That Contain Ions

80 Chapter 3 Chemical Foundations Elements, Atoms, and Ions 

The atomic-level view shows the arrangement of sodium ions (Na ) and chloride ions (Cl ) in the ionic compound sodium chloride. The photo shows solid sodium chloride highly magnified. 

To explore further the significance of the electrical conductivity results, we need to discuss briefly the nature of electric currents. An electric current can travel along a metal wire because electrons are free to move through the wire the moving electrons carry the current. In ionic substances, the ions carry the current. Therefore, substances that contain ions can conduct an electric current only if the ions can move—the current travels by the movement of the charged ions. In solid NaCl, the ions are tightly held and cannot move. When the solid melts and becomes a liquid, however, the structure is disrupted and the ions can move. As a result, an electric current can travel through the melted salt. 

The same reasoning applies to NaCl dissolved in water. When the solid dissolves, the ions can move around and are dispersed throughout the water, allowing it to conduct a current. 

Thus we recognize substances that contain ions by their characteristic properties. They often have very high melting points, and they conduct an electric current when melted or dissolved in water. 

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A solute may be present as ions or as molecules. We can find out if the solute is present as ions 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 " mol-L ) that pure water itself does not conduct electricity significantly. 

The corrosion of iron occurs particularly rapidly when an aqueous solution is present. This is because water that contains ions provides an oxidation pathway with an activation energy that is much lower than the activation energy for the direct reaction of iron with oxygen gas. As illustrated schematically in Figure 19-21. oxidation and reduction occur at different locations on the metal surface. In the absence of dissolved ions to act as charge carriers, a complete electrical circuit is missing, so the redox reaction is slow, hi contrast, when dissolved ions are present, such as in salt water and acidic water, corrosion can be quite rapid. 

A straightforward method is to incorporate ahovalent impurity ions into the crystal. These impurities can, in principle, be compensated structurally, by the incorporation of interstitials or vacancies, or by electronic defects, holes, or electrons. The possibility of electronic compensation can be excluded by working with insulating solids that contain ions with a fixed valence. 

The removal of ionic contaminants from ion chromatographic eluents is often necessary and has traditionally been accomplished using trap columns that contain ion-exchange resins of appropriate functionality. 

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. 

Ionic liquids, a new type of solvent, are salts that contain ions (an organic cation and an anion, usually inorganic) and occur as liquids at room temperature. 

The ionic polarization mechanism refers to a polarization mechanism in materials that contain ions. The dipole moment in this case is produced by the separation of the positive and negative ions included in the structure of these materials. The dipole moment for the ionic polarization mechanism can be expressed as... 

The usefulness of soaps is limited by their tendency to precipitate out of solution in hard water. Hard water is water that is acidic or that contains ions of calcium, magnesium, or iron. In acidic water (such as the acid rain of environmental concern), soap molecules are protonated to the free fatty acids. Without the ionized carboxylate group, the fatty acid floats to the top as a greasy acid scum precipitate. 

This is the process of applying a DC current through electrodes to a solution that contains ions. The ions then begin to migrate to the oppositely charged electrodes and are discharged. 

Electrostatic interactions are present in materials that contain ions. These include aqueous solutions of acids, bases, salts, polyelectrolytes (i.e., charged polymers), as well as colloidal suspensions of charged particles or droplets. Although fluids containing charged surfaces and mobile ions maintain electroneutrality overall, locally there are often eharge imbalances. 

When two aqueous solutions that contain ions as solutes are combined, the ions may react with one another. These reactions are always double-replacement reactions. The solvent molecules, which are all water molecules, do not usually react. Three types of products can form from the double-replacement reaction precipitate, water, or gas. You can observe a precipitate forming when you do the miniLAB for this chapter. 

When two aqueous solutions that contain ions as solutes are combined, the ions may react with one another. The solvent molecules do not usually react. 

Compounds that contain ions of alkali metals (e.g., K+ or Na+). 

If we refer to the general solubility rules, in this case, we are not really able to identify the precipitate, because there seem to be conflicting trends. We notice that compounds that contain ions of alkali metals tend to be soluble, which would seem to suggest that the sodium chlorate is probably not the precipitate, but we also read that compounds that contain halogens also tend to be soluble, which would seem to let the lead (II) iodide off the hook How is it possible for us to have a solid precipitate and then have no suspect ... 

The most important concept for this question from Section 3.15(c) Solubility is the general rule that compounds that contain ions with widely different radii are more soluble in water than compounds containing ions with similar radii. The six-coordinate radii of Na" and K are 1.02 and 1.38 A, respectively (see Table 1.4), whereas the thermochemical radius of the perchlorate ion is 2.36 A (see Table 3.10). Therefore, because the radii of Na and CIO4 differ more than the radii of K and CIO4", the salt NaC104 should be more soluble in water than KCIO4. 

E21.23 The intense band at -250 nm is an LMCT transition (specifically a Cl -to-Cr charge transfer). Irradiation at this wavelength should produce a population of [CrCl(NH3)5] ions that contain ions and Cl atoms instead of Cr ions and Cl" ions. Irradiation of the complex at wavelengths between 350 and 600 nm will not lead to photoreduction. The bands that are observed between these two wavelengths are ligand field transitions the electrons on the metal are rearranged, and the electrons on the Cl ion are not involved. 

Some chemical substances, when dissolved in water, break apart into positive and negative ions such that free ions exist in solution and give the solution certain characteristic properties. One very important such property is the ability of the solution to conduct an electric current. Solutions that contain ions will conduct an electric current. Solutions that do not contain ions will not conduct an electric current. This property is the basis for a number of laboratory and industrial processes, including the manufacture of certain chemicals, electroplating of metals, and the quantitative analysis of dissolved chemical species. 

The traditional electrolytic technologies are those that pass direct current electricity between electrodes in contact with phases that contain ions. Electrolysis is caused to occur by the interaction of... 

The word polar was introduced in Unit 5 (see page 71). A polar substance is a substance that contains ions or consists of polar molecules. A polar solvent is a solvent which consists of polar molecules. 

Let us return to the cubic closest-packing arranganent and take a look at a mineral that contains ions in all of the tetrahedral voids. The unit ceU of fluorite, CaFj, is shown in Figure 48. 

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