Section 7.3.3 Solids Dissolving

Many of the d-block elements form characteristically colored solutions in water. For example, although solid copper(II) chloride is brown and copper(II) bromide is black, their aqueous solutions are both light blue. The blue color is due to the hydrated copper(II) ions, [Cu(H20)fJ2+, that form when the solids dissolve. As the formula suggests, these hydrated ions have a specific composition they also have definite shapes and properties. They can be regarded as the outcome of a reaction in which the water molecules act as Lewis bases (electron pair donors, Section 10.2) and the Cu2+ ion acts as a Lewis acid (an electron pair acceptor). This type of Lewis acid-base reaction is characteristic of many cations of d-block elements. 

As discussed in Section 3-, whenever an ionic solid dissolves in water, the salt breaks apart to give a solution of cations and anions. Thus, in any aqueous salt solution the major species are water molecules and the cations and anions generated by the salt. For example, a solution of potassium chloride contains K and Cl ions and H2 O molecules as major species. Likewise, the major species in a solution of ammonium nitrate are NH4 , NO3, and H2 O. 

The formation of a complex can also remove an ion and disturb the solubility equilibrium until more solid dissolves. We first met complexes in Section 2.13, where we saw that they were species formed by the reaction of a Lewis acid and a Lewis base. In this section, we consider complexes in which the Lewis acid is a metal cation, such as Ag+. An example is the formation of Ag(NH3)2+ when an aqueous solution of the Lewis base ammonia is added to a solution of silver bromide ... 

A rapid, alternative method of crystal separation is the addition of solid potassium chloride (an amount necessary to fill the crucible) directly into the untreated, fused mass (obtained after cooling to room temperature), followed by heating the new mixture to its fusion point by means of a Meker burner in a fume hood and maintaining this temperature for 1 or 2 hours. The molten potassium chloride will rapidly dissolve the magnesium fluoride and decompose the tungstate. The crucible is allowed to cool, and the fused mass is treated as described in the previous section. However, dissolving of the flux will now be much more rapid. 

In this section we consider the equilibria associated with solids dissolving in water to form aqueous solutions. When an ionic solid dissolves in water, we typically assume that it dissociates into separate hydrated cations and anions. For example, when calcium fluoride dissolves in water, we typically represent the situation as follows ... 

So far we have considered solids dissolving in aqueous solutions. Now we will consider the reverse process—the formation of precipitates. When solutions are mixed, various reactions can occur. We have already considered acid-base reactions in some detail. In this section we show how to predict whether a precipitate will form when two solutions are mixed. We will use the ion product, which is defined just like the Ksp expression for a given solid except that initial concentrations are used instead of equilibrium concentrations. For solid CaF2 the expression for the ion product (Q) is written... 

In a typical preparation, 100 g (0.178 mole) of the linear quadridentate nickel-(II) complex prepared in Section 7-C is placed in 1-L flask. To this is added 150 mL of dry ethylenediamine (dried by distillation over KOH). The resultant slurry is stirred and heated to reflux ( 160°) under a blanket of nitrogen in an oil bath. After about % hour, all the solid dissolves to give a red solution, which is refluxed for an additional 15 minutes or until red crystals form. The solution is... 

Consider a solid dissolving in a liquid to form an ideal solution (Fig. 6.3). The solute in its solid state will have the lower energy but the dissolution process is favoured by the gain in entropy as the solute becomes dispersed in the solution. At equilibrium the chemical potential of the solid must be equal to the chemical potential of the same substance in the solution. For an ideal solution Section 6.1)... 

The previous two sections reviewed characteristics of polymers that, in general, are not soluble in water and, therefore, are typically used as solid materials fibers, matrices, microspheres, or foams. Water-soluble polymers are also useful as biomaterials. Water-soluble polymers can be used in their molecular, water-soluble form as agents to modify other materials or as solid, dissolvable matrices (see the example of copolymers of methyl vinyl ether and maleic anhydride in Section A.2.2). Alternatively, water-soluble polymers may be cross-linked, by chemical or physical means, into solid materials (gels) that swell in water but do not dissolve. 

Solubility is an oft ill-defined term, used rather indiscriminately to refer to small amounts of a solute of one phase dissolved in a solvent of another phase. Invariably, the solvent is a liquid or dense fluid, though it may contain any number of components, while the solute may be gas, liquid, or solid. Solubility problems are really phase-equilibrium problems and are attacked using the general strategies presented in Chapter 10. In this section we describe the three common solubility problems gas solubility, which refers to supercritical gases dissolved in liquids ( 12.2.1) solid solubility, which refers to solids dissolved in liquids ( 12.2.2) and solubilities in near-critical... 

You are presented with a white solid and told that due to careless labeling it is not clear if the substance is barium chloride, lead chloride, or zinc chloride. When you transfer the solid to a heaker and add water, the solid dissolves to give a clear solution. Next a Na2S04(white precipitate forms. What is the identity of the unknown white solid [Section 4.2]... 

It is important to distinguish carefully between solubility and the solubility-product constant. The solubility of a substance is the quantity that dissolves to form a saturated solution. (Section 13.2) Solubility is often expressed as grams of solute per liter of Solution (g/L). Molar solubility is the number of moles of solute that dissolve in forming 1 L of saturated solution of the solute (mol/L). The solubility-product constant (K p) is the equilibrium constant for the equilibrium between an ionic solid and its saturated solution and is a unitless number. Thus, the magnitude of is a measure of how much of the solid dissolves to form a saturated solution. 

When an ionic solid dissolves in water, a mixture of water and ions replaces the pure solid and pure water, as shown for KCl in FIGURE 19.10. The ions in the liquid move in a volume that is larger than the volume in which they were able to move in the crystal lattice and so possess more motional energy. This increased motion might lead us to conclude that the entropy of the system has increased. We have to be careful, however, because some of the water molecules have lost some freedom of motion because they are now held around the ions as water of hydration. (Section 13.1) These water... 

Sections 7.3.1 to 7.3.4 consider solids suspension, solids dispersion, solids dissolving, and solids flocculating respectively. General issues related to mixing using a fluidized bed are given in Section 7.3.5. 

The circulating liquor in the first effect is heated with 10-bar steam. The product is 42% NaOH with about 10% suspended solids. Flashing this from the operating pressure of 175 down to 7 kPa, the fourth-effect pressure, produces a 15% slurry in 49.3% NaOH solution (b.p. 82°C). Part of the pumped stream goes to sulfate removal. The rest is split by a cyclone into a nearly clear solution and a 40% solids underflow. The underflow goes to the slurry tank. The clear stream is the final product of the evaporation section. Its dissolved NaCl concentration is about 2.5%. Product coolers deliver a solution that is 50% NaOH and 1% NaCl, with about 2.5% suspended solids. Cooling and solids removal are the subjects of Sections 9.3.2.4-9.3.2.6. 

The technique of liquid-liquid extraction was described in Sections 12.1-12.8. In this section, solid-liquid extraction is described. Solid-liquid extraction is often used to extract a solid natural product from a natural source, such as a plant. A solvent is chosen that selectively dissolves the desired compound but that leaves behind the undesired insoluble solid. A continuous solid-liquid extraction apparatus, called a Soxhlet extractor, is commonly used in a research laboratory. 

The AgCl minicell described in Section 25.3 may be used to determine the infrared spectrum of a solid dissolved in carbon tetrachloride. Prepare a 5-10% solution (5-10 mg in 0.1 mL) in carbon tetrachloride. If it is not possible to prepare a solution of this concentration because of low solubility, dissolve as much solid as possible in the solvent. Following the instructions given in Section 25.3, position the AgCl plates as shown in Figure 25.4C to obtain the maximum possible path length of 0.20 mm. When the cell is tightened firmly, the cell will not leak. 

Section 11.7) When the solid dissolves, the ions are free to move about in the water. They are in a much more random and disordered state than before. At the same time, however, water molecules are held around the ions as water of hydration, as shown in Figure 19.8 . (Section 13.1) These water molecules are in a more ordered state than before because they are now confined to the immediate environment of flie ions. Therefore, the dissolving of a salt involves bofli disordering and ordering processes. The disordering processes are usually dominant, and so the overall effect is an increase in disorder of the system upon dissolving most salts in water. 

Consider the chemical dissolution of a slightly soluble solid reactant A in a liquid with excess reactant B, This type of reaction is encountered frequently in organic syntheses. This situation resembles the solid surface reaction that was treated in section 5,3,2,2, but the difference is that now the solid dissolves first and then reacts in the liquid phase (therefore the solid reactant is now denoted as A). When the reaction is relatively slow (tp < 0.3), eq. (5.21) is to be applied here. For the situation where the rate is completely determined by mass transfer of the slightly soluble A into the solution with a large excess of B, the effective rate constant is now the mass transfer coefficient of A, and the calculations similar to those in section 5,3,2,2 apply here also. The mass balance for n particles per unit volume is then... 

The latex form of Heveaplus is compounded in the usual way with dispersions of required additives to formulations as exemplified in a latter section. Solid Heveaplus MG is best milled on a two-roll mill to bring about some breakdown and elimination of gel material before dissolving in solvent. The degree of milling should be carefully controlled, as excessive milling will lead to the elimination of grafted material and increase the amount of homopolymer. A recommended solvent is made of... 

When an ionic solid dissolves in water, the crystal lattice breaks up and the ions separate. It requires a large amount of energy to overcome the attractive forces between the ions. How does this happen, even when the water is not heated We will answer this question in this section. 

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