Even though Arrhenius equation seems to imply that A and E are independent of temperature, kineticists have all along been aware of a more complex temperature dependence. Some of the equations that have been used are |
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| k = AT m |
(29.8) |
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| k = A e DT |
(29.9) |
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| k = AT m e -B / T |
(29.10) |
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| k = A T m e - (B - D T * T ) / T |
(29.11) |
| The temperature dependence of k in eq (29.11) is determined by the temperature dependence of the three parameters m, B and D. |
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| Let us interpret the graph of Fig. 29.1. When the reactants collide in the normal course of their motion, they do not necessarily react and remain as reactants. Certain collisions which occur with sufficient energy (the activation energy) to enable the bond breaking (in the reactants) and the bond making (in the products) steps so that the reaction goes in the forward direction. |
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| There is a minimum or threshold energy for this process which has been referred to as the activation energy. This explanation does not imply that all reactions have a positive activation energy. Reactions such as precipitation reactions seem to occur "instantly", i.e., as soon as the reactants approach one another within a certain distance, the reaction is complete. These are called diffusion controlled reactions. There are some reactions for which the activation energy is negative ! |
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