Debye-HOckel model

CHEMICAL THERMODYNAMICS DEBYE-HOCKEL MODEL GIBBS FREE ENERGY... 

Let us specify the different hypotheses laid out by the Debye-HOckel model and the subsequent approximations which help one develop a means of expressing the activity coefficient of various ions in an electrolyte. Nearly all the assumptions, expressed below, are based on the fact that this theory is developed for dilute solutions ... 

In reality, these experimental results are not wholly inconsistent with the Debye-HOckel model, whereby the mean activity coefficient decreases as the molality increases. This comes down to taking into account the fact that a usual solute/solvent description is no longer satisfactory for concentrated electrolytes. Indeed, when dealing with concentrated electrolytes, the number of solvent molecules involved in the solvation sphere, close to the ions, cannot be ignored when compared to the total number of solvent molecules. Once one takes this phenomenon into account, then three types of adjustment emerge, each of which are laid out in detail below. 

Contrary to the hypothesis made in the Debye-HOckel model (see appendix A.3.2). 

Based on the simplified Debye-HOckel model, if you take the mean activity coefficient of a solute in a solution containing only NaCI, and compare it to the mean activity coefficient in a solution with the same ionic strength containing only Cu(NC>3)2 then the former coefficient is... 

Figure 1. Experimental and calculated solubiliti ofSr(OH) 8H20 in NaOK Patterned line represents calculations with Sr OH interactions described solely with the use ofPitzer s form of the extended Debye-HOckel equation. Solid line represents the calculations of our final thermodynamic model, which includes values for the Pitzer ion interaction parameters. Total concentrations in units of molarity. From (3).

The theoretical model put forward by Debye and HOckel helps one to understand the experimental law outlined above. In order to determine any deviation from the ideal situation, the approach consists firstly in calculating the interactions exerted by all other ions on a central ion d . These interactions then appear as a corrective term in the electrostatic potential around the central ion, and are subsequently expressed as an activity coefficient for the ion. 

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