Experimental energy of activation

Pearson and Edwards98 report that the thermal conversion of tetraborane without diborane being present is also first order, but is about one order of magnitude slower. The experimental energies of activation for both reactions, however, have been found to be almost the same. 

If we now inspect the results of the various theories we have just considered, we will see that they can all be cast into the form demanded by Eq. (XII.5.1), and the thermodynamic significance of the terms can now be interpreted in terms of the equilibria proposed by the various models. From laboratory experiments we can calculate specific rate constants at different temperatures, and following the Arrhenius method we can define an experimental energy of activation by the equation... 

Fig. 5.28. The dependence of the experimental energy of activation for self-diffusion on the melting point (1 cal = 4.184 J).

Experimental Energies of Activation for Various Transport Process ... 

In the following sections, a value ofA will be calculated. Used in the right-hand side of Eq. (5.101), it gives there the theoretical prediction of the experimental energy of activation, i.e., the left-hand side ofEq. (5.101) [cf. Eq. (5.100)]. 

Although rate constants are independent of concentration, they depend on temperature. The Arrhenius equation relates the rate constant of a reaction to the experimental energy of activation and to the temperature at which the reaction is carried out. A good rule of thumb is that an increase of 10 °C in temperature will double the rate constant for a reaction and, therefore, double the rate of the reaction. 

Do not confuse the free energy of activation, AG, with the experimental energy of activation, E, in the Arrhenius equation. The free energy of activation (AG = AH — TAS ) has both an enthalpy component and an entropy component, whereas the experimental energy of... 

Rate constants for a reaction were determined at five temperatures. From the following data, calculate the experimental energy of activation and then calculate AG, A//, and A5 for the reaction at 30 °C ... 

The, quantity AH differs slightly from the experimental energy of activation E, obtained as previously described (p. 388) from an application of the Arrhenius law the relationship between the two quantities is, for a reaction in solution,... 

From the theoretical point of view, energies of activation, as defined either by Eqs. (16) and (17) or by Eqs. (19) and (20), are experimentally accessible. Various definitions of the experimental energies of activation are presented in Table 1. 

With this technique it is possible to determine an experimental energy of activation (or rather a temperature coefficient) using the crenel method developed by Rouquerol and Rouquerol [ROU 71], The crenel method is very close to the switch method discussed in Chapter 11. 

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