Exothermic process entropy changes

FIGURE 7.17 (a) In an exothermic process, heat escapes into the surroundings and increases their entropy, (b) In an endothermic process, the entropy of the surroundings decreases. The red arrows represent the transfer of heat between system and surroundings, and the green arrows indicate the entropy change of the surroundings. 

The thermodynamic data of Tables 2 and 3 are remarkably selfconsistent, and correlate well with data for the various phenyltrimethyl ammonium perchlorates in 1,2-dichloroethane originally studied by Denison and Ramsey (44). Clearly dissociation is nominally exothermic though the actual enthalpy changes are quite small. Solvation of the free ions, therefore, is also small (in these solvents) but exceeds that of the ion pair species. The overall entropy changes confirm some ordering of solvent molecules on dissociation, the loss in entropy for this process being more than sufficient to balance the entropy increase associated with... 

Any solid state reaction, however complex, must resolve itself into interactions between pairs of solid phases, the elementary processes occurring successively or simultaneously to give a variety of intermediate and final products. Because the entropy change is small, all solid state reactions are exothermic. This property forms the basis of the heating curve method for detecting reactivity in solid mixtures. 

The ease of dissolution of a solute depends on two factors (1) the change in energy and (2) the change in disorder (called entropy change) that accompanies the process. In the next chapter we will study both of these factors in detail for many kinds of physical and chemical changes. For now, we point out that a process is favored by (1) a decrease in the energy of the system, which corresponds to an exothermic process, and (2) an increase in the disorder, or randomness, of the system. 

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