-7 lives 2.5 days, Z,Z-9 about 5 days, and Z,Z-10 about 1 year at room temperature. Activation energies are available in the publications. The Z,E E,E isomerization in most azobenzenophanes is very fast. However, in 2,19-Dioxo[3.3](3,3 )azobenzolophane 12, the Z,E-form is relatively stable, The remarkable differences in these and other structures are not due to different activation enthalpies but to different activation entropies. 

Much less stable at heating is poly(vinyl bromide), which starts decomposing as low as 100° C with dehydrobromination. Since the dehydrobromination occurs at lower temperatures (activation energy of only 17 kJ mol ), long chains of unsaturated hydrocarbons are generated and do not decompose until the temperature is further increased. Some literature reports regarding thermal decomposition of these polymers are summarized in Table 6.3.8. 

Effect of Temperature. Activation Energies of Surface Diffusion... 

The actual rates of change of property with time in the accelerated tests are not sensibly contrasted to those from natural ageing from simple observation of the curves. This is simply because it is the rate at which the rates of change with time change with temperature (activation energy) that will largely determine predictions made to lower temperatures. 

The effect of temperature was not specifically studied this work, but previous work in the literature shows that the activation energy for the central polymerization mechanism is about 80,000 J/mole. This means that the temperature coefficient will be about a factor of two in the gel time for every 10°C. change in the temperature. Activation energies will probably be different on the acid and the basic side, since the temperature coefficient for the charge repulsion effect will enter into the total activation energy in basic solution. 

Isothermal Percent Conversion versus Time Isoconversion Time versus Temperature Activation Energy versus % Conversion Table of Results... 

We see that the rate of a reaction usually increases with temperature. Activation energy, which is the minimum amount of energy required to initiate a chemical reaction, also influences the rate. (13.4)... 

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