Chemical kinetics activation energy

A similar slow evolution from energy to entropy with a final synthesis of both concepts can also be observed in the historical development of chemical kinetics. The energy factor was first pointed out by Arrhenius (1889) when he explained the temperature effect on reaction rates. But in spite of the early work of Kohnstamm and Scheffer (1911) who introduced the idea of activation entropy, the importance of entropy was generally recognized only after Eyring (1935) formulated clearly the thermodynamic treatment of the transition state method. 

The applications of this simple measure of surface adsorbate coverage have been quite widespread and diverse. It has been possible, for example, to measure adsorption isothemis in many systems. From these measurements, one may obtain important infomiation such as the adsorption free energy, A G° = -RTln(K ) [21]. One can also monitor tire kinetics of adsorption and desorption to obtain rates. In conjunction with temperature-dependent data, one may frirther infer activation energies and pre-exponential factors [73, 74]. Knowledge of such kinetic parameters is useful for teclmological applications, such as semiconductor growth and synthesis of chemical compounds [75]. Second-order nonlinear optics may also play a role in the investigation of physical kinetics, such as the rates and mechanisms of transport processes across interfaces [76]. 

There are two main applications for such real-time analysis. The first is the detemiination of the chemical reaction kinetics. Wlien the sample temperature is ramped linearly with time, the data of thickness of fomied phase together with ramped temperature allows calculation of the complete reaction kinetics (that is, both the activation energy and tlie pre-exponential factor) from a single sample [6], instead of having to perfomi many different temperature ramps as is the usual case in differential themial analysis [7, 8, 9, 10 and H]. The second application is in detemiining the... 

Ca.ta.lysts, A catalyst has been defined as a substance that increases the rate at which a chemical reaction approaches equiHbrium without becoming permanently involved in the reaction (16). Thus a catalyst accelerates the kinetics of the reaction by lowering the reaction s activation energy (5), ie, by introducing a less difficult path for the reactants to foUow. Eor VOC oxidation, a catalyst decreases the temperature, or time required for oxidation, and hence also decreases the capital, maintenance, and operating costs of the system (see Catalysis). 

FIGURE 13.25 (a) In the collision theory of chemical reactions, reaction may take place only when two molecules collide with a kinetic energy at least equal to a minimum value, /rmn (which later we identify with the activation energy), (b) Otherwise, they simply bounce apart. 

The rate of a chemical reaction and the extent to which it proceeds play an important role in analytical chemistry. The fundamental problem which faces the analyst arises because thermodynamic data will indicate the position of equilibrium that can be reached, but not the time taken to reach that position. Similarly, a compound may be thermodynamically unstable because its decomposition will lead to a net decrease in free energy, whilst a high activation energy for the decomposition reaction restricts the rate of decomposition. In practical terms such a compound would be stable, e.g. NO. It is thus essential to consider all analytical reactions from both thermodynamic and kinetic viewpoints. 

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