Entropy change explanation

In this discussion, entropy factors have been ignored and in certain cases where the difference between lattice energy and hydration energy is small it is the entropy changes which determine whether a substance will or will not dissolve. Each case must be considered individually and the relevant data obtained (see Chapter 3), when irregular behaviour will often be found to have a logical explanation. 

A more general explanation of the temperature independence of the phase-transition quantities will be illustrated with ASa—although it could be illustrated equally as well with AThe integral entropy change for the phase transition is related to the partial entropy changes by Equation 21 (53). 

The chelate effect is often attributed to a favorable entropy change for multidentate binding. Recent work does not support this explanation V. Vallet,... 

Bell and co-workers84 confirmed the lowering of osmotic pressure in the presence of salts as observed by Oakley and Young.32 These authors suggested that as disaggregation and the Donnan effect were unlikely, an explanation might be that the salt caused a decreased asymmetry in the molecule with a resultant entropy change. 

It should be pointed out, however, that the thermodynamic explanation of the chelate effect, in particular the contribution of entropy as presented above, is actually not as straightforward as it might appear. The entropy change for a reaction depends on the standard state chosen for reference and for very concentrated solutions one might chose unit mole fraction instead of one molal and the chelate effect would disappear. However, this is not realistic and for solutions one molal (or less) there is a real chelate effect. In very dilute solutions (0.1 M or less) where complexation of metal ions is generally most important, the chelate effect is of major importance and is properly understood as entropically driven. 

The continuous succession of equilibrium states represented by the line in Figure 3.7b is one example of a reversible process. The explanation of reversible processes takes up considerable space in most texts, and usually it seems to have some connection with entropy changes. However, the importance of reversible processes is much more fundamental than furnishing an explanation for the entropy. It is a direct result of our desire to apply mathematics to physical properties. 

Repeat the calculation in Problem 8.10 for constant-volume heating. Which one results in the greater entropy change Give a physical explanation for the difference. 

The same problem arises to an even greater extent when alkyl substitution occurs very close to the acid-base centre, as in the alkylamines, since the separation into internal and external effects will be a poor approximation in this case. Thus in aqueous solution the sequence of acid strength is NH4 > Me3NH > MeNHj > Me2NHj. The unexpected position of trimethylamine cannot be explained in terms of an internal effect of the methyl groups, but could arise if the hydration of the ions or molecules introduces a complicating factor. This explanation is supported by the observed entropy changes (Table 5) and also by measurements in aprotic solvents we shall return to the problem in Chapter 10 in connection with basic catalysis by amines. 

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