Solutes 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 well-known fact of fundamental solution science is that the presence of ions in any solution gives the solution a low electrical resistance and the ability to conduct an electrical current. The absence of ions means that the solution would not be conductive. Thus, solutions of ionic compounds and acids, especially strong acids, have a low electrical resistance and are conductive. This means that if a pair of conductive surfaces are immersed into the solution and connected to an electrical power source, such as a simple battery, a current can be detected flowing in the circuit. Alternatively, if the resistance of the solution between the electrodes were measured (with an ohmmeter), it would be low. Conductivity cells based on this simple design are in common use in nonchromatography applications to determine the quality of deionized water, for example. Deionized water should have no ions dissolved in it and thus should have a very low conductivity. The conductivity detector is based on this simple apparatus. 

When equilibrium is reached, solubility product constants are used to describe saturated solutions of ionic compounds of relatively low solubility. When the ion concentration in solution reaches saturation, equilibrium between the solid and dissolved ions is established. 

The equilibrium constant is given by the product of the concentration of ions present in a saturated solution of ionic compounds. 

The electrical conductance of liquid water is very low compared with the values given by solutions of ionic compounds. Typically, the conductance of a I mol dm-3 solution of sodium chloride is about one... 

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). 

Ion-dipole forces are important for solutions of ionic compounds in dipolar solvents, where solvated species such as Na(OH2) and C1(H20) (for solutions of NaCl in H2O) exist. In the case of some metal ions, these solvated species can be sufficiently stable to be considered as discrete species, such as [Co(NH3)6] or Ag(CFl3CN) 4. 

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)... 

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. 

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. 

As opposed to solutions of ionic compounds (such as table salt), which are always excellent conductors of electricity, acidic solutions have electrical conductivities ranging from strong to weak. The range of electrical conductivities exhibited by different acidic solutions distinguishes acid ionization from ionic dissociation. The range also indicates that acids vary in their ability to produce ions. 

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. 

Hydrates often form as solutions of ionic compounds evaporate to dryness. All hydrates are solids. The water in a hydrate can be driven off with heating, leaving the water-free compound behind. 

It is well known that cation-anion pairs can form in concentrated solutions of ionic compounds (18). The extent of ion... 

When discussing solutions of ionic compounds, molarity emphasizes the number of individual ions. A one molar solution of Na+ contains Avogadro s number, 6.022 X 10, of Na per liter. In contrast, equivalents per liter emphasize charge one equivalent of Na+ contains Avogadro s number of positive charge. 

Fig. 3.9. Tbe effect of polarity (of dipole character) of water molecules on the solution of ionic compounds, a — crystalline lattice of NaCl, b — water molecules surround the chloride anions and sodium cations, c — water molecules remove the Na and Cl ions from crystalline lattice, surround them and prevent their return to the lattice...

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. 

Write the ions separately for solutions of ionic compounds (salts, strong acids and bases). 

Knowledge Required (1) The nature of ionic bonding and properties of ionic compounds. (2) The attributes of solutions of ionic compounds in water. 

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. 

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 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. 

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). 

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) ... 

Nothing says you will have a reaction every time you mix solutions of ionic compounds ... 

Sections 9.2 and 9.3 Solutions of Ionic Compounds and Strong and Weak Acids... 

We have been discussing the formation of monatomic ions as neutral atoms that gain or lose electrons. For most elements this is not a common event, but an accomplished fact. The natural occurrence of many elements is in ionic compounds—compounds made up of ions—or solutions of ionic compounds. Nowhere in natme, for example, are sodium or chlorine atoms to be found, but there are large natural deposits of sodium chloride (table salt) that are made up of sodium ions and chloride ions. The compound, along with other ionic compounds, may also be obtained by evaporating seawater, which contains the ions in solution. 

The bonds in an ionic crystal are very strong, which is why nearly all ionic compounds are solids at room temperature. A high temperature is required to break the many ionic bonds, free the ions from one another, and melt the crystal to become a liquid. Solid ionic compounds are poor conductors of electricity because the ions are locked in place in the crystal (Fig. 12.4[a]). When the substance is melted or dissolved, the crystal is destroyed. The ions are then free to move and able to carry electric current (Fig. 12.4[b]). Liquid ionic compounds and water solutions of ionic compounds are good conductors. 

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