Absolute Rate Transition State Theory and the Activated Complex

The ratio of the rates RJR, then equals Dp/rk. Diffusion coefficients of ions typically range from 3 X 10 to 2 X 10cm-/s (Lerman 1979). Assuming D = 10 cm Vs, a porosity of 0.2, crystal radius of 1 cm, and k -2x 10 cm/s at 25 C (based on James and Lupton 1978), we find R,i/Rs, = 1/1000. In other words / . R, and the slower rate, diffusion (a transport process), will control the dissolution rate. 

6 ABSOLUTE RATE (TRANSITION STATE) THEORY AND THE ACTIVATED COMPLEX 

Considerable insight into reaction mechanisms has been derived from the concept of absolute-rate theory. The theory is based on two assumptions first, that there is an energy maximum (barrier) between products and reactants in a reaction, and that an activated complex exists at that maximum and second, that chemical equilibrium always exists among reactants, products, and the activated complex, C-. Thus for the reaction 

The equilibrium constant involving reactants A and B and the activated complex (see Fig. 2.6) is 

The effect of ionic strength on the second-order rate constant may be obtained as follows. We define a specific rate constant k , for conditions of infinite dilution when the activity coefficients equal unity, so that 

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Sec. 2.6 Absolute Rate (Transition State) Theory and the Activated Complex... 

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