Calculations of Hydration Heats

In the 1970s Bockris and Saluj a developed models incorporating and extending ideas proposed by Eley and Evans, Frank and Wen, and Bockris and Reddy. Three basic models of ionic hydration that differ from each other in the structure in the first coordination shell were examined. The features of these models are given in Table 2.16. The notations chosen for the models were lA, IB, 1C 2A, 2B, 2C and 3A, 3B, 3C, where 1, 2, and 3 refer to three basic hydration models, and A, B, and C refer to the subdivision of the model for the structure-broken (SB) region. These models are all defined in Table 2.16. A model due to Bockris and Reddy (model 3 in Table 2.16 and Fig. 2.37) recognizes the distinction between coordination number (CN) and solvation number (SN). 

Molecules in the first shell are SB region consists of dimers H bonded to those in the SB formed from monomers region rest of the solvent as drawn from liquid water bulk SB region consists of monomers, some of which librate with respect to molecules in the first shell 

In the concept of nonsolvated coordinated water, it is assumed that an ion exists in two states in the solution. It is either stationary or engaged in diffusional movement. As an ion arrives at a new site after a movement, its situation in respect to coordination by water is of two types. Clearly, the water molecules must always have some degree of orientation in the direction of their dipoles in the first layer around the ion. It is practical to divide these molecules into two types. There will be those oriented so that these dipoles interact maximally with the ion, and another type that is oriented so that the dipoles are oriented at 90° to the ion and do not interact with it at all. The coordination number refers to the total number of water molecules in the first layer around the ion, independently of how they are oriented. 

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