12.2 Electrolytes

When some substances are dissolved in water, they undergo either a physical or a chemical change that yields ions in solution. These substances constitute an important class of compounds called electrolyte. Substances that do not yield ions when dissolved are called nonelectrolyte. If the physical or chemical process that generates the ions is essentially 100% efficient (all of the dissolved compound yields ions), then the substance is known as a strong electrolyte. If only a relatively small fraction of the dissolved substance undergoes the ion-producing process, it is called a weak electrolyte.

Substances may be identified as strong, weak, or nonelectrolytes by measuring the electrical conductance of an aqueous solution containing the substance. To conduct electricity, a substance must contain freely mobile, charged species. Most familiar is the conduction of electricity through metallic wires, in which case the mobile, charged entities are electrons. Solutions may also conduct electricity if they contain dissolved ions, with conductivity increasing as ion concentration increases. Applying a voltage to electrodes immersed in a solution permits assessment of the relative concentration of dissolved ions, either quantitatively, by measuring the electrical current flow, or qualitatively, by observing the brightness of a light bulb included in the circuit (Figure 12.2.1).

Ionic Electrolytes

Water and other polar molecules are attracted to ions, as shown in Figure 12.2.2. The electrostatic attraction between an ion and a molecule with a dipole is called an ion-dipole attraction. These attractions play an important role in the dissolution of ionic compounds in water.

When ionic compounds dissolve in water, the ions in the solid separate and disperse uniformly throughout the solution because water molecules surround and solvate the ions, reducing the strong electrostatic forces between them. This process represents a physical change known as dissociation. Under most conditions, ionic compounds will dissociate nearly completely when dissolved, and so they are classified as strong electrolytes.

Consider what happens at the microscopic level when we add solid $\mathrm{KCl}$ to water. Ion-dipole forces attract the positive (hydrogen) end of the polar water molecules to the negative chloride ions at the surface of the solid, and they attract the negative (oxygen) ends to the positive potassium ions. The water molecules surround individual $\mathrm{K}{\phantom{A}}^{+}$ and $\mathrm{Cl}{\phantom{A}}^{-}$ ions, reducing the strong interionic forces that bind the ions together and letting them move off into solution as solvated ions, as Figure 12.2.2 shows. Overcoming the electrostatic attraction permits the independent motion of each hydrated ion in a dilute solution, as the ions transition from fixed positions in the undissolved compound to widely dispersed, solvated ions in solution.

Covalent Electrolytes

Pure water is an extremely poor conductor of electricity because it is only very slightly ionized—only about two out of every 1 billion molecules ionize at 25 °C. Water ionizes when one molecule of water gives up a proton to another molecule of water, yielding hydronium and hydroxide ions.

$\mathrm{H}{\phantom{A}}_2\mathrm{O}(l)+\mathrm{H}{\phantom{A}}_2\mathrm{O}(l)\rightleftharpoons \mathrm{H}{\phantom{A}}_3\mathrm{O}{\phantom{A}}^{+}(aq)+\mathrm{OH}{\phantom{A}}^{-}(aq)$

In some cases, solutions prepared from covalent compounds conduct electricity because the solute molecules react chemically with the solvent to produce ions. For example, pure hydrogen chloride is a gas consisting of covalent $\mathrm{HCl}$ molecules. This gas contains no ions. However, when we dissolve hydrogen chloride in water, we find that the solution is a very good conductor. The water molecules play an essential part in forming ions: Solutions of hydrogen chloride in many other solvents, such as benzene, do not conduct electricity and do not contain ions.

Hydrogen chloride is an acid, and so its molecules react with water, transferring H⁺ ions to form hydronium ions ($\mathrm{H}{\phantom{A}}_3\mathrm{O}{\phantom{A}}^{+}$) and chloride ions ($\mathrm{Cl}{\phantom{A}}^{-}$):

This reaction is essentially 100% complete for $\mathrm{HCl}$ (i.e., it is a strong acid and, consequently, a strong electrolyte). Likewise, weak acids and bases that only react partially generate relatively low concentrations of ions when dissolved in water and are classified as weak electrolytes. The reader may wish to review the discussion of strong and weak acids provided in the earlier chapter of this text on reaction classes and stoichiometry.

Glossary

dissociation: physical process accompanying the dissolution of an ionic compound in which the compound’s constituent ions are solvated and dispersed throughout the solution

electrolyte: substance that produces ions when dissolved in water

ion-dipole attraction: electrostatic attraction between an ion and a polar molecule

nonelectrolyte: substance that does not produce ions when dissolved in water

strong electrolyte: substance that dissociates or ionizes completely when dissolved in water

weak electrolyte: substance that ionizes only partially when dissolved in water