Dissolution of ionic compounds

We examine a semiconductor of ionic compound MX composed of cation and anion X. As shown in Fig. 9-12, the dissolution of ionic compound MX involves the transfer of M and X " ions at the electrode interface as shown in Eqn. 9-39 and 9-40 ... 

Furthermore, in the dissolution of ionic compounds where vu = v, we obtain from Eqns. 9—41 and 9—42 the potential of the compact layer as shown in Eqn. 9-44 ... 

Figure 9-16 illustrates the polarization curves for the anodic oxidative and the cathodic reductive dissolution of ionic compound semiconductors. The anodic oxidative dissolution proceeds readily at p-type semiconductor electrodes in which the mqjority charge carriers are holes whereas, the cathodic reductive dissolution proceeds readily at n-type semiconductor electrodes in which the majority charge carriers are electrons. 

A.V. (1997) Adsorption of a corticoid on colloidal hematite particles of different geometries. J. Colloid Interface Sd. 187 429-434 Verdonck, L. Hoste, S. Roelandt, F.F. Van der Kelen, G.P. (1982) Normal coordinate analysis of a-FeOOH - a molecular approach. J. Molecular Structure 79 273-279 Vermilyea, D.A. (1966) The dissolution of ionic compounds in aqueous media. J. Electro-chem. Soc. 113 1067-1070 Vermohlen, K. Lewandowski, H. Narres, H-D. Schwager, M.S. (2000) Adsorption of polyelectrolytes onto oxides - the influence of ionic strength, molar mass and Ca " ions. Coll. Surf. A 163 45-53... 

For example, the energy necessary for the dissolution of ionic compounds is supplied by the interaction between the ions and the solvent molecules, i.e., by the energy of solvation of the ions. 

Water is the most common solvent used to dissolve ionic compounds. Principally, the reasons for dissolution of ionic crystals in water are two. Not stated in any order of sequence of importance, the first one maybe mentioned as the weakening of the electrostatic forces of attraction in an ionic crystal known, and the effect may be alternatively be expressed as the consequence of the presence of highly polar water molecules. The high dielectric constant of water implies that the attractive forces between the cations and anions in an ionic salt come down by a factor of 80 when water happens to be the leaching medium. The second responsible factor is the tendency of the ionic crystals to hydrate. 

Fig. 9-13. Reaction rate of simultaneous dissolution of surface cations and anions from a semiconductor electrode of ionic compound as a iimction of potential of a compact layer 4 )=potmitial of acorn-...

Fig. 9-15. Oxidative and reductive dissolution reactions of semiconductor electrodes of ionic compounds (a) cation dissolution coupled with anodic hole oxidation of surface anions, (b) anion dissolution coupled with cathodic electron reduction of surface cations.

The solubility product, Ksp, for an ionic compound is the equilibrium constant for dissolution of the compound in water. The solubility of the compound and Ksp are related by the equilibrium equation for the dissolution reaction. The solubility of an ionic compound is (1) suppressed by the presence of a common ion in the solution (2) increased by decreasing the pH if the compound contains a basic anion, such as OH-, S2-, or CO32- and (3) increased by the presence of a Lewis base, such as NH3, CN-, or OH-, that can bond to the metal cation to form a complex ion. The stability of a complex ion is measured by its formation constant, Kf. 

For each of the following compounds, write a balanced net ionic equation for dissolution of the compound in water, and write the equilibrium expression for Ksp ... 

Corrosion refers to the loss or conversion into another insoluble compound of the surface layers of a solid in contact with a fluid. The solid is normally a metal, but the term corrosion is also used to refer to the dissolution of ionic crystals or semiconductors. In the majority of cases the fluid is water, but an important exception is the reaction of metallic surfaces with air at high temperature, leading to oxide formation, or, in industrial environments, to sulphides, etc. In the context of this book, corrosion of metals or semiconductors in contact with aqueous solution or humid air at normal temperatures is of predominant interest. 

In general, the ionic composition of molten salt systems depends on the solvent used for the dissolution of the compound, which contains the metal to be deposited, and the chemical nature of this compound. Usually, chemical reactions take place between this compound and the solvent. At these chemical reactions, new complex anions are formed, atomic composition and stability of which depend on the electronic state of the central metallic atom and the polarization ability of the alkali metal cations. The chemical nature of the anions present also plays a non-negligible role. The above-mentioned phenomena will be explained in the following chapters. 

Determining fCjp from Solubility The solubilities of ionic compounds are determined experimentally, and several chemical handbooks tabulate them. Most solubility values are given in units of grams of solute dissolved in 100 grams of H2O. Because the mass of compound in solution is small, a negligible error is introduced if we assume that TOO g of water is equal to 100 mL of solution. We then convert the solubility from grams of solute per 100 mL of solution to molar solubility, the amount (mol) of solute dissolved per liter of solution (that is, the molarity of the solute). Next, we use the equation for the dissolution of the solute to find the molarity of each ion and substitute into the ion-product expression to find the value of K p. 

The equilibria we have considered thus far in this chapter have involved acids and bases. Furthermore, they have been homogeneous that is, all the species have been in the same phase. Through the rest of the chapter, we will consider the equilibria involved in the dissolution or precipitation of ionic compounds. These reactions are heterogeneous. 

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. 

The solubility of solids in liquids is an important process for the analyst, who frequently uses dissolution as a primary step in an analysis or uses precipitation as a separation procedure. The dissolution of a solid in a liquid is favoured by the entropy change as explained by the principle of maximum disorder discussed earlier. However it is necessary to supply energy in order to break up the lattice and for ionic solids this may be several hundred kilojoules per mole. Even so many of these compounds are soluble in water. After break up of the lattice the solute species are dispersed within the solvent, requiring further energy and producing some weakening of the solvent-solvent interactions. 

The same disciission may apply to the anodic dissolution of semiconductor electrodes of covalently bonded compounds such as gallium arsenide. In general, covalent compoimd semiconductors contain varying ionic polarity, in which the component atoms of positive polarity re likely to become surface cations and the component atoms of negative polarity are likely to become surface radicals. For such compound semiconductors in anodic dissolution, the valence band mechanism predominates over the conduction band mechanism with increasing band gap and increasing polarity of the compounds. 

Fig. 9-16. Polarization curves of anodic oxidative dissolution and cathodic reductive dissolution of semiconductor electrodes of an ionic compound MX iiixcp) (iMxh )== anodic oxidative (cathodic reductive) dissolution current solid curve = band edge level pinning at the electrode interface, dashed curve = Fermi level pinning.

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