Dissolution of ionic salts in aqueous solution

Compounds with the ionic bond (salts) that form in the solid state the ion crystalline lattice dissociate to ions. Being dissolved, acids and bases undergo complete or partial dissociation where a noticeable chemical interaction of ions with solvents occurs. Each ion in the solvent, e.g., in water, is surrounded by the dense solvate shell of polar molecules. This shell appears due to the ion-dipole interaction. Solvation is manifested, first, in that the dissolution of a salt in H2O is accompanied by a decrease in the volume and, second, liberation of a great amount of heat. This is seen from the AH values where the ion from the gas phase is transfoied to an aqueous solution (A//Li+ = A/fp-, AH in kj/mol)... 

Dissolution of an ionic salt is essentially a separation process carried out by the interaction of the salt with water molecules. The separation is relatively easy in water because of its high dielectric constant. Comparison of the energies needed to separate ions of NaCl from 0-2 nm to infinity shows that it takes 692-89 kJ mol" in vacuum, but only 8-82 kJ moF in aqueous solution (Moore, 1972). Similar arguments have been used to try to estimate solvation energies of ions in aqueous solution, but there are difficulties caused by the variations in dielectric constant in the immediate vicinity of individual ions. 

As explained previously, electrodissolution in ionic liquids is a simple and efficient process, particularly in chloride-based eutectics. Type III eutectics based on hydrogen bond donors are particularly suitable for this purpose. However, it has been noted that the polishing process only occurs in very specific liquids and even structurally related compounds are often not effective. It has been shown that 316 series stainless steels can be electropolished in choline chloride ethylene glycol eutectics [19] and extensive electrochemical studies have been carried out. The dissolution process in aqueous solutions has been described by two main models the duplex salt model, which describes a compact and porous layer at the iron surface [20], and an adsorbate-acceptor mechanism, which looks at the role of adsorbed metallic species and the transport of the acceptor which solubilises... 

Teachers need to be aware of two different uses of the term electrolyte . In the strict sense, an electrolyte is a liquid that cm undergo electrolysis. This can be a single substance, as in the case of a molten salt, or a solution. The most typical electrolytes are the aqueous solutions of salts (in general of ionic compounds), of acids, and of bases. By extension, we also call electrolytes the pure substances (solid, liquid, or gaseous) that, when dissolved in water, provide liquid electrolytes. Some biological substances (such as DNA or polypeptides) and synthetic polymers (such as polystyrene sulfonate) contain multiple charged functional groups and their dissolution leads to electrolyte solutions these are termed polyelectrolytes. 

In its modern form, the model assmnes that solid electrolytes, such as the salts, are composed of ions which are held together by electrostatic forces of attraction. In other words, the ions do not form during the dissolution process, but pre-exist in the solid state (Chapter 3). Remind students that sodium chloride and calcium chloride are ionic compounds. When such an electrolyte is dissolved in a solvent, these forces are overcome and the electrolyte undergoes dissociation into ions. In the solution, the ions are moving about, each surrormded and followed in its movement by a number of solvent molecules, and so we say that in the solution the ions are solvated (hydrated in the case of aqueous solutions). 

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