Solubilities of Ionic Compounds in Water

The various ways in which solubility data are presented are discussed in Section 1.1. This section consists of a description of the factors governing the solubility of inorganic compounds in water. The discussion is restricted mainly to the Group 1 halides with some concluding generalizations. 

Q Calculate the molar concemralions in mol dm 1 of the sodium halides (that lor NaC l is calculated in Worked Problem 1.1). given that their saturated solutions at 25 °C have the compositions  

A I he numbers of moles of the compounds in 100 g of water are obtained by dividing the mass of the compounds by their relative formula masses. The volumes of the solutions in dm arc obtained hy dividing the total mass bv the density. Dividing the numbers of moles of the halides by the volumes of their solutions gives the required molar concemralions. The answers are  

The results given in the text and the worked problem indicate that the solubilities of the sodium halides vary from the low value for the fluoride to the much higher values of the other halides. 

This variation is associated with the Gibbs energies of solution. Ast, G of the compounds. These, and the values of the enthalpies of solution, ASoi H - and the entropy changes of solution. when the compounds 

1 Relationship between Solubility and Gibbs Energy Change for 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 ... 

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. 

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

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 ... 

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. 

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... 

SECTION 17.5 Several experimental factors, including temperature, affect the solubilities of ionic compounds in water. The solubility of a slightly soluble ionic compound is decreased by the presence of a second solute that furnishes a common ion (the common-ion effect). The solubility of compounds containing basic anions increases as the solution is made more acidic (as pH decreases). Salts with anions of negligible basicity (the anions of strong acids) are unaffected by pH changes. 

The general mles for predicting the solubility of ionic compounds in water were introduced in Section 4.2. Although useful, these solubility rules do not enable us to make quantitative predictions about how much of a given ionic compound will dissolve in water. To develop a quantitative approach, we start with what we already know about chemical equilibrium. 

Let ns snmmarize the main points in this section. Compounds that dissolve in water are soluble those that dissolve little, or not at all, are insoluble. Soluble substances are either electrolytes or nonelectrolytes. Nonelectrolytes form noncon-dncting aqneons solutions because they dissolve completely as molecules. Electrolytes form electrically conducting solutions in water because they dissolve to give ions in solntion. Electrolytes can be strong or weak. Almost all soluble ionic substances are strong electrolytes. Soluble molecular substances usually are nonelectrolytes or weak electrolytes the latter solution consists primarily of molecules, but has a small percentage of ions. Ammonia, NH3, is an example of a molecular substance that is a weak electrolyte. A few molecular substances (such as HCl) dissolve almost entirely as ions in the solution and are therefore strong electrolytes. The solubility rules can be used to predict the solubility of ionic compounds in water. 

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