Hydration negative

Table IV shows the values for both polyols. The hydration numbers are a consequence of molecular shape. Following Samoilov, we classified both compunds as negatively hydrated since their hydration time ratios are less than one, with sorbitol being more negatively hydrated.

Self-Test 8.9A Why does Cl have a more negative hydration enthalpy than I has ... 

Self-Test 8.9B Would you expect Rb+ to have a more negative or a less negative hydration enthalpy than Sr2+ Explain your answer. 

PROBLEM 11.2 Which would you expect to have the larger (more negative) hydration energy ... 

The concept [72] of the positive and negative hydration is inherently confirmed in our calculations by the fitted t(Cm) dependences which are increasing for NaCl and decreasing for KC1 (Table XVI). 

In the presence of electrolyte solutions, hydrated cations and anions participate in the formation of contacts and determine their strength. The addition of electrolytes can either increase the structuring of water layers or, vice versa, weaken this process (positive and negative hydration). Some electrolytes strengthen water structure, the others weaken it. In the first case, the viscosity of water increases in the second case it decreases. The first group consists of polyvalent electrolytes and the electrolytes, such as halides of alkaline elements, while the second group - of some electrolytes, such as KI. 

Geiger A. Molecular dynamics simulation study of the negative hydration effect in aqueous electrolyte solutions. Ber. Bunsenges. Phys. Chem. 1981 85 52-63. 

From a kinetic viewpoint, salinity action on the water solution structure is similar to the action of temperature and pressure. This was a reason to compare the effect of temperature and pressure, on the one hand, and salinity, on the other, on the mobility of solution components, and therefore, on its structure. In this connection John Desmond Bernal (1901-1971) and Ralph Howard Fowler (1889-1944) introduced the concept of structural temperature of the solution. Under their definition, structural temperature of a given solution is equal to the temperatme of pine water with the solution s structural properties (viscosity, density, refraction, etc.). Ions with positive hydration work as lowering of temperature and have structural temperature below the solution temperature ions with negative hydration - as increase of temperature, and their structural temperature is higher than the solution s temperature. Non-polar compounds occupy plentiful space, thereby lowering the intensity of translation motion of the water molecules, lowering the structural temperature of the solution, as in a case of positive hydration. 

Figure l.lf Samoilov s model of lon-hydration ASi > 0, positive hydration AEi < 0, negative hydration. 

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