4.3 Acid-Base Reactions

Acids and bases are among the most familiar compounds that we encounter (Figure 4.5). Many are industrial and household substances, and some are important components of biological fluids. For example, hydrochloric acid is not only an important industrial chemical but is also the main constituent of gastric juice in our stomach. Acids and bases also happen to be common electrolytes.

Acids

Acids are substances that are able to ionize in aqueous solutions to form a hydrogen ion and thereby increase the concentration of H+(aq) ions. Because a hydrogen atom consists of a proton and an electron, H+ is simply a proton. Thus, acids are often called proton donors. Molecular models of three common acids, HCl, HNO3, and HC2H3O2, are shown below.

AAAUAVD0

Molecules of different acids can ionize to form different numbers of H+ ions. Both HCl and HNO3 are examples of monoprotic acids, which yield one H+ per molecule of acid. Sulfuric acid, H2SO4, is an example of a diprotic acid, one that yields two H+ per molecule of acid. The ionization of H2SO4 and other diprotic acids occurs in two steps:

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[4.9]

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[4.10]

Although H2SO4 is a strong electrolyte, only the first ionization is complete. Thus, aqueous solutions of sulfuric acid contain a mixture of H+(aq), HSO4(aq), and SO42–(aq).

Bases

Bases are substances that accept (react with) H+ ions. Hydroxide ions, OH, are basic because they readily react with H+ ions to form water:

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[4.11]

Any substance that increases the concentration of OH(aq) when added to water is a base. Ionic hydroxide compounds such as NaOH, KOH, and Ca(OH)2 are among the most common bases. When dissolved in water, they dissociate into their separate ions, introducing OH ions into the solution.

Compounds that do not contain OH ions can also be bases. For example, ammonia, NH3, is a common base. When added to water, it accepts an H+ ion from the water molecule and thereby increases the concentration of OH ions in the water (Figure 4.6):

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[4.12]

Because only a small fraction of the NH3 (about 1 percent) forms NH4+ and OH ions, ammonia is a weak electrolyte.

Figure 4.6 An H2O molecule acts as a proton donor (acid), and NH3 as a proton acceptor (base). Only a fraction of the NH3 reacts with H2O; NH3 is a weak electrolyte.

Strong and Weak Acids and Bases

Acids and bases that are strong electrolytes (completely ionized in solution) are called strong acids and strong bases. Those that are weak electrolytes (partly ionized) are called weak acids and weak bases. Strong acids are more reactive than weak acids when the reactivity depends only on the concentration of H+(aq). The reactivity of an acid, however, can depend on the anion as well as on H+(aq). For example, hydrofluoric acid, HF, is a weak acid, being only partly ionized in aqueous solution. However, HF is very reactive and vigorously attacks many substances, including glass. This reactivity is due to the combined action of H+(aq) and F(aq).

Table 4.2 lists the common strong acids and bases. You should commit these to memory. As you examine this table, notice first that some of the most common acids, such as HCl, HNO3, and H2SO4, are strong. Second, three of the strong acids result from combining a hydrogen atom and a halogen atom. (HF, however, is a weak acid.) Third, the list of strong acids is very short. Most acids are weak. Fourth, the only common strong bases are the hydroxides of Li+, Na+, K+, Rb+, and Cs+ (the alkali metals, group 1A) and the hydroxides of Ca2+, Sr2+, and Ba2+ (the heavy alkaline earths, group 2A). The most common weak base is NH3.

Identifying Strong and Weak Electrolytes

If we remember the common strong acids and bases (Table 4.2) and we remember that NH3 is a weak base, we can make reasonable predictions about the electrolytic behavior of a great number of water-soluble substances. Table 4.3 summarizes our observations about electrolytes. To classify a soluble substance as a strong electrolyte, weak electrolyte, or nonelectrolyte, we simply work our way down and across this table. We first ask ourselves whether the substance is ionic or molecular. If it is ionic, it is a strong electrolyte. If it is molecular, we ask whether it is an acid. (Does it have H first in the chemical formula?) If it is an acid, we rely on the memorized list from Table 4.2 to determine whether it is a strong or weak electrolyte. If an acid is not listed in Table 4.2, it is probably a weak electrolyte. For example, H3PO4, H2SO3, and HC7H5O2, which are not listed in the table are weak acids. If the substance is not an acid, we next ask whether it is NH3, which is a weak electrolyte. (There are compounds called amines that are related to NH3 and are also molecular bases, but we will not consider them until later chapters.) Finally, if a molecular substance is not an acid or NH3, it is probably a nonelectrolyte. This approach to classifying substances is summarized in the flowchart shown in Figure 4.7.

Figure 4.7 Flowchart for classifying water-soluble compounds as strong electrolytes, weak electrolytes, or nonelectrolytes.

SAMPLE EXERCISE 4.4

Classify each of the following dissolved substances as a strong electrolyte, weak electrolyte, or nonelectrolyte: CaCl2, HNO3, C2H5OH (ethanol), HCHO2 (formic acid), KOH.

SOLUTION We are given several chemical formulas and asked to classify each substance as a strong electrolyte, weak electrolyte, or nonelectrolyte. The approach we take is outlined in Table 4.3 and in Figure 4.7. We can predict whether a substance is ionic or molecular, based on its composition. As we saw in Section 2.6, most ionic compounds we encounter in this text are composed of both a metal and a nonmetal, whereas most molecular ones are composed only of nonmetals.

Two compounds fit the criteria for ionic compounds: CaCl2 and KOH. Both are strong electrolytes. The remaining compounds are molecular. Two, HNO3 and HCHO2, are acids. Nitric acid, HNO3, is a common strong acid (a strong electrolyte), as shown in Table 4.2. Because most acids are weak acids, our best guess would be that HCHO2 is a weak acid (weak electrolyte). This is correct.

One molecular compound remains, C2H5OH. Though it has an OH group, it is not a metal hydroxide; thus, it is not a base. It is a nonelectrolyte.

PRACTICE EXERCISE

Consider solutions is which 0.1 mol of each of the following compounds is dissolved in 1 L of water: Ca(NO3)2 (calcium nitrate), C6H12O6 (glucose), NaC2H3O2 (sodium acetate), HC2H3O2 (acetic acid). Rank the solutions in order of increasing electrical conductivity, based on the fact that the greater the number of ions in solution, the greater the conductivity. Answer: C6H12O6 (nonelectrolyte) < HC2H3O2 (weak electrolyte, existing mainly in the form of molecules with few ions) < NaC2H3O2 (strong electrolyte that provides two ions, Na+ and C2H3O2) < Ca(NO3)2 (strong electrolyte that provides three ions, Ca2+ and 2NO3)

Neutralization Reactions and Salts

Solutions of acids and bases have very different properties. Acids have a sour taste, whereas bases have a bitter taste. (Tasting chemical solutions is, of course, not a good practice. However, we have all had acids such as ascorbic acid [vitamin C], acetylsalicylic acid [aspirin], and citric acid [in citrus fruits] in our mouths, and we are familiar with their characteristic sour taste. It differs from the taste of soaps, which are mostly basic.) Acids can change the colors of certain dyes in a specific way that differs from the effect of a base (Figure 4.8). For example, the dye known as litmus is changed from blue to red by an acid, and from red to blue by a base. In addition, acidic and basic solutions differ in chemical properties in several important ways that we will explore in this chapter and in later chapters.

When a solution of an acid and that of a base are mixed, a neutralization reaction occurs. The products of the reaction have none of the characteristic properties of either the acidic or the basic solutions. For example, when hydrochloric acid is mixed with a solution of sodium hydroxide, the following reaction occurs:

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[4.13]

Here, water and table salt, NaCl, are the products of the reaction. By analogy to this reaction, the term salt has come to mean any ionic compound whose cation comes from a base (here, Na+ from NaOH) and whose anion comes from an acid (here, Cl from HCl). In general, a neutralization reaction between an acid and a metal hydroxide produces water and a salt.

Because HCl, NaOH, and NaCl are all soluble strong electrolytes, the complete ionic equation associated with Equation 4.13 is

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[4.14]

Therefore, the net ionic equation is

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[4.15]

Equation 4.15 summarizes the essential feature of the neutralization reaction between any strong acid and any strong base: H+(aq) and OH(aq) ions combine to form H2O.

Figure 4.9 shows the reaction between hydrochloric acid and another base, Mg(OH)2, which is insoluble in water. A milky white suspension of Mg(OH)2 called milk of magnesia is seen dissolving as the neutralization reaction occurs:

Molecular equation:

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[4.16]

Net ionic equation:

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[4.17]

Notice that the OH ions (this time in a solid reactant) and H+ ions combine to form H2O. Because the ions exchange partners, neutralization reactions between acids and metal hydroxides are metathesis reactions.

SAMPLE EXERCISE 4.5

(a) Write a balanced complete chemical equation for the reaction between aqueous solutions of acetic acid, HC2H3O2, and barium hydroxide, Ba(OH)2. (b) Write the net ionic equation for this reaction.

SOLUTION (a) We are given the chemical formulas for an acid and a base and asked to write a balanced chemical equation for their neutralization reaction. As Equation 4.13 and the italicized statement that follows it indicate, such reactions form H2O and a salt. The salt that forms will contain the cation of the base (Ba2+) and the anion of the acid (C2H3O2). Thus, the formula of the salt is Ba(C2H3O2)2. According to the solubility guidelines in Table 4.1, this compound is soluble. The unbalanced equation for the neutralization reaction is

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To balance the equation, we must provide two molecules of HC2H3O2 to furnish the two C2H3O2 ions and to supply the two H+ ions needed to combine with the two OH ions of the base. The balanced equation is

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We can check to be sure that the equation is correctly balanced by counting the number of atoms of each kind on both sides of the arrow. (There are 10 H, 6 O, 4 C, and 1 Ba on each side.) However, it is often easier to check equations by counting groups: There are 2 C2H3O2 groups as well as 1 Ba, and 4 additional H atoms and 2 additional O atoms on each side of the equation.

(b) To write the ionic equation, we must determine whether each compound in aqueous solution is a strong electrolyte or not. HC2H3O2 is a weak electrolyte (weak acid), Ba(OH)2 is a strong electrolyte, and Ba(C2H3O2)2 is also a strong electrolyte. Thus, the complete ionic equation is

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Eliminating the spectator ions gives

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The coefficients in this equation can be simplified:

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The equation is checked by noting that the numbers of each kind of element and the net charge are the same on both sides of the equation.

PRACTICE EXERCISE

(a) Write a balanced equation for the reaction of carbonic acid, H2CO3, and potassium hydroxide, KOH. (b) Write the net ionic equation for this reaction. Answer: (a) H2CO3(aq) + 2KOH(aq) 2H2O(l) + K2CO3(aq); (b) H2CO3(aq) + 2OH(aq) 2H2O(l) + CO32–(aq) (H2CO3 is a weak electrolyte, whereas KOH and K2CO3 are strong electrolytes)

Acid-Base Reactions with Gas Formation

There are many bases besides OH that react with H+ to form molecular compounds. Two of these that you might encounter in the laboratory are the sulfide ion and the carbonate ion. Both of these anions react with acids to form gases that have low solubilities in water. Hydrogen sulfide, H2S, the substance that gives rotten eggs their foul odor, forms when an acid such as HCl(aq) reacts with a metal sulfide such as Na2S:

Molecular equation:

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[4.18]

Net ionic equation:

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[4.19]

Carbonates and bicarbonates react with acids to form CO2 gas. Reaction of CO32– or HCO3 with an acid first gives carbonic acid, H2CO3. For example, when hydrochloric acid is added to sodium bicarbonate, the following reaction occurs:

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[4.20]

Carbonic acid is unstable; if present in solution in sufficient concentrations, it decomposes to form CO2, which escapes from the solution as a gas:

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[4.21]

The decomposition of H2CO3 into H2O and CO2 causes bubbles to form, as shown in Figure 4.10. The overall reaction is summarized by the following equations:

Molecular equation:

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[4.22]

Net ionic equation:

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[4.23]

Both NaHCO3 and Na2CO3 are used as acid neutralizers in acid spills. The bicarbonate or carbonate salt is added until the fizzing due to the formation of CO2(g) stops. Sometimes sodium bicarbonate is used as an antacid to soothe an upset stomach. In that case the HCO3 reacts with stomach acid to form CO2(g). The fizz when Alka-Seltzer tablets are added to water is due to the reaction of tablet ingredients sodium bicarbonate and citric acid.