Ionic solids, dissolving

As pointed out in Chapter 2, when an ionic solid dissolves in water, the cations and anions separate from each other. This process can be represented by a chemical equation in which the reactant is the solid and the products are the positive and negative ions in water (aqueous) solution. For the dissolving of MgCl2, the equation is... 

Each of the following ionic solids dissolves in water to form conducting solutions. Write equations for each reaction. 

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. 

Many substances that participate in aqueous reactions are soluble salts. These ionic solids dissolve in water to give solutions of cations and anions. For almost all salts, there is an upper limit to the amount that will dissolve in water. A salt solution is saturated when the amount dissolved has reached this upper limit of solubility. Any additional salt added to a saturated solution remains undissolved at the bottom of the vessel. When excess solid... 

Sodium chloride and other soluble ionic solids dissolve in polar solvents such as water because of ion-dipole forces. An ion-dipole force is the force of attraction between an ion and a polar molecule (a dipole). For example, NaCl dissolves in water because the attractions between the Na and Cl ions and the water molecules provide enough energy to overcome the forces that bind the ions together. Figure 4.14 shows how ion-dipole forces dissolve any type of soluble ionic compound. 

In aqueous solution, ionic solids dissolve as the negative ends and the positive ends of the water molecules hecome oriented with the corresponding oppositely charged ions that make up the ionic compound, pulling them away from the solid into solution. 

A solute may be present as ions or as molecules. We can identify the form of the solute by noting whether the solution conducts an electric current. Because a current is a flow of electric charge, only solutions that contain ions conduct electricity. There is such a tiny concentration of ions in pure water (about 10-7 m) that water alone does not conduct electricity. A substance that dissolves to give a solution that conducts electricity is called an electrolyte. Electrolyte solutions (solutions of electrolytes), which conduct electricity because they contain ions, include aqueous solutions of ionic compounds, such as sodium chloride and potassium nitrate. The ions are not formed when an ionic solid dissolves they exist as separate ions in the solid but become free to move apart in the presence of water (Fig. 1.1). Acids also are electrolytes. Unlike salts, they are molecular compounds in the pure state but form ions when they dissolve. One example is hydrogen chloride, which exists as gaseous HC1 molecules. In solution, however, HCl is called hydrochloric acid and is present as hydrogen ions and chloride ions. 

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 ionic solids dissolved in pure water. We will now see what happens when the water contains an ion in common with the dissolving salt. For example, consider the solubility of solid silver chromate (Ag2Cr04, Ksp = 9.0 X 10-12) in a 0.100 M solution of AgN03. Before any Ag2Cr04 dissolves, the solution contains the major species Ag+, N03, and H20. Since N03 is not found in Ag2Cr04, we can ignore it. The relevant initial concentrations (before any Ag2Cr04 dissolves) are... 

Some ionic solids dissolve with the release of heat. Examples are anhydrous sodium sulfate, Na2S04 calcium acetate, Ca(CH3COO)2 calcium chloride, CaCl2 and lithium sulfate hydrate, Li2S04 H20. 

Many ionic solids dissolve by endothermic processes. Their solubilities in water usually increase as heat is added and the temperature increases. For example, KCl dissolves endothermically. 

In Chapter 2 we saw that mixtures are either heterogeneous or homogeneous. A homogeneous mixture is a solution. Many ionic solids dissolve in solvents, such as water. An ionic solid, if soluble, will form positive and negative ions in solution by dissociation. 

Many ionic solids dissolve in water, dissociating into positive and negative ions (an electrolytic solution). Because these ions can carry (conduct) a current of electricity, they are called electrolytes. Covalent solids in solution usually retain their neutral character and are nonelectrolytes. 

These ions with their opposite charges attract each other in the same way as do the simple ions in binary ionic compounds. However, the individual polyatomic ions are held together by covalent bonds, with all of the atoms behaving as a unit. For example, in the ammonium ion, NH +, there are four N—H covalent bonds. Likewise, the nitrate ion, N03, contains three covalent N—O bonds. Thus, although ammonium nitrate is an ionic compound because it contains the NH " and N03 ions, it also contains covalent bonds in the individual polyatomic ions. When ammonium nitrate is dissolved in water, it behaves as a strong electrolyte like the binary ionic compounds sodium chloride and potassium bromide. As we saw in Chapter 8, this occurs because when an ionic solid dissolves, the ions are freed to move independently and can conduct an electric current. 

When an ionic solid dissolves, what is present in the solution ... 

In Chapter 14, you learned that the bonding forces in ionic solids such as NaCl are very strong, yet many ionic solids dissolve readily in water. Explain. 

We have learned in this chapter that many ionic solids dissolve in water as strong electrolytes, that is, as separated ions in solution. What properties of water facilitate this process Would you expect ionic compounds to be soluble in elemental Uquids like bromine or mercury, just as they are in water Explain. 

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... 

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