Lattice energies 1:1 salts

The above results indicate that complexation is favored for low-lattice-energy salts. Also, the lower the salt s lattice energy the more... 

Although the data for the silver halides suggest that silver(I) fluoride is likely to be more soluble than the other silver halides (which is in fact the case), the hydration enthalpies for the sodium halides almost exactly balance the lattice energies. What then is the driving force which makes these salts soluble, and which indeed must be responsible for the solution process where this is endothermic We have seen on p. 66 the relationship AG = — TAS and... 

The fluorite stmcture, which has a large crystal lattice energy, is adopted by Ce02 preferentially stahi1i2ing this oxide of the tetravalent cation rather than Ce202. Compounds of cerium(IV) other than the oxide, ceric fluoride [10060-10-3] CeF, and related materials, although less stable can be prepared. For example ceric sulfate [13590-82-4] Ce(S0 2> certain double salts are known. 

The overall lattice energies of ionic solids, as treated by the Born-Eande or Kaputin-sldi equations, thus depends on (i) the product of the net ion charges, (ii) ion-ion separation, and (iii) pacldng efficiency of the ions (reflected in the Madelung constant, M, in the Coulombic energy term). Thus, low-melting salts should be most... 

Unfortunately, the requirements cannot be fulfilled by a single salt, because they are partially contradicting. For example, the last two requirements would be best fulfilled with simple halides such as LiF or LiCl. However, their solubility in every suitable solvent is low, due to their high lattice energies. In addition, these salts show strong ion-ion interaction forces, even in solvents of high permittivity. 

More recently considered candidates are large molecular anions with delocalized anionic charge, which offer low lattice energies, relatively small ion-ion interaction, and hence sufficient solubility and relatively large conductivity. Delocalization of the charge is achieved by electron-with drawing substituents such as -F or - CF3. Furthermore, these anions show a good electrochemical stability to oxidation. In contrast to Lewis acid-based salts they are chemically more stable with various solvents and often also show excellent thermal stability. 

Madelung constant (A) A number that appears in the expression for the lattice energy and depends on the type of crystal lattice. Example A = 1.748 for the rock-salt structure. 

The lattice energy is the sum of all Ion interactions, each of which is described by Equation. Looking at this equation, we can predict that lattice energy will increase as ionic charge increases and that it will decrease as ionic size Increases. A third trend occurs in the summing of all the ion contributions Lattice energy increases with the number of ions in the chemical formula of the salt. 

The possibility of measuring the Volta potential in the system metal-solid-state electrolyte and using the data obtained to determine ionic components of the free lattice energy has been shown in our papers. Earlier, Copeland and Seifert measured the Volta potential between Ag and solid AgNOj in the temperature range between 190 and 280 °C. They investigated the potential jump during the phase transition from solid to liquid salt. 

The fact that the water molecules forming the hydration sheath have limited mobility, i.e. that the solution is to certain degree ordered, results in lower values of the ionic entropies. In special cases, the ionic entropy can be measured (e.g. from the dependence of the standard potential on the temperature for electrodes of the second kind). Otherwise, the heat of solution is the measurable quantity. Knowledge of the lattice energy then permits calculation of the heat of hydration. For a saturated solution, the heat of solution is equal to the product of the temperature and the entropy of solution, from which the entropy of the salt in the solution can be found. However, the absolute value of the entropy of the crystal must be obtained from the dependence of its thermal capacity on the temperature down to very low temperatures. The value of the entropy of the salt can then yield the overall hydration number. It is, however, difficult to separate the contributions of the cation and of the anion. 

Conventional electrolytes applied in electrochemical devices are based on molecular liquids as solvents and salts as sources of ions. There are a large number of molecular systems, both pure and mixed, characterized by various chemical and physical properties, which are the liquids at room temperatures. This is the reason why they dominate both in laboratory and industrial scale. In such a case, solid salt is reacted with a molecular solvent and if the energy liberated during the reaction exceeds the lattice energy of the salt, the solid is liquified chemically below its melting point, and forms the solution. Water may serve as an example of the cheapest and most widely used molecular solvent. 

Incorporation of heavier alkali metals into ate complexes is usually disfavored because of the large lattice energies of the simple salt elimination products MX. However, Stephan et al. have reported that reaction of Cp2ZrHCl with KPH(mes ) in the presence of KH gives the... 

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