Strong electrolyte Compounds that ionize

Strong electrolytes Compounds that ionize (or dissociate into their constituent ions) completely, or nearly completely, when dissolved in water to produce aqueous solutions that conduct an electric current. 

Strong electrolytes (those that ionize in water) include with minor exceptions, the following compounds ... 

Thus, compounds must be both soluble and ionic to be written in the form of their separate ions. A listing of water-soluble compounds was given in Table 8.3. In addition to the compounds listed there, all strong acids are water soluble. In summary strong electrolytes—compounds that dissociate or ionize extensively in aqueous solution—include the following ... 

In Chapter 8, you learned that ionic compounds are called electrolytes because they dissociate in water to form a solution that conducts electric current. Some molecular compounds ionize in water and also are electrolytes. Electrolytes that produce many ions in solution are called strong electrolytes those that produce only a few ions in solution are called weak electrolytes. 

Acids, first encountered in Section 3.6, are molecular compounds, but they do ionize—form ions—when they dissolve in water. Hydrochloric acid (HCl) is a molecular compound that ionizes into and Cl when it dissolves in water. HCl is an example of a strong acid, one that completely ionizes in solution. Since strong acids completely ionize in solution, they are also strong electrolytes. We represent the complete ionization of a strong acid with a single reaction arrow between the acid and its ionized form ... 

Strong electrolyte A compound that is completely ionized to ions in dilute water solution, 37 Strontium, 543 Strontium chromate, 434 Structural formula A formula showing the arrangement of bonded atoms in a molecule, 34,579-580,586,590,593, 597... 

Many of the reactions that you will study occur in aqueous solution. Water readily dissolves many ionic compounds as well as some covalent compounds. Ionic compounds that dissolve in water (dissociate) form electrolyte solutions— solutions that conduct electrical current due to the presence of ions. We may classify electrolytes as either strong or weak. Strong electrolytes dissociate (break apart or ionize) completely in solution, while weak electrolytes only partially dissociate. Even though many ionic compounds dissolve in water, many do not. If the attraction of the oppositely charged ions in the solid is greater than the attraction of the water molecules to the ions, then the salt will not dissolve to an appreciable amount. 

Table I contains a list of some of the compounds that have been submitted to this type of analysis. The recovery data is intended to be illustrative only since recoveries depend strongly on several important method variables. Recoveries are expressed as a percentage of the amount added to organic free water. The purge time was 11-15 minutes with helium or nitrogen, the purge rate was 20 ml/minute at ambient temperature, and the trap was Tenax followed by Silica Gel. Data from the 5 ml sample was obtained with a custom made purging device and either flame ionization, microcoulo-metric, or electrolytic conductivity GC detectors. Data from the 25 ml sample was obtained with a Tekmar commercial liquid sample concentrator and a mass spectrometer GC detector using CRMS.

NaCI is present in solution as ions. Compounds that are completely ionized in water are called strong electrolytes because these solutions easily conduct electricity. Most salts are strong electrolytes. Other compounds (including many acids and bases) may dissolve in water without completely ionizing. These are referred to as weak electrolytes and their state of ionization is at equilibrium with the larger molecule. Those compounds that dissolve with no ionization (e.g., glucose, C6H12O6) are called nonelectrolytes. 

As we have emphasized, colligative properties depend on the number of solute particles in a given mass of solvent. A 0.100 molal aqueous solution of a covalent compound that does not ionize gives a freezing point depression of 0.186°C. If dissociation were complete, 0.100 m KBr would have an ejfective molality of 0.200 m (i.e., 0.100 m K+ + 0.100 m Br ). So we might predict that a 0.100 molal solution of this 1 1 strong electrolyte would have a freezing point depression of 2 X 0.186°C, or 0.372°C. In fact, the observed depression is only 0.349°C. This value for ATf is about 6% less than we would expect for an effective molarity of 0.200 m. 

Any substance whose aqueous solution contains ions is called an electrolyte. Any substance that forms a solution containing no ions is a nonelectrolyte. Electrolytes that are present in solution entirely as ions are strong electrolytes, whereas those that are present partly as ions and partly as molecules are weak electrolytes. Ionic compounds dissociate into ions when they dissolve, and they are strong electrolytes. The solubility of ionic substances is made possible by solvation, the interaction of ions with polar solvent molecules. Most molecular compounds are nonelectrolytes, although some are weak electrolytes, and a few are strong electrolytes. When representing the ionization of a weak electrolyte in solution, half-arrows in both directions are used, indicating that the forward and reverse reactions can achieve a chemical balance called a chemical equilibrium. 

The list of molecular compounds that are strong electrolytes is fairly short. It comprises the seven strong acids, which are listed in Table 4.1. A strong acid ionizes completely, resulting in a solution that contains hydrogen ions and the corresponding anions but essentially no acid molecules. 

Mo.st of the molecular compounds that are electrolytes are weak electrolytes. A weak electrolyte is a compound that produces ions upon dissolving but exists in solution predominantly as molecules that are nor ionized. Most adds (except those listed in Table 4.1) are weak electrolytes. Acetic acid (HC2H3O2) is not one of the strong acids listed in Table 4.1, so it is a weak acid. Its ionization in water is represented by the following chemical equation ... 

The colligative properties of an electrolyte solution can be used to determine percent dissociation. Percent dissociation is the percentage of dissolved molecules (or formula units, in the case of an ionic compound) that separate into ions in solution. For a strong electrolyte such as NaCl, there should be complete, or 100 percent, dissociation. However, the data in Table 13.4 indicate that this is not necessarily the case. An experimentally determined van t Hoff factor smaller than the corresponding calculated value indicates less than 100 percent dissociation. As the experimentally determined van t Hoff factors for NaCl indicate, dissociation of a strong electrolyte is more complete at lower concentration. The percent ionization of a weak electrolyte, such as a weak acid, also depends on the concentration of the solution. 

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