CHEMISTRY THE CENTRAL SCIENCE

4 REACTIONS IN AQUEOUS SOLUTION

CHAPTER SUMMARY AND KEY TERMS

INTRODUCTION AND SECTION 4.1 Solutions in which water is the dissolving medium are called aqueous solutions. The component of the solution that is present in the greatest quantity is the solvent. The other components are solutes.

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 achemical equilibrium.

SECTION 4.2 Precipitation reactions are those in which an insoluble product, called a precipitate, forms. Solubility guidelines help determine whether or not an ionic compound will be soluble in water. (The solubility of a substance is the amount that dissolves in a given quantity of solvent.) Reactions such as precipitation reactions, in which cations and anions appear to exchange partners, are called exchange reactions, or metathesis reactions.

Chemical equations can be written to show whether dissolved substances are present in solution predominantly as ions or molecules. When the complete chemical formulas of all reactants and products are used, the equation is called a molecular equation. A complete ionic equationshows all dissolved strong electrolytes as their component ions. In a net ionic equation, those ions that go through the reaction unchanged (spectator ions) are omitted.

SECTION 4.3 Acids and bases are important electrolytes. Acids are proton donors; they increase the concentration of H+(aq) in aqueous solutions to which they are added. Bases are proton acceptors; they increase the concentration of OH(aq) in aqueous solutions. Those acids and bases that are strong electrolytes are called strong acids and strong bases, respectively. Those that are weak electrolytes are weak acids and weak bases. When solutions of acids and bases are mixed, a neutralization reaction occurs. The neutralization reaction between an acid and a metal hydroxide produces water and a salt. Gases can also be formed as a result of neutralization reactions. The reaction of a sulfide with an acid forms H2S(g); the reaction between a carbonate and an acid forms CO2(g).

SECTION 4.4 Oxidation is the loss of electrons by a substance, whereas reduction is the gain of electrons by a substance. Oxidation numbers keep track of electrons during chemical reactions and are assigned to atoms using specific rules. The oxidation of an element results in an increase in its oxidation number, whereas reduction is accompanied by a decrease in oxidation number. Oxidation is always accompanied by reduction, giving oxidation-reduction, or redoxreactions.

Many metals are oxidized by O2, acids, and salts. The redox reactions between metals and acids as well as those between metals and salts are called displacement reactions. The products of these displacement reactions are always an element (H2 or a metal) and a salt. Comparing such reactions allows us to rank metals according to their ease of oxidation. A list of metals arranged in order of decreasing ease of oxidation is called an activity series. Any metal on the list can be oxidized by ions of metals (or H+) below it in the series.

SECTION 4.5 The concentration of a solution expresses the amount of a solute dissolved in the solution. One of the common ways to express the concentration of a solute is in terms of molarity. The molarity of a solution is the number of moles of solute per liter of solution. Molarity makes it possible to interconvert solution volume and number of moles of solute. Solutions of known molarity can be formed either by weighing out the solute and diluting it to a known volume or by the dilution of a more concentrated solution of known concentration (a stock solution). Adding solvent to the solution (the process of dilution) decreases the concentration of the solute without changing the number of moles of solute in the solution (Mconc × Vconc = Mdil × Vdil).

SECTION 4.6 In the process called titration, we combine a solution of known concentration (a standard solution) with a solution of unknown concentration to determine the unknown concentration or the quantity of solute in the unknown. The point in the titration at which stoichiometrically equivalent quantities of reactants are brought together is called the equivalence point. An indicator can be used to show the end point of the titration, which coincides closely with the equivalence point.

KEY SKILLS

• Recognize compounds as acids or bases, and as strong, weak, or nonelectrolytes. (Sections 4.1 and 4.3)

• Recognize reactions by type and be able to predict the products of simple acid–base, precipitation, and redox reactions. (Sections 4.2–4.4)

• Be able to calculate molarity and use it to convert between moles of a substance in solution and volume of the solution. (Section 4.5)

• Understand how to carry out a dilution to achieve a desired solution concentration. (Section 4.5)

• Understand how to perform and interpret the results of a titration. (Section 4.6)

KEY EQUATIONS

Molarity is the most commonly used unit of concentration in chemistry.

When adding solvent to a concentrated solution to make a dilute solution, molarities and volumes of both concentrated and dilute solutions can be calculated if three of the quantities are known.