Aqueous solution of ionic compounds

A number of other attempts have been made to account for the properties of concentrated aqueous solutions of ionic compounds by procedures that represent further improvements on the simple Debye-Huckel approach. However, they lie outside the scope of the present chapter. The important point to emphasize is that the concentrated aqueous solutions that are generally employed in the preparation of AB cements tend to exhibit significant ion-ion interactions such interactions lead to significant deviations from ideality which may be accounted for by substantial extension of the ideas of simple dilute solution theory. 

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. 

Liu and colleagues found that they could switch between the two forms of s-surf by changing the gas that they bubbled through a solution of the surfactant. They demonstrated this switch by measuring the electrical conductivity of the s-surf solution aqueous solutions of ionic compounds have higher conductivity than solutions of nonionic compounds. They started with a solution of the ami-dine form of s-surf in water. Their results are shown below dotted lines indicate the switch from one gas to another. 

Aqueous solutions of ionic compounds will conduct electricity if positive and negative electrodes are connected to a DC source and inserted into the solution (Figure 7.2). The positive metal ions, cations, slowly migrate to the negative electrode (cathode) and the negative ions, anions, migrate to the positive electrode (anode). 

In aqueous solutions of ionic compounds, the ions act independently of each other. Soluble ionic compounds are written as their separate ions. We must be familiar with the solubility rules presented in Chapter 8 and recognize that the following types of compounds are strong electrolytes strong acids in solution, soluble metallic hydroxides, and salts. (Salts, which can be formed as the products of reactions of acids with bases, include all ionic compounds except strong acids and bases and metalhc oxides and hydroxides.) Compounds must be both ionic and soluble to be written in the form of their separate ions. (Section 9.1)... 

Some substances conduct electricity and some cannot. The conductivity of a substance depends on whether it contains charged particles, and these particles must be able to move. Electrons move freely within a metal, thus allowing it to conduct electricity. Solid NaCl contains ions, but they cannot move, so solid NaCl is a nonconductor by itself But an aqueous solution of ionic compounds such as NaCl contains charged ions, which can move about. Solutions of ionic compounds conduct electricity. Pure water does not conduct electricity. 

In an aqueous solution, ionic compounds are completely dissociated into ions. For example, an aqueous solution of barium nitrate, Ba(N03)2, contains Ba + ions and NO3 ions. If aqueous solutions of ionic compounds are mixed, some ions may interact to form an insoluble product called a precipitate. For example, if aqueous solutions of barium nitrate and sodium sulfate are mixed, insoluble barium sulfate will precipitate. The complete formula equation for this reaction is written as follows. 

Tip-off You are asked to predict whether a precipitation reaction will take place between two aqueous solutions of ionic compounds, and if the answer is yes, to write the complete equation for the reaction. 

Aqueous solutions of ionic compounds Recall that water molecules are polar molecules and are in constant motion, as described by the kinetic-molecular theory. When a crystal of an ionic compound, such as sodium chloride (NaCl), is placed in water, the water molecules collide with the surface of the crystal. The charged ends of the water molecules attract the positive sodium ions and negative chloride ions. This attraction between the dipoles and the ions is greater than the attraction among the ions in the crystal, so the ions break away from the surface. The water molecules surround the ions, and the solvated ions move into the solution, shown in Figure 14.10, exposing more ions on the surface of the crystal. Solvation continues until the entire crystal has dissolved. 

Aqueous solutions of ionic compounds contain dissolved positive, and negative ions. When two such solutions are mixed, the ions may take part in a double-replacement reaction. One outcome of a double-replacement reaction is the formation of a precipitate. By writing ionic equations and knowing the solubilities of specific ionic compounds, you can predict whether a precipitate will be formed. 

In this subsection, we consider two closely related aspects of aqueous solutions of ionic compounds— how they occur and how they behave. We also use a compound s formula to calculate the amount (mol) of each ion in solution. 

The electrolysis of aqueous solutions of ionic compounds is more complicated than the electrolysis of molten ionic compounds (Chapter 9) since the water itself will undergo electrolysis. This occurs because water is slightly dissociated into hydrogen and hydroxide ions (Chapter 8) ... 

The ion-dipole force occurs when an ionic compound is mixed with a polar compound it is especially important in aqueous solutions of ionic compounds. For example, when sodium chloride is mixed with water, the sodium and chloride ions interact with water... 

To assess the energy requirements for the formation of aqueous solutions of ionic compounds, we turn to the process pictured in Figure 14-6. Water... 

Chemistry is often conducted in aqueous solutions. Soluble ionic compounds dissolve into their component ions, and these ions can react to form new products. In these kinds of reactions, sometimes only the cation or anion of a dissolved compound reacts. The other ion merely watches the whole affair, twiddling its charged thumbs in electrostatic boredom. These uninvolved ions cire called spectator ions. 

In Exercises 89 and 90, write balanced (i) formula unit, (ii) total ionic, and (iii) net ionic equations for the reactions that occur when aqueous solutions of the compounds are mixed. 

Figure 9.9 Electrical conductance and ion mobility. A, No current flows in the ionic solid because ions are immobile. B, In the molten compound, mobile ions flow toward the oppositely charged electrodes and carry a current. C, In an aqueous solution of the compound, mobile solvated ions carry a current.

The displacement of hydrogen from water or acids is just one type of single-displacement reaction. Other elements can also be displaced from their compounds. For example, copper metal reduces aqueous solutions of ionic silver compounds, such as silver nitrate, to deposit silver metal. The copper is oxidized. 

The strongest intermolecular interactions are those between ions and between ions and dipoles. We have encountered this ion-ion interaction before in the context of chemical bonding in ionic solids, but it is also a major intermolecular interaction in solutions of ionic compounds, such as aqueous sodium chloride. As discussed in Section 0.1, the interaction between two ions of charge qp, and separated by a distance r is given by the Coulomb potential. 

In the previous section, we used a precipitation reaction to illustrate how to convert a molecular equation to an ionic equation. A precipitation reaction occurs in aqueous solution because one product is insoluble. A precipitate is an insoluble solid compound formed during a chemical reaction in solution. To predict whethCT a precipitate will form when you mix two solutions of ionic compounds, you need to know whether any of the potential products that might form are insoluble or not This is another application of the solubility rules (Section 4.1). 

Earlier treatment of reactions in aqueous solution a number of our users and non-user reviewers told us that they wanted students to learn about this topic before stoichiometry. Chapter 9 addresses this request. We now have a two-chapter treatment of chemical reactivity with a qualitative emphasis, preceding the quantitative chapter on stoichiometry. Chapter 8 provides an introduction to chemical reactivity, with an emphasis on writing and balancing chemical equations and recognizing reaction types based on the nature of the equation. After students have become confident with the fundamentals, we then increase the level of sophistication of our presentation on chemical change by introducing solutions of ionic compounds and net ionic equations. 

Many chemical reactions in aqueous solution involve ionic compounds. Only some of the ions in solution take part in these reactions. 

The case of the solutions is clearly complicated by the existence of interactions between the stracture elements of the solid in solution or at the interface with solvent. This phenomenon is particularly important in the case of the aqueous solutions of ionic sohds because of the polar character of the water molecule that strongly solvates the ions. In this case, the enthalpy of condensation is not necessarily any more negative many ionic compounds have an exothermic dissolution in water (e.g. sodium hydroxide, lithium chloride, or potassium fluoride). 

The stability of chalconide (ionic) compounds decreases from oxygen to tellurium treatment with water produces XH- with X = O or S (hydroxyl and thiol radicals, respectively), but XH2 with X = Se or Te. On warming, aqueous solutions of HS evolve hydrogen sulfide, evidencing that the hydrosulfide ion is much less stable than hydroxide. 

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