Chemical reactions Arrhenius behavior

The time required to produce a 50% reduction in properties is selected as an arbitrary failure point. These times can be gathered and used to make a linear Arrhenius plot of log time versus the reciprocal of the absolute exposure temperature. An Arrhenius relationship is a rate equation followed by many chemical reactions. A linear Arrhenius plot is extrapolated from this equation to predict the temperature at which failure is to be expected at an arbitrary time that depends on the plastic s heat-aging behavior, which... 

The rate of a chemical reaction may be affected by temperature in several ways, but the most common behavior by far is that observed by Arrhenius some 100 years ago. The empirical... 

Example 5.3.2 demonstrates how the heat of adsorption of reactant molecules can profoundly affect the kinetics of a surface catalyzed chemical reaction. The experimentally determined, apparent rate constant Ikj/Ki) shows typical Arrhenius-type behavior since it increases exponentially with temperature. The apparent activation energy of the reaction is simply app = E2 - AHadsco = - A//adsco (see Example 5.3.2), which is a positive number. A situation can also arise in which a negative overall activation energy is observed, that is, the observed reaction rate... 

The effects of temperature on chemical reactions, including respiratory rate, traditionatly quantified by Qio, which is a coefficient by which it is pessible to calculate how many times increases the rate of a reaction for each increase in temperature of 10 °C. The effect of temperature can also be quantified by the Arrhenius model, where the effect of temperature increase is given by the activation energy (Ea) (Cameron et al., 1995). The temperature quotient is useful because it allows us to calculate the respiration rates at one temperature from a known rate at another temperature. However, the resparation rate does not follow ideal behavior, and the Qio can vary considerably with temperature. At higher temperatures, the Qio is usually smaller than at lower temperatures. 

Reaction limitation obtains if the rate coefficients of the surface reactions [(4), (5), (6)] are very low compared to the transport steps. Chemical reactions are thermally activated and rates of simple reactions follow Arrhenius behavior. Thus a plot of the logarithm of the deposition rate vs. 1/T is a straight line in the temperature range corresponding to the reaction-limited regime. The slope of this line is determined by the adsorption heats of the reactants and the activation energy of the rate-determining surface reaction step. 

The ability to model the detailed chemistry of ignition and combustion of energetic materials requires the simultaneous treatment of the chemical kinetics behavior of large chemical reaction systems combined with convective and diffusive transport of mass, momentum, and energy. Such models require the evaluation of equations of state, thermodynamic properties, chemical rate expressions, and transport properties. The computer software used to evaluate these quantities is referred to as the Chemkin package.33-35 includes an interpreter for the chemical reactions, a thermochemical data base, a linking file, and gas-phase subroutine libraries. The interpreter reads in the list of elementary chemical reactions. The forward reaction rates are given in the form of the Arrhenius rate expression... 

Arrhenius kinetics applies to many physical processes, not just to chemical reactions. One example is the diffusion of atoms in solids. Figure 19.6 shows that the diffusion rates of carbon atoms through solid iron metal follow Arrhenius behavior over a remarkable 14 orders of magnitude. This evidence supports the interstitial model, in which the carbon atoms occupy the interstices in the iron lattice, and Jump over energy barriers to travel from one interstitial site to another. 

In the temperature range between -323 K and 423 K, it is found that the time to fracture and temperature follow an Arrhenius relation. The activation energy for soda-lime-silica glass was found to be equal to 18.1 kcal/mole. This kind of behavior shows that the static fatigue is a temperature-dependent activated process, similar to any chemical reaction or diffusion. 

The electron transfer in the photosynthetic reaction center of Rh, viridis involves three initial steps RsRlQaQb- s lQaQb s lQaQb- Rs lQaQb- Different from Arrhenius behavior of most chemical reactions, one observes that electron transfer rates vary little with temperature, in some instances even increase when temperature is lowered. In order to explain the redox and temperature dependence of electron transfer rates, previous interpretations have assumed that quantum mechanical behavior of electron transfer arises through a small number of nuclear degrees of freedom, such as one or two, particularly strongly coupled to the electron transfer re-... 

The overwhelming majority of chemical reactions really complies with the Arrhenius law and, in the logarithmic scale, is schanatically expressed by a straight line (Figure 7.13). However, not long ago, the effect of the limitation of chanical reactions ratio at low temperatures was observed, the ratio became constant (Figure.7.13), in contradiction to Arrhenius theory. An explanation of this behavior can be given within the framework of the tunnel mechanism. 

An example of this approach was presented earlier in Figure 3.34, which contains Arrhenius plots (rate vs. l/T cf. Section 3.0.2) at different total pressures. Figure 3.34 clearly shows the two types of deposition rate behavior. At low temperatures (higher 1/r) the reaction kinetics are slow compared to mass transport, and the deposition rate is low. At higher temperatures (lower HT) chemical kinetic processes are rapid compared to mass transport, resulting in a distinct change in slope and a higher deposition rate. 

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