Ionic compounds in water

Which solution, NaCI(aQ) or CHsOHCaQ), conducts electricity  

Water is a very effective solvent for ionic compounds. Although H2O is an electrically neutral molecule, the O atom is rich in electrons and has a partial negative charge, denoted by S. Each H atom has a partial positive charge, denoted by Cations are attracted by the negative end of H2O, and anions are attracted by the positive end. 

We can usually predict the nature of the ions in a solution of an ionic compound from the chemical name of the substance. Sodium sulfate (Na2S04), for example, dissociates into sodium ions (Na ) and sulfate ions (804 ). You must remember the formulas and charges of common ions (Tables 2.4 and 2.5) to understand the forms in which ionic compounds exist in aqueous solution. 

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As we noted in Chapter 4, the solubility of ionic compounds in water varies tremendously from one solid to another. The extent to which solution occurs depends on a balance between two forces, both electrical in nature ... 

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 ... 

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. 

TABLE 2.1 Solubility Rules for Ionic Compounds in Water... 

Water. It should come as no surprise that ordinary water can be an excellent solvent for many samples. Due to its extremely polar nature, water will dissolve most substances of likewise polar or ionic nature. Obviously, then, when samples are composed solely of ionic salts or polar substances, water would be an excellent choice. An example might be the analysis of a commercial iodized table salt for sodium iodide content. A list of solubility rules for ionic compounds in water can be found in Table 2.1. 

In this chapter, you will continue your study of acid-base reactions. You will find out how ions in aqueous solution can act as acids or bases. Then, by applying equilibrium concepts to ions in solution, you will be able to predict the solubility of ionic compounds in water and the formation of a precipitate. 

The solubility of ionic solids in water covers a wide range of values. Knowing the concentration of ions in aqueous solution is important in medicine and in chemical analysis. In this section, you will continue to study equilibrium. You will examine the solubility equilibria of ionic compounds in water. 

The considerable solubility of polar molecules and many ionic compounds in water can be explained by dipole-dipole or ion-dipole interactions between the dissolved species and the solvent, water. 

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. 

Solubilities of ionic compounds in water were discussed, and trends explained. The effects of ionic charges and sizes were explained. 

In order for an ionic compound to dissolve, the Madelung energy or electrostatic attraction between the ions in the lattice must be overcome. In a solution in which the ions are separated by molecules of a solvent with a high dielectric constant ( H 0 81.7 ) the attractive force will be considerably less. The process of solution of an ionic compound in water may be considered by a Bom-Haber type of cycle. The overall enthalpy of the process is the sum of two terms, the enthalpy of dissociating the ions from the lattice (the lattice energy) and the enthalpy of introducing the dissociated ions into the solvent (the solvation energy) ... 

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. 

Table 1.1 summarizes the observed solubility patterns of common ionic compounds in water. Notice that all nitrates and all common com-... 

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 solution of an ionic compound in water is a good conductor of electricity - such ionic substances are electrolytes. 

Many interrelated factors affect the solubility of substances in water. This makes it challenging to predict which ionic substances will dissolve in water. By performing experiments, chemists have developed guidelines to help them make predictions about solubility. In Investigation 9-A, you will perform your own experiments to develop quidelines about the solubility of ionic compounds in water. 

How can you develop guidelines to help you predict the solubility of ionic compounds in water ... 

O (a) CHD Name the two factors that affect the solubility of an ionic compound in water. 

The driving force behind double substitution reactions is the formation of a covalent compound (including water or a gaseous compound) or an insoluble ionic compound from ions in solution. A solid formed from ions in solution is called a precipitate. We can thus predict that a reaction will occur if soluble ionic compounds yield at least one insoluble ionic compound or one covalent compound. We need to be familiar with the solubilities of some common ionic compounds in water. Some types of ionic compounds that are soluble or insoluble in water are listed in Table 8.3. A more comprehensive tabulation of solubilities is presented in Table 8.4 for reference, not necessarily to be memorized. 

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. 

Although water is a polar molecule, pure water does not carry an electric current. It is a good solvent for many ionic compounds, and solutions of ionic compounds in water do carry electric currents. The charged particles in solution move freely, carrying electric charges. Even a dilute solution of ions in water becomes a good conductor. Without ions in solution, there is very little electrical conductivity. 

The product of and the concentration of the undissolved solid creates a new constant called the solubility product constant, The solubility product constant is an equilihrium constant for the dissolving of a sparingly soluble ionic compound in water. The solubility product constant expression is... 

Solubility Guidelines for Common Ionic Compounds in Water Generally Soluble Exceptions... 

Ionic compounds are composed of positive and negative ions in a ratio that will provide an electrically neutral compound. The atoms of a covalent compound are attached to one another to form molecules. Dissolution of an ionic compound in water produces solvated ions whereas covalent compounds have solvated molecules. 

Describe the process for dissolving an ionic compound in water. Your description should include mention of the nature of the particles in solution and the attractions between the particles in the solution. 

The heat of solution of an ionic compound in water is the sum of the lattice energy of the compound and the heat of hydration. The relative magnitudes of these two quantities determine whether the solution process is endothermic or exothermic. The heat of dilution is the heat absorbed or evolved when a solution is diluted. 

The rules given in Table 4.2 (p. 113) allow us to predict the solubility of a particular ionic compound in water. When sodium chloride dissolves in water, the ions are stabilized in solution by hydration, which involves ion-dipole interaction. In general, we predict that ionic compounds should be much more soluble in polar solvents, such as water, liquid ammonia, and liquid hydrogen fluoride, than in nonpolar solvents, such as benzene and carbon tetrachloride. Since the molecules of nonpolar solvents lack a dipole moment, they cannot effectively solvate the Na and Cl ions. Solvation is the process in which an ion or a molecule is surrounded by solvent molecules arranged in a specific manner. The process is called hydration when the solvent is water.) The predominant intermolecular interaction between ions and nonpolar compounds is ion-induced dipole interaction, which is much weaker than ion-dipole interaction. Consequently, ionic compounds usually have extremely low solubility in nonpolar solvents. 

Figure 12.3 shows the temperature dependence of the solubility of some ionic compounds in water. In most but certainly not all cases, the solubility of a solid substance increases with temperature. However, there is no clear correlation between the sign of A/ZsoIj, and the variation of solubility with temperature. For example, the solution process of CaCl2 is exothermic, and that of NH4NO3 is endothermic. But the solubility of both compounds increases with increasing temperature. In general, the effect of temperature on solubility is best determined experimentally. 

FIGURE 12.3 Temperature dependence of the solubility of some ionic compounds in water. 

How do the solubilities of most ionic compounds in water change with temperature With pressure 1Z26 Describe the fractional crystallization process and its application. 

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