Ionic compounds water-insoluble

Many, but not all, ionic compounds (e.g., NaCl but not CaC03) are soluble in water, a polar solvent In contrast, ionic compounds are insoluble in nonpolar solvents such as benzene (C6H6) or carbon tetrachloride (CCI4). 

Some ionic compounds are insoluble in water. This observation can be explained by inferring that the ionic bonding within the lattice is stronger than the ion-dipole bonds formed when the ionic solid dissolves and hydrates. The increase in entropy is insufficient to make the process favourable. Insoluble ionic compounds usually have small and highly charged ions and high lattice energies. 

Sometimes when water solutions of two different ionic compounds are mixed, an insoluble solid separates out of solution. The precipitate that forms is itself ionic the cation comes from one solution, the anion from the other. To predict the occurrence of reactions of this type, you must know which ionic substances are insoluble in water. 

Table 1.1 summarizes the solubility patterns of common ionic compounds in water. Notice that all nitrates and all common compounds of the Group 1 metals are soluble so they make useful starting solutions for precipitation reactions. Any spectator ions can be used, provided that they remain in solution and do not otherwise react. For example, Table 1.1 shows that mercury(I) iodide, Hg2I2, is insoluble. It is formed as a precipitate when solutions containing Hg22+ ions and I ions are mixed ... 

Use the information in Table 1.1 to classify each of the following ionic compounds as soluble or insoluble in water ... 

Because the fluoride ion is so small, the lattice enthalpies of its ionic compounds tend to be high (see Table 6.6). As a result, fluorides are less soluble than other halides. This difference in solubility is one of the reasons why the oceans are salty with chlorides rather than fluorides, even though fluorine is more abundant than chlorine in the Earth s crust. Chlorides are more readily dissolved and washed out to sea. There are some exceptions to this trend in solubilities, including AgF, which is soluble the other silver halides are insoluble. The exception arises because the covalent character of the silver halides increases from AgCl to Agl as the anion becomes larger and more polarizable. Silver fluoride, which contains the small and almost unpolarizable fluoride ion, is freely soluble in water because it is predominantly ionic. 

In the first reaction, two ionic compounds in water are mixed. The AgCl formed by the swapping of anions is insoluble, causing the reaction to proceed. The solid AgCl formed from solution is an example of a precipitate. In the second reaction, a covalent compound, HzO, is formed from its ions in solution, H+ and OH, causing the reaction to proceed. In the third reaction, a solid reacts with the acid in solution to produce two covalent compounds. 

When a substance made up of ions is dissolved in water, the dissolved ions behave independently. That is, they undergo their own characteristic reactions regardless of what other ions may be present. For example, barium ions in solution, Ba2, always react with sulfate ions in solution, S04 , to form an insoluble ionic compound, BaS04(s), no matter what other ions are present in the barium solution. If a solution of barium chloride, BaCK, and a solution of sodium sulfate, Na S04, arc mixed, a white solid, barium sulfate, is produced. The solid can be separated from the solution by filtration, and the resulting solution contains sodium chloride, just as it would if solid NaCl were added to water. In other words, when the two solutions are mixed, the following reaction occurs ... 

These problems involve mixing two solutions. Each solution is a water solution of an ionic compound. From the mixture of the two solutions, at least one insoluble precipitate will form. The other ions present are probably soluble and are called spectator ions they are not included in the net ionic equation. You must know your solubility rules to do these problems. 

According to the electrostatic model the solvation is due to electrostatic interaction between the charged ions and the dipolar solvent molecules. Thus the solvating and ionizing properties of a solvent are considered as being due primarily to the dipole moment of the solvent molecules. Thus, ionic compounds such as sodium chloride are insoluble in non-polar solvents such as carbon tetrachloride. Actually, rather than the dipole moment the field action of the dipoles should be considered. This approach might explain why acetonitrile (p = 3.2) is poor in its ionizing properties compared to water (p = 1.84). However, no numerical values are available for this quantity. 

One way to identify cations in solution is hy selectively precipitating them out of solution. As you know, cations may form soluble or insoluble ionic compounds, depending on the anions that are present. For example, copper(II) chloride, CuCl2 is soluble in water. Copper(II) sulfide, CuS, is insoluble in an acidic solution. Knowing about the relative solubility of cations when combined with various anions helps chemists identify them. 

In many everyday needs, one should be able to apply organic-water-insoluble compounds in industry and biology. It has been found that micelles (both ionic and... 

While many ionic compounds are soluble in water, many are not. The term solubility is somewhat subjective. There are actually degrees of solubility. A substance is considered soluble if 0.1 moles of it can dissolve in 1 liter of water. If less than 0.001 mole of the substance dissolves in water, a substance is considered insoluble. Partially soluble substances fall between these two extremes. Table 11.3 summarizes the solubility of some major groups of ionic compounds in water. 

In many cases the enthalpy of solution for ionic compounds in water is positive. In these cases we find the solution cooling as the solute dissolves. The mixing tendency of entropy is forcing the solution to do work to pull the ions apart, and since in an adiabatic process such work can be done only at the expense of internal energy, the solution cools. If the enthulpy of solution is sufficiently positive, favorable entrapy may not be able to overcome it and the compound will be insoluble. Thus some ionic compounds, such as KCI04, are essentially insoluble in water at room temperature. 

Use the information in Table 1.1 to classify the following ionic compounds as soluble or insoluble in water (a) lead(II) nitrate, Pb(N03)2 (b) lead(II) chloride, PbCl2 (c) silver nitrate, AgNOa (d) sodium sulfate, Na2S04. 

J.12 Identify the following as a strong acid, a weak add, a base, a soluble ionic compound, or an insoluble ionic compound in water (a) HN03 (b) KOH ... 

The solubility of an ionic compound increases dramatically if the solution contains a Lewis base that can form a coordinate covalent bond (Section 7.5) to the metal cation. Silver chloride, for example, is insoluble in water and in acid, but it dissolves in an excess of aqueous ammonia, forming the complex ion Ag(NH3)2 + (Figure 16.13). A complex ion is an ion that contains a metal cation bonded to one or more small molecules or ions, such as NH3, CN-, or OH-. In accord with Le Chatelier s principle, ammonia shifts the solubility equilibrium to the right by tying up the Ag+ ion in the form of the complex ion ... 

As shown in Figure 1.2, the solvent strength of supercritical carbon dioxide approaches that of hydrocarbons or halocarbons. As a solvent, C02 is often compared to fluorinated solvents. In general, most nonpolar molecules are soluble in C02, while most polar compounds and polymers are insoluble (Hyatt, 1984). High vapor pressure fluids (e.g., acetone, methanol, ethers), many vinyl monomers (e.g., acrylates, styrenics, and olefins), free-radical initiators (e.g., azo- and peroxy-based initiators), and fluorocarbons are soluble in liquid and supercritical C02. Water and highly ionic compounds, however, are fairly insoluble in C02 (King et al., 1992 Lowry and Erickson, 1927). Only two classes of polymers, siloxane-based polymers and amorphous fluoropolymers, are soluble in C02 at relatively mild conditions (T < 100 °C and P < 350 bar) (DeSimone et al., 1992, 1994 McHugh and Krukonis, 1994). 

One of the great rules of thumb in chemistry is like dissolves like. That means that polar solutes are more soluble in polar solvents, while nonpolar solutes are more soluble in nonpolar solvents. We have discussed molecular polarity in a previous chapter, but you can consider most organic compounds to be nonpolar. The most polar species are, obviously, ionic compounds, followed by species that can form hydrogen bonds, such as water and ethanol (CH3CH2OH). Therefore, you would expect ionic compounds to be soluble in water, but not very soluble in an organic solvent such as ether or hexane. On the other hand, you would expect an organic compound, like the vast majority of the pure form of injectable medications, to be relatively insoluble in a water-based medium such as blood. 

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