Entropy, activation internal

Figure 2. Activation energies and activation entropies of internal diffusion in cation exchange on type A zeolite...

The activation entropies were considerably different from the large negative values expected for a second-order reaction and this was attributed to the effect of the internal return mechanism. 

The steric environment of the atoms in the vicinity of the reaction centre will change in the course of a chemical reaction, and consequently the potential energy due to non-bonded interactions will in general also change and contribute to the free energy of activation. The effect is mainly on the vibrational energy levels, and since they are usually widely spaced, the contribution is to the enthalpy rather than the entropy. When low vibrational frequencies or internal rotations are involved, however, effects on entropy might of course also be expected. In any case, the rather universal non-bonded effects will affect the rates of essentially all chemical reactions, and not only the rates of reactions that are subject to obvious steric effects in the classical sense. 

The increase in the length of the side chain results normally in an internal plasticization effect caused by a lower polarity of the main chain and an increase in the configurational entropy. Both effects result in a lower activation energy of segmental motion and consequently a lower glass transition temperature. The modification of PPO with myristoyl chloride offers the best example. No side chain crystallization was detected by DSC for these polymers. 

Humphreys and Hammett have estimated that in solution the entropy of acetic acid or its derivative is about 4-6 e.u. greater than the entropy of formic acid or its corresponding derivative due to the internal freedom of the methyl group. On this basis the authors concluded that the entropy of the acetate ion must be about the same as that of the formate ion, meaning that the internal motion of the methyl group is frozen out in the ionic species. It would appear from the data, however, that the entropy of the activated complex for acetate hydrolysis is more negative than that for formate hydrolysis by another 5 e.u. A possible explanation is that the charge becomes more concentrated in the acetate complex with a resultant increase in solvent electrostriction. 

In the gas phase it was shown that as the complexity of the reacting molecules increased, A S decreased see Sections 4.3.5 and 4.4.2. This is a consequence of including the effect of the internal motions of rotation and vibration in reactants and activated complex. The change in the number of degrees of freedom is a major contribution to the entropy of activation see Problems 4.12-4.15. 

For reactions between ions of like sign, solvation effects give a decrease in entropy on activation. Consideration of the internal structure leads again to a decrease in entropy on activation. The two effects reinforce each other, and also are in the same direction as predicted by the electrostatic treatment as given in Section 7.4.5. This would indicate p factors of less than unity. 

For reactions between ions of unlike sign, solvation effects give an increase in entropy on activation. Consideration of internal structure gives a decrease in A S and the two effects could balance, or partially balance, each other. The electrostatic modifications would, however predict an increase in A S, indicating p factors greater than unity. Since the other two effects are in opposite directions to each other, the electrostatic modifications must dominate, with this being the major factor influencing the A factors. 

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