Acids, Bases, and ph

Transcription

1 CHAPTER 9 1 SECTION Acids, Bases, and Salts Acids, Bases, and ph KEY IDEAS As you read this section, keep these questions in mind: What properties do acids have? What properties do bases have? How can ph tell you if something is an acid or a base? What Are the Properties of Acids? Does the thought of eating a lemon cause your mouth to pucker? You expect a lemon to taste sour. All the foods in the figure below taste sour because they contain acids. READING TOOLBOX Outline Make an outline of the material in this section. Use the headers in the section to help you organize your outline. Citrus fruits, such as lemons, grapefruits, limes, and oranges, contain citric acid. Apples contain malic acid. Grapes contain tartaric acid. When acids dissolve in water, they ionize, or form ions. When acids ionize, they form hydrogen ions, H. The hydrogen ions bond to water molecules, H 2 O, to form hydronium ions, H 3 O.Anacid is a compound that increases the number of hydronium ions when it dissolves in water. Hydronium ions give acids their properties. Some properties of acids are listed below. sour taste conduct electricity turn blue litmus paper red corrosive (able to destroy or eat away certain materials) 1. Define What is an acid? Interactive Reader 181 Acids, Bases, and Salts

2 2. Identify What determines whether an acid is strong or weak? 3. Identify What happens to the total charge of the solution when HNO 3 dissolves? What Is the Difference Between Strong and Weak Acids? All acids ionize when they are dissolved in water. However, some acids ionize completely, while others do not. The relative numbers of acid molecules that ionize in water determine if the acid is strong or weak. STRONG ACIDS The figure below shows what happens to nitric acid when it is dissolved in water. The arrow pointing to the right shows that all of the nitric acid ionizes. Nitric acid is a strong acid. When strong acids are dissolved in water, they ionize completely. This means that all the dissolved acid molecules break apart into ions. Nitric acid Water Hydronium ion Nitrate ion _ HNO 3 H 2 O H 3 O NO 3 4. Define What is an electrolyte? When nitric acid ionizes, it forms hydronium ions and nitrate ions. The ions move around freely in the water and can conduct electricity. A substance that conducts electricity when it is dissolved in water is called an electrolyte. Strong acids are strong electrolytes. This is because solutions of these acids contain as many hydronium ions as the acid can possibly form. WEAK ACIDS Weak acids, such as acetic acid, do not completely ionize. Only some of the molecules of acid ionize in water. Examine the figure at the top of the next page. The arrow pointing to the right shows that dissolved acetic acid molecules break apart into ions. The arrow pointing to the left shows that ions are also recombining to form molecules. Therefore, a solution of a weak acid contains fewer hydronium ions than a solution of a strong acid with the same concentration. Interactive Reader 182 Acids, Bases, and Salts

3 Acetic acid Water Hydronium ion Acetate ion _ CH 3 COOH H 2 O H 3 O CH 3 COO Because there are few ions in a solution of a weak acid, the solution does not conduct electricity very well. Thus, weak acids are weak electrolytes. 5. Infer Which of the two beakers contains more ions? How do you know? 6. Identify Which beaker contains a strong acid? Explain your answer. Both of these beakers contain acids. One contains a weak acid and one contains a strong acid. ACID DANGERS Although many products you use every day contain acids, strong and concentrated acids can be very dangerous. For example, your stomach makes a very strong acid called hydrochloric acid. You need this acid to help you digest food. Normally, your stomach has a thick lining that protects it from the strong acid. However, if your body makes too much acid, the acid can destroy your stomach. Even weak acids are not always safe to handle. Acids can damage your skin. The vapors are also harmful to your eyes, lungs, and mouth. This is why you should always wear goggles, gloves, and a laboratory apron when you work with acids. Never taste a chemical to determine if it is an acid. 7. List Identify three pieces of safety equipment you should use when you work with an acid. Interactive Reader 183 Acids, Bases, and Salts

4 Some Common Acids Acid Formula Strength Uses Hydrochloric acid HCl strong Cleaning masonry; treating metals before plating or painting; adjusting ph of swimming pools Sulfuric acid H 2 SO 4 strong making fertilizers; the acid in car batteries Nitric acid HNO 3 strong making fertilizers and explosives 8. Identify Which common acid is found in car batteries? 9. Identify Is the acid in vinegar strong or weak? Acetic acid CH 3 COOH weak making chemicals, plastics, and medicines; the acid in vinegar Formic acid HCOOH weak dying cloth; the acid used by stinging ants Citric acid H 3 C 6 H 5 O 7 weak making flavorings and soft drinks; the acid in citrus fruits All these products contain acids. 10. Identify What kind of ions do bases form when they are dissolved in water? What Are the Properties of Bases? Baking soda is an example of a common base. It is used to make cakes and cookies and may be found in toothpastes and cleaning supplies. A base is a compound that forms hydroxide ions, OH, when it is dissolved in water. Hydroxide ions give bases their properties. Some of these properties are listed below. bitter taste feel slippery conduct electricity turn red litmus paper blue corrosive Interactive Reader 184 Acids, Bases, and Salts

5 STRONG BASES When strong bases are dissolved in water, they ionize completely. Most strong bases are made up of metal ions and hydroxide ions. These kinds of bases are called metal hydroxides. Sodium hydroxide is an example of a metal hydroxide. Some drain cleaners contain sodium hydroxide. Sodium hydroxide completely ionizes when it is put into water. Sodium hydroxide and other strong bases are strong electrolytes. That is, they conduct electricity well when they are dissolved in water. WEAK BASES Ammonia is an example of a weak base. Ammonia, NH 3, does not contain hydroxide ions. Where do the oxygen and hydrogen that make up the hydroxide ion come from? Ammonia reacts with water to form OH. This reaction is shown in the figure below. Ammonia Water Ammonium ion Hydroxide ion _ 11. Compare In terms of ionization, how are strong acids and strong bases similar? 12. Explain How do bases that do not contain hydroxide ions form hydroxide ions? NH 3 H 2 O NH 4 OH The double arrow tells you that ammonia does not ionize completely when it is dissolved in water. Therefore, ammonia is a weak base and a weak electrolyte. DANGERS OF BASES The table on the next page gives examples of common bases. Many bases are used in soaps, dyes, and fertilizer. However, concentrated bases can be very dangerous. They may dissolve many materials, including skin. When you work with bases, you should wear safety goggles, gloves, and a laboratory apron to protect yourself. Interactive Reader 185 Acids, Bases, and Salts

6 All these products contain bases. Some Common Bases Base Formula Strength Uses 13. Identify Identify two common bases used in making soap. Potassium hydroxide Sodium hydroxide Calcium hydroxide KOH strong making soap; dyeing products NaOH strong making soap; refining petroleum; cleaning drains; making synthetic fibers Ca(OH) 2 strong treating acid soil; treating lakes polluted by acid rain; making mortar, plaster, and cement Ammonia NH 3 weak fertilizing soil; making fertilizers; making nitric acid; making cleaning solutions Methylamine CH 3 NH 2 weak making dyes and medicines; tanning leather Aniline C 6 H 5 NH 2 weak making dyes and varnishes; used as a solvent What Is the Difference Between Strong and Concentrated? Many people think the words concentrated and strong mean the same thing. Similarly, many people think that weak and dilute mean the same thing. In chemistry, however, these words have different meanings. Recall that the concentration of a solution refers to the number of solute particles in a particular amount of solution. A solution with a large number of solute particles is concentrated. A solution with a small number of solute particles is dilute. Interactive Reader 186 Acids, Bases, and Salts

7 STRONG AND DILUTE Suppose a scientist added a very small amount of hydrochloric acid to a liter of water. Because the solution contains only a small amount of solute, you could describe it as dilute. However, when any amount of hydrochloric acid is dissolved in water, it ionizes completely. At any concentration, hydrochloric acid is a strong acid. Thus, a strong acid may form a dilute solution. What Is ph? How can you tell if a solution is acidic or basic? One way is to use an indicator. An indicator is a substance that changes color if a solution is acidic or basic. Litmus paper and ph paper are examples of indicators. An indicator can tell you something is an acid or a base. However, in many cases, you need to know how acidic or basic a solution is. The ph of a solution is a value that expresses how acidic or basic a solution is. The ph scale ranges from 0 to 14. Solutions with a ph less than 7 are acids. Solutions with a ph greater than 7 are bases. Solutions with a ph of 7 are neutral. A neutral solution is not acidic or basic. Acidic: ph < 7 Basic: ph > 7 Neutral: ph = Infer Can a weak base form a concentrated solution? Explain your answer. 15. Identify Is a solution with a ph greater than 7 basic or acidic? Increasing acidity Increasing basicity Lemon juice Soft drink Milk Human saliva Sea water Tap water Detergents Household ammonia 16. Identify Is a soft drink more acidic or less acidic than milk? 17. Analyze Relationships What happens to acidity as ph increases? What happens to basicity as ph increases? Acid rain Clean rain Human stomach contents Interactive Reader 187 Acids, Bases, and Salts

8 18. Describe Describe the relative concentrations of hydronium and hydroxide ions in an acidic solution. RELATIVE CONCENTRATIONS OF IONS The ph of a solution indicates the concentration of hydronium ions. An acidic solution has a greater concentration of hydronium ions than hydroxide ions. A basic solution has a greater concentration of hydroxide ions than hydronium ions. A neutral solution has equal concentrations of hydronium ions and hydroxide ions. CONCENTRATION AND ph Recall that you can use molarity (M) to describe the concentration of a substance in a solution. You can use the concentration of hydronium ions in a solution to determine the solution s ph. In a solution, strong acids like HCl and HNO 3 produce one hydronium ion for each dissolved acid particle. The concentration of a strong acid indicates how many particles are in the solution. Because every particle of a strong acid that dissolves will ionize, the hydronium ion concentration equals the acid concentration. Writing the H 3 O concentration in scientific notation can help you determine ph. When the concentration of hydronium ions is one times ten to a power, the ph is the negative of the power of 10. For example, pure water has a hydronium ion concentration of mol/l, or M. Thus, pure water has a ph of 7. CALCULATING ph A hydrochloric acid solution has a H 3 O concentration of M. What is the ph of the solution? Math Skills 19. Calculate HNO 3 is a strong acid. What is the ph of a 0.01 M solution of HNO 3? Step 1: List the given and unknown values. Step 2: Write the molar concentration of hydroxide ions in scientific notation. Step 3: The ph is the negative of the power of 10 in the H 3 O concentration. Given: concentration of HCl = M concentration of H 3 O = M = M Unknown: ph concentration of H 3 O ions = M ph = -(-4) = 4 The solution has a ph of 4. Interactive Reader 188 Acids, Bases, and Salts

9 INTERPRETING ph DIFFERENCES As you have seen, ph is based on powers of 10. Because of this, small differences in ph reflect big differences in hydronium concentrations. For example, the ph of apple juice is about 3. The ph of coffee is about 5. The difference in ph of the two liquids seems small. However, the difference of two ph units is actually quite large. Because each ph unit represents a power of 10, a ph difference of two represents a difference in acidity of 10 2, or 100. Thus, apple juice is 100 times more acidic than coffee. Antacid tablets are basic. When antacid tablets are dissolved in water, the solution they form has a ph of about 8. Thus, coffee is 10 3, or 1,000 times, as acidic as an antacid tablet solution. Math Skills 20. Calculate Solution A has a ph of 9. Solution B has a ph of 3. How much more acidic is solution B than solution A? MEASURING ph Indicators such as litmus paper do not measure ph very precisely. To measure ph precisely, you can use a ph meter. Because ions in a solution have electric charge, they can conduct electric current. A ph meter measures ph by determining the electric current created by the movement of ions. 21. Apply Concepts Is the tomato acidic or basic? Explain your answer. A ph meter can measure the H 3 O concentration precisely. 22. Infer How is the ph value shown on the ph meter more precise than ph values you have seen so far? Interactive Reader 189 Acids, Bases, and Salts

10 Section 1 Review SECTION VOCABULARY acid any compound that increases the number of hydronium ions when dissolved in water base any compound that increases the number of hydroxide ions when dissolved in water electrolyte a substance that dissolves in water to give a solution that conducts an electric current indicator a compound that can reversibly change color depending on conditions such as ph ph a value that is used to express the acidity or basicity of a system; each whole number on the scale 1. Write Equations Write an equation showing the ionization of hydrochloric acid in water. 2. Explain How do a strong acid and a weak acid behave differently when each is dissolved in water? 3. Identify Relationships What is the relationship between the strength of an acid or base and the strength of the electrolyte that it forms? 4. Classify Suppose you have a solution that has 4 times as many hydronium ions as hydroxide ions. Is this solution acidic, basic, or neutral? 5. Arrange Arrange the following substances in order of increasing acidity: vinegar (ph = 2.8), stomach acid (ph = 2.0), and a soft drink (ph = 3.4). 6. Graph Trends Create two graphs to describe how ph changes as acidity and basicity increase. Be sure to label the axes of your graphs. Interactive Reader 190 Acids, Bases, and Salts