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SGCh04 - Chapter Four REACTIONS IN AQUEOUS SOLUTION General...

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62 Chapter Four REACTIONS IN AQUEOUS SOLUTION General Properties of Aqueous Solutions Precipitation Reactions Acid-Base Reactions Oxidation-Reduction Reactions Solution Concentration Gravimetric Analysis Acid-Base Titrations Redox Titrations GENERAL PROPERTIES OF AQUEOUS SOLUTIONS STUDY OBJECTIVES 1. Define solute, solvent, and solution. 2. Distinguish between nonelectrolytes, weak electrolytes, and strong electrolytes. Solutions. Many chemical reactions occur in aqueous solutions. Solutions are homogeneous mixtures of two or more substances. Solutions have two components, the solute which is the substance present in the smaller amount, and the solvent which is the substance present in the larger amount. A solute can be a solid, liquid, or gaseous substance. In aqueous solutions, water is the solvent. Electrolytes and Nonelectrolytes. Many aqueous solutions of ionic compounds conduct electricity whereas water itself essentially does not conduct. These solutions are called electrolyte solutions. An electrolyte is a substance that, when dissolved in water, yields an electrically conducting solution. Electrolytes produce ions in solution that are capable of carrying an electrical current through the solution. For example, salts such as KCl(s), which already consist of ions, separate into individual ions when dissolved in water. Salts dissociate in water. KCl(aq) K + (aq) + Cl (aq) Acids can ionize in water. Hydrochloric acid even though it is a molecule will completely ionize in water. HCl(aq) H + (aq) + Cl (aq) Thus a solution of HCl, for instance, contains H + ions and Cl ions, but no HCl molecules! Strong electrolytes are 100% dissociated into ions in solution. A nonelectrolyte is a substance that does not ionize in aqueous solution. Such a solution does not conduct an electrical current. Sugar is an example of a nonelectrolyte. In solution, sugar exists as C 6 H 12 O 6
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Reactions in Aqueous Solution / 63 molecules. These are neutral (no charge) and cannot conduct an electrical current. All nonelectrolytes exist as molecules in aqueous solution. Between electrolytes and nonelectrolytes are the weak electrolytes. These substances ionize only partially to yield a few ions capable of conducting a weak electrical current. Compounds that are incompletely dissociated into ions are called weak electrolytes . Acetic acid (CH 3 COOH) is an example of a weak electrolyte. You will see in Chapter 16 that in a solution of CH 3 COOH about 99% of the CH 3 COOH molecules are not ionized. Only 1% are ionized into H + and CH 3 COO ions as shown below. CH 3 COOH(aq) CH 3 COO (aq) + H + (aq) Recall that a double arrow means the reaction is reversible. This means that the reaction can occur in the reverse direction as well as the forward direction. The reverse reaction removes ions from the solution and leads to incomplete ionization. Table 4.2 of the text lists examples of substances that are strong electrolytes, weak electrolytes, and nonelectrolytes.
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