Solubility of ionic substances

Through many years of experience and research, chemists have discovered patterns in the solubilities of ionic substances. Most salts are insoluble. The soluble salts are summarized in Table 4-1. and the flowchart in Figure 4-6 shows how to determine if a salt is soluble or insoluble. 

Water solubility of ionic substances is dependent on a fine balance between lattice energy, hydration energy and entropy of ions. The scheme shown hereunder as ... 

In section 9.2, you worked with concentrations of ions in solutions at equilibrium. You used Kgp to determine the solubility of ionic substances in pure water and in solutions of common ions. The systems you worked with were saturated solutions that contained excess, undissolved solute. 

Factors That Affect the Solubility of Ionic Substances... 

The solubility of ionic substances in water varies greatly. For example, sodium chloride is quite soluble in water, whereas silver chloride (contains Ag+ and Cl- ions) is only very slightly soluble. The differences in the solubilities of ionic compounds in water typically depend on the relative affinities of the ions for each other (these forces hold the solid together) and the affinities of the ions for water molecules [which cause the solid to disperse (dissolve) in water]. Solubility is a complex issue that we will explore in much more detail in Chapter 17. However, the most important thing to remember at this point is that when an ionic solid does dissolve in water, the ions are dispersed and are assumed to move around independently. 

In Lesson 6-2,1 mentioned that displacement reactions are a bit misleading at times. Sometimes, a product that is shown on the product side of the equation does not really appear in the physical chemical reaction. The reason for this has to do with the solubility of ionic substances in water. If a particular product is soluble, it will stay dissolved in the aqueous solution. If a product is insoluble, it will appear as a solid precipitate in the test vessel. It is important to know which products stay dissolved in the water, so we can make proper identification of the precipitates that do form as the result of the chemical reactions. Ionic equations are more realistic representations of these reactions that take place in aqueous solution. Ionic equations show the individual ions that exist in solution. When we take an ionic displacement reaction and remove the information that is misleading, we produce a net ionic equation. 

Lattice energies may be used to understand many important chemical trends, including the characteristic oxidation states of metallic elements, the stabilization of high oxidation states by oxide and fluoride, and trends in the thermal stability of oxoanion salts such as carbonates. Stability and reactivity (B3) Solubility of ionic substances (E4)... 

Highest dielectric constant of any common liquid that means high solubility of ionic substances and their ionization ... 

The solubility of ionic substances in water varies greatly. For example, sodium chloride is quite soluble in water, whereas silver chloride (contains Ag" and Cl ions) is only very slightly soluble. The differences in the solubilities of ionic compounds in water... 

It is convenient to define the solubility of ionic substances in units of g per 1000 g of saturated solution, or as the number of moles of substance needed to produce 1 dm of saturated solution. Molar solubility is defined as follows ... 

Any substance whose aqueous solution contains ions is called an electrolyte. Any substance that forms a solution containing no ions is a nonelectrolyte. Electrolytes that are present in solution entirely as ions are strong electrolytes, whereas those that are present partly as ions and partly as molecules are weak electrolytes. Ionic compounds dissociate into ions when they dissolve, and they are strong electrolytes. The solubility of ionic substances is made possible by solvation, the interaction of ions with polar solvent molecules. Most molecular compounds are nonelectrolytes, although some are weak electrolytes, and a few are strong electrolytes. When representing the ionization of a weak electrolyte in solution, half-arrows in both directions are used, indicating that the forward and reverse reactions can achieve a chemical balance called a chemical equilibrium. 

The liquid range of liquid anunonia is only between 19SK to 240K and has a low dielectric constant. While the solubility of ionic substances is decreased in ammonia, it is a better solvent than water for certain nonpolar molecules. 

The solubility of ionic substances in relatively nonpolar aprotic solvents can be greatly enhanced by using catalytic quantities of macrocyclic polyethers, such as 18-crown-6, the structure of which is shown in Fig. 5.5. These macrocyclic ethers selectively solvate the cation, both enhancing solubility and also leaving the anion in a very weakly solvated state. The anions behave under these conditions as highly reactive species, sometimes termed naked anions. A study of the relative rates of nucleophilic substitution on benzyl tosylate by potassium salts in acetonitrile in the presence of 18-crown-6 revealed a pronounced leveling effect. " All the potassium halides (fluoride, chloride, bromide, and iodide) were approximately equal in their reactivity. Potassium acetate was observed to be almost ten times more reactive than potassium iodide under these conditions—a reversal of the normal reactivity of acetate ion versus iodide ion in nucleophilic substitution reactions. As measured by cHji values in Table 5.5, iodide is 3 log units, i.e., 10 times, more reactive than acetate ion in the protic solvent methanol. 

As a general observation, the solubilities of ionic substances (about 95% of them) increase with increasing temperature. Exceptions to this generalization tend to be found among compounds containing the anions SOs , S04 , As04, and P04 . 

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