Experimental Procedures and Accuracy

Determination of both the energy (or enthalpy) of activation and the entropy of activation involves measurement of the temperature coefficient of the rate constant. The most usual procedure is to plot log k against I/T for a series of temperatures and to establish the best straight line. The slope is equal to — EJ2.303 R. With Ea established, one can obtain A8 for reactions in solution from equation (10) 

Perhaps the most common failing in reporting AS values is to list them with more significant figures than the experimental data justify. It is for example common, but hardly ever justified, to list AS values to 0.01 e.u. One can assess the problem by considering quantitatively the contribution to errors in the Arrhenius parameters.2 Considering 

2 Much of this analysis has been taken directly from Taft et al. (1955). 

Assume that the temperatures and especially the temperature difference are known with high precision and label the errors (or precision measures) for the rate coefficients k2 and kx as + r2 and r1 respectively. Since r2jk2 and r1jk1 are small compared to unity one has 

The same analysis applies directly to entropies of activation. In this case the error is the last term of the equation, 

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