5.5 pH and Acidic and. Basic Solutions. 5.6 Arrhenius Acid-. Base Reactions. 5.7 Brønsted-Lowry. Acids and Bases t’s test day in chemistry class—they’ve
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CHAPTER 5ACIDS, BASES, AND ACID-BASE REACTIONS1595.1 Acids 5.2 Acid Nomenclature 5.3 Summary of Chemical Nomenclature5.4 Strong and Weak Bases5.5 pH and Acidic and Basic Solutions5.6 Arrhenius Acid- Base Reactions5.7 Brønsted-Lowry Acids and Basest™s test day in chemistry classŠthey™ve been learning about acids and basesŠand Fran unwisely skips breakfast in order to have time for some last-minute studying. As she reads, she chews on a candy bar and sips a cup of co˜ee. Fran is well aware that the sugary candy sticking to her molars is providing breakfast for the bacteria in her mouth, which in turn produce an acid that will dissolve some of the enamel on her teeth. Feeling a little guilty about all that sugar from the candy, Fran drinks her co˜ee black, even though she doesn™t like the taste. ˚e ca˜eine in her co˜ee is a base, and like all bases, it tastes bitter. Fran™s junk -food breakfast and her worrying about the exam combine to give her an annoying case of acid indigestion, which she calms by drinking some baking soda mixed with water. ˚e baking soda contains a base that fineutralizesfl some of her excess stomach acid. After taking the exam, Fran feels happy and con˛dent. All those hours working problems, reviewing the learning objectives, and participating in class really paid o˜. Now she™s ready for some lunch. Before eating, she washes her hands with soap made from the reaction of a strong base and animal fat. One of the reasons the soap is slippery is because all bases feel slippery on the skin. To compensate for her less-than-healthy breakfast, Fran chooses salad with a piece of lean meat on top for lunch. Like all acids, the vinegar in her salad dressing tastes sour. Her stomach produces just enough additional acid to start the digestion of the protein from the meat. Read on to learn more about the acids and bases that are important in Fran™s life and your own: what they are, how to construct their names and recognize their formulas, and how they react with each other. The vinegar in salad dressing tastes sour, as do all acids. Review Skills˚e presentation of information in this chapter assumes that you can already perform the tasks listed below. You can test your readiness to proceed by answering the Review Questions at the end of the chapter. ˚is might also be a good time to read the Chapter Objectives, which precede the Review Questions. Describe the structure of liquid water. (Section 3.3) Convert between the names and formulas for common polyatomic ions. (Table 3.5) Given a chemical name or formula, decide whether or not it represents an ionic compound. (Section 3.5) Convert between names and formulas for ionic compounds. (Section 3.5) Write a description of the changes that take place when an ionic compound is dissolved in water. (Section 4.2) Predict ionic solubility. (Section 4.2) Predict the products of double-displacement reactions. (Section 4.2)
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5.1 AcidsAcids have many uses. For example, phosphoric acid is used to make gasoline additives and carbonated beverages. ˚e textile industry uses oxalic acid (found in rhubarb and spinach) to bleach cloth, and glass is etched by hydro˝uoric acid. Dyes and many other chemicals are made with sulfuric acid and nitric acid, and corn syrup, which is added to a variety of foods, is processed with hydrochloric acid. ˚e chemical reactions of acids often take place in water solutions, so after discussing what acids are, we will explore a model for visualizing the particle structure of water solutions of acids.Acids have many uses, including making car batteries, Arrhenius Acids You may have already noticed, in your ˛rst few weeks of studying chemistry, that the more you learn about matter, the more ways you have of grouping and classifying the di˜erent substances. ˚e most common and familiar way of classifying substances is by their noteworthy properties. For example, people long ago decided that any substance that has a sour taste is an acid. Lemons are sour because they contain citric acid, and old wine that has been exposed to the air tastes sour due to acetic acid. As chemists learned more about these substances, however, they developed more speci˛c de˛nitions that allowed classi˛cation without relying on taste. A good thing, too, because many acids and bases should not be tastedŠor even touched. ˚ey speed the breakdown of some of the substances that form the structure of our bodies or that help regulate the body™s chemical changes. Two di˜erent de˛nitions of acid are going to be of use to us. For example, chemists conduct many laboratory experiments using a reagent known as finitric acid,fl a substance that has been classi˛ed as an acid according to the Arrhenius de˛nition of acid (named after the Swedish Nobel prize -winning chemist, Svante August Arrhenius). Arrhenius recognized that when ionic compounds dissolve, they form ions in solution. (˚us, when sodium chloride dissolves, it forms sodium ions and chloride ions.) He postulated that acids dissolve in a similar way to form H ions and some kind of anion. For example, he predicted that when HCl is added to water, H ions and Cl ions form. We now know that H ions do not persist in water; they combine with water molecules to form hydronium ions, H 3O. ˚erefore, according to the modern form of the Arrhenius theory, an acid is a substance that produces hydronium ions , H3O, when it is added to water. On the basis of this de˛nition, an acidic solution is a solution with a signi˛cant concentration of H3O. For reasons that are described in Section 5.7, chemists often ˛nd this de˛nition too limiting, so another, broader de˛nition of acids, called the Brønsted-Lowry de˛nition, which we describe later, is commonly used instead. To get an understanding of how hydronium ions are formed when Arrhenius acids are added to water, let™s consider the dissolving of gaseous hydrogen chloride, HCl(g), in water. ˚e solution that forms is called hydrochloric acid. When HCl molecules dissolve in water, a chemical change takes place in which water molecules OBJECTIVE 3OBJECTIVE 2160 Chapter 5 Acids, Bases, and Acid-Base Reactions
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pull hydrogen atoms away from HCl molecules. In each case, the hydrogen atom is transferred without its electron, that is, as an H ion, and because most uncharged hydrogen atoms contain only one proton and one electron, most hydrogen atoms without their electrons are just protons. For this reason, the hydrogen ion, H , is often called a proton. We say that the HCl donates a proton, H , to water, forming hydronium ion, H 3O, and chloride ion, Cl (Figure 5.1). Figure 5.1HCl Reaction with Water Because HCl produces hydronium ions when added to water, it is an acid according to the Arrhenius de˛nition of acids. Once the chloride ion and the hydronium ion are formed, the negatively charged oxygen atoms of the water molecules surround the hydronium ion, and the positively charged hydrogen atoms of the water molecules surround the chloride ion. Figure 5.2 shows how you can picture this solution. 5.1 Acids 161OBJECTIVE 3Figure 5.2Hydrochloric Acid in Water OBJECTIVE 3Hydrochloric acid solutions are used in the chemical industry to remove impurities from metal surfaces (this is called pickling), to process food, to increase the permeability of limestone (an aid in oil drilling), and to make many important chemicals.
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OBJECTIVE 4OBJECTIVE 5Types of Arrhenius Acids In terms of chemical structure, Arrhenius acids can be divided into several di˜erent subcategories. We will look at three of them here: binary acids, oxyacids, and organic acids. ˚e binary acids are HF( aq), HCl(aq), HBr( aq), and HI(aq); all have the general formula of HX(aq), where X is one of the ˛rst four halogens. ˚e formulas for the binary acids will be followed by ( aq) in this text to show that they are dissolved in water. ˚e most common binary acid is hydrochloric acid, HCl( aq). Oxyacids (often called oxoacids) are molecular substances that have the general formula HaXbOc. In other words, they contain hydrogen, oxygen, and one other element represented by X; the a, b, and c represent subscripts. ˚e most common oxyacids in the chemical laboratory are nitric acid, HNO 3, and sulfuric acid, H2SO4.Acetic acid, the acid responsible for the properties of vinegar, contains hydrogen, oxygen, and carbon and therefore ˛ts the criteria for classi˛cation as an oxyacid, but it is more commonly described as an organic (or carbon-based) acid. It can also be called a carboxylic acid. (˚is type of acid is described in more detail in Section 17.1.) ˚e formula for acetic acid can be written as either HC2H3O2, CH3CO2H, or CH3COOH. ˚e reason for keeping one H in these formulas separate from the others is that the hydrogen atoms in acetic acid are not all equal. Only one of them can be transferred to a water molecule. ˚at hydrogen atom is known as the acidic hydrogen. We will use the formula HC2H3O2 because it is more consistent with the formulas for other acids presented in this chapter. ˚e Lewis structure, space-˛lling model, and ball-and-stick model for acetic acid (Figure 5.3) show why CH 3CO2H, and CH3COOH are also common. ˚e acidic hydrogen is the one connected to an oxygen atom. Figure 5.3Acetic AcidPure acetic acid freezes at 17 C (63 F). ˚erefore, it is a liquid at normal room temperature, but if you put it outside on a cold day, it will freeze. ˚e solid has layered crystals that look like tiny glaciers, so pure acetic acid is called glacial acetic acid. ˚e chemical industry uses acetic acid to make several substances necessary for producing latex paints, safety glass layers, photographic ˛lm, cigarette ˛lters, magnetic tapes, and clothing. Acetic acid is also used to make esters, which are substances that have very pleasant odors and are added to candy and other foods. Acids can have more than one acidic hydrogen. If each molecule of an acid can donate one hydrogen ion, the acid is called a monoprotic acid . If each molecule can donate two or more hydrogen ions, the acid is a polyprotic acid . A diprotic acid , such as sulfuric acid, H 2SO4, has two acidic hydrogen atoms. Some acids, such as phosphoric acid, 162 Chapter 5 Acids, Bases, and Acid-Base Reactions
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H3PO4, are triprotic acids . Most of the phosphoric acid produced by the chemical industry is used to make fertilizers and detergents, but it is also used to make pharmaceuticals, to re˛ne sugar, and in water treatment. ˚e tartness of some foods and beverages comes from acidifying them by adding phosphoric acid. ˚e space-˛lling model in Figure 5.4 shows the three acidic hydrogen atoms of phosphoric acid.5.1 Acids 163Figure 5.4The phosphate in this fertilizer was made from phosphoric acid. Strong and Weak Acids Although hydrochloric acid and acetic acid are both acids according to the Arrhenius de˛nition, the solutions created by dissolving the same numbers of HCl and HC 2H3O2 molecules in water have very di˜erent acid properties. You wouldn™t hesitate to put a solution of the weak acid HC 2H3O2 (vinegar) on your salad, but putting a solution of the strong acid HCl on your salad would have a very di˜erent e˜ect on the lettuce. With hydrochloric acid, you are more likely to get a brown, fuming mess rather than a crisp, green salad. Strong acids form nearly one H 3O ion in solution for each acid molecule dissolved in water, whereas weak acids yield signi˛cantly less than one H3O ion in solution for each acid molecule dissolved in water. When an acetic acid molecule, HC 2H3O2, collides with an H 2O molecule, an H can be transferred to the water to form a hydronium ion, H 3O, and an acetate ion, C2H3O2. ˚e acetate ion, however, is less stable in solution than the chloride ion formed when the strong acid HCl dissolves in water. Because of this instability, the C2H3O2 reacts with the hydronium ion, pulling the H ion back to reform HC 2H3O2 and H2O. A reaction in which the reactants are constantly forming products and, at the same time, the products are re-forming the reactants is called a reversible reaction . ˚e chemical equations for reactions that are signi˛cantly reversible are written with double arrows as illustrated in Figure 5.5. Figure 5.5Reversible Reaction of Acetic Acid and Water If you were small enough to be riding on one of the carbon atoms in HC 2H3O2 or C2H3O2, you would ˛nd that your atom was usually in the HC 2H3O2 form but often in the C2H3O2 form and continually changing back and forth. ˚e forward and reverse reactions would be taking place simultaneously all around you. When acetic OBJECTIVE 6OBJECTIVE 6OBJECTIVE 6
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acid is added to water, the relative amounts of the di˜erent products and reactants soon reach levels at which the opposing reactions proceed at equal rates. (We will see why in Chapter 16.) ˚is means that the forward reaction is producing C 2H3O2 as quickly as the reverse reaction is producing HC 2H3O2(aq). At this point, there is no more net change in the amounts of HC 2H3O2, H2O, C2H3O2, or H3O in the solution. For example, for each 1000 molecules of acetic acid added to water, the solution will eventually contain about 996 acetic acid molecules (HC 2H3O2), four hydronium ions (H 3O), and four acetate ions (C2H3O2). Acetic acid is therefore a weak acid , a substance that is incompletely ionized in water because of the reversibility of its reaction with water that forms hydronium ion, H 3O. Figure 5.6 shows a simple model that will help you to picture this solution. Figure 5.6Acetic Acid in Water OBJECTIVE 6˚e products formed from the reaction of a strong acid and water do not recombine at a signi˛cant rate to re-form the uncharged acid molecules and water. For example, when HCl molecules react with water, the H 3O and Cl ions that form do not react to a signi˛cant degree to reform HCl and H 2O. (Look again at Figure 5.2 to see the behavior of a strong acid in solution.) Reactions like this that are not signi˛cantly reversible are often called completion reactions . ˚e chemical equations for completion reactions are written with single arrows to indicate that the reaction proceeds to form almost 100% products. OBJECTIVE 7164 Chapter 5 Acids, Bases, and Acid-Base Reactions
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Sulfuric acid, H 2SO4, is a strong diprotic acid. When added to water, each H 2SO4 molecule loses its ˛rst hydrogen ion completely. ˚is is the reason that H 2SO4 is classi˛ed as a strong acid. Notice the single arrow to indicate a completion reaction. H2SO4(aq) H2O(l) H3O(aq) HSO4(aq)˚e hydrogen sulfate ion, HSO 4, which is a product of this reaction, is a weak acid. It reacts with water in a reversible reaction to form a hydronium ion and a sulfate ion. Notice the double arrow to indicate a reversible reaction. HSO4(aq) H2O(l) 2(aq)For each 100 sulfuric acid molecules added to water, the solution will eventually H3O(aq) SO4contain about 101 hydronium ions (H 3O), 99 hydrogen sulfate ions (HSO 4), and 1 sulfate ion (SO42). Sulfuric acid, H 2SO4, is produced by the United States chemical industry in greater mass than any other chemical. Over 40 billion kilograms of H 2SO4 are produced each year, to make phosphate fertilizers, plastics, and many other substances. Sulfuric acid is also used in ore processing, petroleum re˛ning, pulp and paper-making, and for a variety of other purposes. Most cars are started by lead-acid storage batteries, which contain about 33.5% H2SO4.To do the Chapter Problems at the end of this chapter, you will need to identify important acids as being either strong or weak. ˚e strong acids that you will be expected to recognize are hydrochloric acid, HCl( aq), nitric acid, HNO3, and sulfuric acid, H2SO4. An acid is considered weak if it is not on the list of strong acids. Table 5.1 summarizes this information. Table 5.1 Arrhenius AcidsStrong Weak Binary Acids hydrochloric acid, HCl(aq)hydro˝uoric acid, HF( aq)Oxyacidsnitric acid, HNO3, sulfuric acid, H2SO4other acids with the general formula HaXbOcOrganic acids None acetic acid, HC2H3O2, and others you will see in Section 17.1 ˜ere is an animation that illustrates the di˚erences between strong and weak acids at the textbook™s Web site. OBJECTIVE 10OBJECTIVE 9OBJECTIVE 11 166 Chapter 5 Acids, Bases, and Acid-Base Reactions Web MoleculesSpecial Topic 5.1 tells how acids are formed in the earth™s atmosphere and how these acids can be damaging to our atmosphere.
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5.1 Acids 167SPECIAL TOPIC 5.1 Acid RainNormal rainwater is very slightly acidic due to several reactions between substances dissolved in the water and the water itself. For example, carbon dioxide, nitrogen dioxide, and sulfur trioxideŠall of which are natural components of airŠreact with water to form carbonic acid, nitric acid, and sulfuric acid.Nitrogen dioxide is produced in nature in many ways, including a reaction between the oxygen and nitrogen in the air during electrical storms.N2(g) O2(g) 2NO(g)2NO(g) O2(g) 2NO2(g)Sulfur dioxide also has natural sources, including the burning of sulfur-containing compounds in volcanic eruptions and forest ˛res. Sulfur dioxide is converted into sulfur trioxide, SO 3, by reaction with the nitrogen dioxide in the air, among other mechanisms. SO2(g) NO2(g) SO3(g) NO(g)We humans have added considerably to the levels of NO2(g) and SO2(g) in our air, causing a steady increase in the acidity of rain. Coal, for example, contains a signi˛cant amount of sulfur; when coal is burned, the sulfur is converted into sulfur dioxide, SO 2(g). ˚e sulfur dioxide is converted into sulfur trioxide, SO 3(g), in the air, and that compound dissolves in rainwater and becomes sulfuric acid, H2SO4(aq). As individuals, we also contribute to acid rain every time we drive a car around the block. When air, which contains nitrogen and oxygen, is heated in the cylinders of the car, the two gases combine to yield nitrogen monoxide, NO(g), which is then converted into nitrogen dioxide, NO2(g), in the air. ˚e NO 2 combines with water in rain to form nitric acid, HNO3(aq). ˚ere are many more H 3O ions in the rain falling in the Northeastern United States than would be expected without human contributions.˚e increased acidity of the rain leads to many problems. For example, the acids in acid rain react with the calcium carbonate in marble statues and buildings, causing them to dissolve. (Marble is compressed limestone, which is composed of calcium carbonate, CaCO3(s).)CaCO3(s) 2HNO3(aq) Ca(NO3)2(aq) CO2(g) H2O(l)A similar reaction allows a plumber to remove the calcium carbonate scale in your hot water pipes. If the pipes are washed in an acidic solution, the calcium carbonate dissolves. The statues on the left were transported by William Randolph Hearst to his home in San Simeon, California. Because it so rarely rains there, and because San Simeon is far from any major sources of pollution, these statues are in much better condition than the similar statues found elsewhere, such as the one on the right, that have been damaged by acid rain.
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5.2 Acid NomenclatureBefore exploring how di˜erent kinds of acids react with compounds other than water, you need a little more familiarity with their names and formulas. Remember that the names of Arrhenius acids usually end in acid (hydrochloric acid, sulfuric acid, nitric acid) and that their formulas ˛t one of two general patterns:HX(aq) X = F, Cl, Br, or I HaXbOc For example, HCl( aq) (hydrochloric acid), H 2SO4 (sulfuric acid), and HNO3 (nitric acid) represent acids. Names and Formulas of Binary Acids Binary acids are named by writing hydro followed by the root of the name of the halogen, then -ic, and ˛nally acid (Table 5.2): hydro(root)ic acid ˚e only exception to remember is that the fiofl in hydro is left o˜ for HI( aq), so its name is hydriodic acid (an acid used to make pharmaceuticals). Most chemists refer to pure HCl gas as hydrogen chloride, but when HCl gas is dissolved in water, HCl( aq), the solution is called hydrochloric acid. We will follow the same rule in this text, calling HCl or HCl( g) hydrogen chloride and calling HCl( aq) hydrochloric acid. ˚e same pattern holds for the other binary acids as well. You will be expected to be able to write formulas and names for the binary acids found on Table 5.2. Remember that it is a good habit to write ( aq) after the formula.Table 5.2 Arrhenius AcidsFormula Named as Binary Covalent Compound Acid Formula Named as Binary acid HF or HF(g)hydrogen mono˝uoride or hydrogen ˝uoride HF(aq)hydro˝uoric acid HCl or HCl(g)hydrogen monochloride or hydrogen chloride HCl(aq)hydrochloric acid HBr or HBr( g)hydrogen monobromide or hydrogen bromide HBr( aq)hydrobromic acid HI or HI(g)hydrogen moniodide or hydrogen iodide HI(aq)hydriodic acid OBJECTIVE 12OBJECTIVE 12168 Chapter 5 Acids, Bases, and Acid-Base Reactions
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5.2 Acid Nomenclature 169Names and Formulas of Oxyacids To name oxyacids, you must ˛rst be able to recognize them by the general formula HaXbOc, with X representing an element other than hydrogen or oxygen (Section 5.1). It will also be useful for you to know the names of the polyatomic oxyanions (Table 3.6), because many oxyacid names are derived from them. If enough H ions are added to a (root)ate polyatomic ion to completely neutralize its charge, the (root)ic acid is formed (Table 5.3). If one H ion is added to nitrate, NO3, nitric acid, HNO3, is formed. If two H ions are added to sulf ate, SO42, sulfuric acid, H2SO4, is formed. If three H ions are added to phosph ate, PO43, phosphoric acid, H3PO4, is formed. Note that the whole name for sulfur, not just the root, sulf-, is found in the name sulfuric acid. Similarly, although the usual root for phosphorus is phosph-, the root phosphor- is used for phosphorus -containing oxyacids, as in the name phosphoric acid.Table 5.3 Relationship Between (Root)ate Polyatomic Ions and (Root)ic Acids Oxyanion Formula Oxyanion Name Oxyacid Formula Oxyacid Name NO3nitrateHNO3nitric acidC2H3O2acetateHC2H3O2acetic acidSO42sulfateH2SO4sulfuric acid (Note that the whole name sulfur is used in the oxyacid name.) CO32carbonateH2CO3carbonic acidPO43phosphateH3PO4phosphoric acid (Note that the root of phosphorus in an oxyacid name is phosphor-.)˜ere is a more complete description of acid nomenclature at the textbook™s Web site. OBJECTIVE 12OBJECTIVE 12
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