Introduction to Chapter 25

Transcription

1 8 Water and Solutions Introduction to Chapter 25 This chapter explores the chemistry of acids and bases and explains the ph scale. In the first Investigation, you will create a ph scale using a natural indicator solution. You will learn about the applications of acid-base chemistry in household products, medicine and the environment. In the second section and Investigation, you will learn about and simulate the effects of acid rain. Investigations to Chapter Acids, Bases, and ph Chapter 25 Acids and Bases What is ph? In this Investigation, a natural indicator and household chemicals are used to create a color-based ph scale. You will use your ph scale to figure out the ph of additional household chemicals and two mystery solutions Acid Rain What is acid rain? In this Investigation, you will model the effects of acid rain on a natural ecosystem. You will observe the effect of different dilutions of an acid on the activity of water fleas (Daphnia magna). Water fleas are an important source of food for fish and other organisms in fresh water environments. In this Investigation, you will also have the opportunity to research topics related to acid rain on the Internet. 427

2 : Acids and Bases Learning Goals In this chapter, you will: Describe ph as a way to measure the strength of acids and bases. Understand the definitions of an acid and a base. Identify ph of common household chemicals. Understand the cause of acid rain. Understand the environmental effects of acid rain. Demonstrate the effect of acid on a natural ecosystem. Vocabulary acid base ph smog acid precipitation electrolytes ph indicator acid rain neutral ph scale 428

3 25.1 Acids, Bases, and ph In the Investigations, you measured several indicators of water quality, including the ph. You may remember that ph values range from 0 to 14. Low values (0 to 6) indicate that a solution is acidic whereas high values (8 to 14) indicate that a solution is basic. Your stomach secretes one of the stronger acids, hydrochloric acid (HCl). As acidic solutions of digested food leave your stomach, other organs in your digestive system secrete bicarbonate, a base. The added base neutralizes the strong, corrosive acid. As you can see, acids and bases play a significant role in how your body works. What exactly are acids and bases? What does ph mean? Why is ph important? The ph of water indicates its quality Acids and bases are defined by ph values Water quality is evaluated using ph values for many reasons. For example, if the ph of your tap water is too high, it might indicate that calcium or magnesium deposits are forming in and may clog your water pipes. On the other hand, if the ph is too low, the water may be corroding your pipes. The ph of water is important to life. The ph of natural bodies of water also has to be just right neither too high nor too low. For example, at a ph of 4 or 5, fish have trouble reproducing. At even lower ph values, they die. The ph of water is related to the action of a class of chemicals called acids and bases. A solution with a ph value that is less than 7 contains an acid, and a solution with ph values greater than 7 contains a base. You will learn how ph is determined later on in this section. What are acids and bases? Acidic solutions have more H+ ions, and basic solutions have more OH- ions An acid is a chemical that contributes hydrogen ions, H+, to a solution. A base is a chemical that contributes hydroxyl ions, OH-, to a solution. Therefore, solutions can be described as acidic or basic according to the concentrations of H+ and OH- ions in the solution. A solution with a high concentration of H+ ions and few OH- ions is strongly acidic. A strongly basic (or alkaline) solution has a high concentration of OH- ions and few H+ ions. Figure 25.1: Acids and bases play important roles in digestion Acids, Bases, and ph 429

4 HCl is an acid NaOH is a base Weak acids and bases Hydrochloric acid (HCl) is a very strong acid which can seriously irritate your nose if you smell it and your skin if it spills on you! When HCl dissolves in water (as indicated by the symbol aq ), it ionizes to become H+ ions and Cl- ions: An acid such as HCl ionizes almost completely when dissolved in water and therefore contributes many H+ ions to a solution. Because of this, HCl is a known as a strong acid. Sodium hydroxide (NaOH) is a base commonly used for rigorous housecleaning and for unclogging drains. As you can see from the chemical formula, NaOH will release OH- ions when it dissolves in water: Since NaOH ionizes almost completely when dissolved in water and contributes many OH- ions to a solution, it is known as a strong base. Many acids and bases do not ionize completely in water and are known as weak acids and bases. Vinegar, also known as acetic acid, is a weak acid because it contributes a few H+ ions to solution. Vinegar is sometimes used for cleaning but is not very irritating to the skin. Your nose may not like it, however! Hydronium Ions In an acid solution, separate hydrogen ions (H+) do not exist by themselves for very long. Each hydrogen ion is attracted to the oxygen end of a water molecule. The two combine to form a hydronium ion. In chemistry, it is common to refer to H 3 O+ ions as H+ ions. A weak acid or base ionizes incompletely when dissolved in water as shown by the double arrows in the chemical equation above. Household ammonia is an example of a weak base because it does not completely ionize in water. 430

5 ph and the ph scale ph is based on the concentration of H+ ions in solution ph 7 means equal H+ and OH- ions Low ph means high H+ concentration The ph scale is based on powers of 10 Acids have more H+ ions, bases have more OHions Just as centimeters describe length, ph describes the exact concentrations of H+ ions in a solution. Most instruments that measure ph use a ph scale that runs from 0 to 14. At ph 1, a solution is strongly acidic. At ph 14, a solution is strongly basic, or alkaline. A solution that has a ph of 7 has equal numbers of H+ and OH- ions and is called a neutral solution. Everyone is familiar with using a ruler to measure things. However, a ph scale is different from a ruler in that the more H+ ions there are in a solution, the lower the ph value. Conversely, fewer H+ ions in a solution means a higher ph value. A ph scale is based on powers of ten. A solution that is ph 1 has 10 times more hydrogen ions than a solution of ph 2. In turn, the ph 2 solution has 10 times more hydrogen ions than a solution of ph 3. Another way of looking at the scale is to see that as the ph numbers increase, the amount of H+ ions decreases by powers of 10. Figure 25.2 illustrates the ph scale. For example, a solution at ph 1 has 10 million times more hydrogen ions than a solution at ph 7, and a solution at ph 7 has 10 million times more hydrogen ions than a solution at ph 14! You now know that the amount of H+ in a solution at ph 6 is 10 times the amount in a solution at ph 7. But a solution at ph 7 is neutral. What is the amount of OHions in a neutral solution? What is the amount of OH- ions in a solution at ph 6? The answers to these questions are that the amount of OH- ions in a neutral solution is equal to the amount of H+ ions. If you go down the ph scale to 6, the amount of OH- ions decreases by a power of 10 whereas the amount of H+ increases by a power of 10. Remember: ph values below 7 means H+ ions outnumber OH- ions, and ph values higher than 7 means OH- ions outnumber H+ ions. Since each ph is an increase or decrease by tenfold, there is a big difference in ion concentration between each ph value. Figure 25.2: The ph scale is based on the concentration of hydrogen ions in solution Acids, Bases, and ph 431

6 The ionization of water Water is both a weak acid and a weak base What does the double-headed arrow mean? Water plays an important role in acid and base chemistry. Water itself is both a weak acid and a weak base. A water molecule can split or ionize into H+ and OHions. Because the H+ ions immediately attach themselves to water molecules, the equation for the ionization of water is written as: 2H 2 O H 3 O + + OH- The double-headed arrow in the equation means that water, if left alone, is always in equilibrium with its ions. The rate at which water breaks down into ions is the same as the rate at which the ions re-form into water. However, water ionizes so slowly that most water molecules exist whole, not as ions. Figure 25.3:.Water ionizes into H+ and OH- ions. Therefore, it acts like both a weak acid and a weak base. Pure water has a neutral ph (ph 7) because the concentrations of H+ and OH- ions are equal. Examples of acid and base chemistry 432 Acids and bases play a role in digestion Acids and bases have many uses Many reactions, such as the ones that occur in your body, work best at specific ph values. For example, acids and bases are very important in the reactions involved in digesting food (see figure 25.1). As you may know, the stomach secretes hydrochloric acid (HCl), a strong acid (ph 1.4). The acidity of our stomachs is necessary to break down the protein molecules in our food so that they can be absorbed. A mucus lining in the stomach protects it from the acid produced. As food and digestive fluids leave the stomach, however, other organs in the digestive system also need to be protected from the acid. This is accomplished by two parts of the system the pancreas and liver. These two organs secrete bicarbonate to neutralize the stomach acid before it reaches other organs. The reactivity of acids and bases means that these chemicals have many uses. They react with each other and other chemicals. Metals and glass can be etched with corrosive acids. Lye (ph 13) is used to unclog drains, and sulfuric acid (ph 1) is used throughout industry. Millions of tons of sulfuric acid are produced each year (40 million tons in 1990). Its uses range from making rayon to cleaning impurities from gasoline. Figure 25.4: Carbon dioxide gas (CO 2 ) escaping from carbonated water. The CO 2 molecules are chains of three atoms, and the water molecules have a triangular shape. CO 2 makes soda fizzy and reacts with water to form carbonic acid, which gives the soda a sharp, or acidic, taste. This and other acids in soda, like phosphoric acid, can dissolve tooth enamel. Be sure to brush your teeth after drinking soda!

7 Electrolytes and nonelectrolytes Electrolytes conduct current Salt dissociates in water Acids and bases are electrolytes Non-electrolytes do not have ions and are not acidic or basic Current is the flow of charge. When a solution contains dissolved ions (charged particles), it can conduct current. Chemicals that conduct current when dissolved in water are called electrolytes. These chemicals form ions when dissolved. Ionic compounds, molecular compounds, and even atoms can contribute ions to a solution. When an ionic compound is dissolved in water, the polar ends of the water molecule attract the positive and negative ions in the solution. In previous sections, you learned how water dissolves table salt (NaCl). Recall that the attraction of the polar ends of the water molecule is strong enough to break the weak ionic bonds of NaCl. When an ionic compound is brought into solution by water it is said to dissociate. The term ionization is used if a molecular compound or atom forms an ion. For example, when NaCl dissolves in water we say it dissociates. When the element Na loses an electron, it ionizes to Na+. All acids and bases are electrolytes because they contribute ions to a solution. Some chemicals, like salt (NaCl), dissociate to form ions in solution but are not acidic or basic. Acids, bases, and salt water are examples of electrolytes. Other chemicals do not form ions when they are dissolved in solution. They are called non-electrolytes. Non-electrolytes are not acidic or basic. Sugar dissolved in water is an example of a non-electrolyte. Electrical appliances and water Because tap water contains small amounts of dissolved ions, it is an electrolyte. Remember that even a small amount of current is dangerous if it enters your body directly? Water provides a way for electric current to enter your body, so always take care when using electrical appliances near water! Electrolytes and your body When you perform a strenuous activity, your body cools itself by sweating. Sweat contains water and dissolved salts (or electrolytes) like sodium and potassium. Before, during, and after exercising, you can replenish fluids and your body s electrolytes by drinking diluted fruit juice, slightly salty water, or by consuming a sports drinks. The water in these fluids helps your body continue to cool itself so that you don t get overheated. By replacing electrolytes, you may be helping your body speed up resorption of fluids. Diluted fruit juice or a sports drink contains small amounts of carbohydrates to give your body the energy boost it may need during strenuous exercise Acids, Bases, and ph 433

8 The ph of substances you use or consume Many foods are acidic and many cleaning products are basic Table 25.1 contains a list of some common chemicals and their ph values. What do you notice about this list of substances? Where would you find acids in your kitchen? Where would you find bases? Table 25.1: The ph of some common chemicals. Household chemical Acid or base ph lemon juice acid 2 vinegar acid 3 soda water acid 4 baking soda base 8.5 bar soap base 10 ammonia base 11 It turns out that many of the foods we consume or use for cooking are acidic. On the other hand, many of our household cleaning products are basic. A ph indicator In the Investigation, you will be testing the ph of common chemicals using another item that you may find in your kitchen. You will measure ph using a ph indicator a chemical that changes color at different ph values. Common foods can also be used as ph indicators. In the Investigation, the indicator you will use is made from the juice of a red cabbage. Acids, bases, and taste Our taste buds are sensitive to acids and bases. We taste acids as sour and bases as bitter. Lemon juice is strongly acidic, and soap is strongly basic. Acids that are stronger than lemon juice and bases that are stronger than ammonia are so reactive that they can harm your skin and damage clothing. Figure 25.5: The ph scale showing common substances. 434

9 Health Perspective: ph and your blood It is very important for your blood ph to stay within the normal range. At higher or lower ph values, your body does not function properly. Fortunately, you can regulate the ph of your blood simply by breathing! Blood is a watery solution that contains many solutes including the dissolved gases carbon dioxide and oxygen. Carbon dioxide appears in your blood because it is produced by respiration. Recall that respiration is the combustion of sugar by your body. You breathe in oxygen to get this process going. The end products of this reaction are energy, water, and carbon dioxide. C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O + energy The rate at which you breathe controls the concentration of carbon dioxide in your blood. For example, if you hold your breath, more carbon dioxide enters your blood. If you hyperventilate, you blow off carbon dioxide, so that significantly less is in your blood. These two processes influence blood ph. The equation below illustrates how carbon dioxide dissolves in an aqueous (watery) solution like blood: CO 2 + H 2 O H 2 CO 3 H + + HCO - 3 carbon dioxide + water carbonic acid hydrogen ion + bicarbonate ion When CO 2 dissolves, H+ ions are produced in solution. Therefore, the more CO 2 in your blood, the more acidic your blood will become. If you breathe slowly, the added CO 2 makes your blood more acidic. However, if you breathe too often and too quickly (hyperventilating), the loss of CO 2 makes your blood more basic. You can offset this effect by breathing into a paper bag. This forces you to re-breathe carbon dioxide. When you breathe normally, your blood ph ranges between 7.35 and Figure 25.6: Some causes of low blood ph (acidosis) include holding your breath, excessive lactic acid produced during heavy exercise, and production of ketone acids because of fasting or having low levels of insulin (diabetes). High blood ph (alkalosis) can be caused by hyperventilating. Under normal conditions, your blood ph ranges between 7.35 and Acids, Bases, and ph 435

10 25.2 Acid Rain You have probably heard about acid rain. You might know that acid rain harms the environment. But what is acid rain? Life occurs within certain ranges of ph. Most plants and animals function best when ph is nearly neutral. At the extreme ends of the ph scale, where you find strong acids and strong bases, many life processes will not occur. For example, fish have trouble reproducing when the ph of their watery environment is too acidic. Also, plants have trouble growing if soil is acidified below 5.1, and aquatic animals have difficulty surviving in water below ph 5.5. It was recently discovered that lakes and forests in certain parts of the world were not as healthy as they once had been. The cause was higher acidity in the lakes, resulting from rainfall, snowfall, or fog that was highly acidic. How can rain or snow be harmful to the environment? The definition of acid rain Rain and snow are naturally acidic 436 Acid rain results from gases produced during combustion reactions Rainfall, snowfall, or any precipitation happens to be naturally acidic. This is because the rain, snow, or fog mixes with carbon dioxide, a gas present in air, and forms small amounts of carbonic acid. The ph of precipitation before the Industrial Revolution (which started in the mid-1700 s) was probably around 5.6. Therefore, the normal ph of rain and other forms of precipitation is considered to be 5.6. Any rain, snow, or fog that has a ph lower than 5.6 is called acid rain or acid precipitation. The chief indicators of acid rain in the atmosphere are the gases sulfur dioxide (SO 2 ), sulfur trioxide (SO 3 ), nitrogen oxide (NO), and nitrogen dioxide (NO 2 ). A large part of SO 2 in the atmosphere is created by the burning of coal and oil that contain sulfur, and by industrial processes such as metal purification. Nitrogen oxide is a leading by-product of fuel combustion from traffic and power plants. Sulfur trioxide and nitrogen dioxide are created from the reactions of sulfur dioxide and nitrogen oxide, respectively, with oxygen-containing compounds in the atmosphere. Both of these reactions speed up due to chemicals present in particulate matter, a common pollutant released by traffic and industrial processes. Figure 25.7: If a body of water has a ph of 4.5 or lower, almost nothing can grow in it. Other animals that live near the water, such as frogs and insects, often die as well. Figure 25.8: Ideally, fish require water within the 6-8 ph range.

11 Sulfur and nitrogen gases mix with water to form sulfuric and nitric acids Sulfur oxides in the air mix with water, ozone, or hydrogen peroxide to form sulfuric acid (H 2 SO 4 ). This strong acid is the number one cause of acid rain. Even before acid rain is formed, the four gases that cause acid rain (SO 2, SO 3, NO, and NO 2 ) create health problems in cities. They are irritants to the respiratory system and may increase the incidence of asthma and other respiratory ailments. Nitrogen oxides react with components in the atmosphere to form nitric acid (HNO 3 ), the second greatest cause of acid rain. Nitric and sulfuric oxides are produced by cars and industry. These airborne chemical react with water to form nitric and sulfuric acids. Weather patterns (i.e., wind) can transport these acids to other areas where they will fall as precipitation. Acid precipitation can affect the health of people, trees, and aquatic life and erodes buildings and statues. Figure 25.9: You can help reduce the emission of acid rain gases by using these alternative means of transportation Acid Rain 437

12 Review Chapter 25 Review Vocabulary review Match the following terms with the correct definition. There is one extra definition in the list that will not match any of the terms. Set One 1. acid a. A chemical that contributes H+ ions to a solution Set Two 1. acid rain a. The naturally occurring rock that protects some areas of the country from acid rain 2. base b. A solution that has no H+ or OH- ions 2. acid precipitation b. Rain with a ph lower than ph c. A solution that has equal numbers of H+ and OH- ions 4. neutral d. A chemical that conducts current when dissolved in a solution 5. electrolyte e. The measurement of the amount of H+ and OH- ions in a solution f. A chemical that contributes OH- ions to a solution 3. smog c. The process of turning an acidic or basic solution into a neutral (ph 7) solution 4. acid shock d. A sudden increase of acid precipitation into a lake or other ecosystem 5. neutralization e. A combination of gases produced by industry and automobiles; a combination of smoke and fog f. Rain, snow, or fog with a ph lower than