H O. (hydrogen ion) transfers from one water molecule to another water molecule. The result is one hydronium ion (H 3 O ) and one hydroxide ion (OH ).

Transcription

1 Hydrogen Ions and Acidity 1 FOCUS Objectives Describe how [H + ] and [OH] are related in an aqueous solution Classify a solution as neutral, acidic, or basic given the hydrogen-ion or hydroxide-ion concentration Convert hydrogen-ion concentrations into ph values and hydroxide-ion concentrations into poh values Describe the purpose of an acid-base indicator. Guide for Reading Build Vocabulary Graphic Organizers Students can compare and contrast the properties of a neutral solution, an acid solution, and a basic solution using a table. Reading Strategy Identify Main Idea/Details Have students write down the titles of the red heads in the section. Have them follow each title with a sentence that expresses the section s main idea. Guide for Reading Key Concepts How are [H ] and [OH ] related in an aqueous solution? How is the hydrogen-ion concentration used to classify a solution as neutral, acidic, or basic? What is the most important characteristic of an acid-base indicator? Vocabulary self-ionization neutral solution ion-product constant for water (K w ) acidic solution basic solution alkaline solutions ph Reading Strategy Relating Text and Visuals As you read about ph, look carefully at the diagram in Figure Make sure that you can explain why the differences in [H ] and [OH ] exist in acidic, neutral, and basic solutions. A patient is brought to a hospital unconscious and with a fruity odor on his breath. The doctor suspects the patient has fallen into a diabetic coma. To confirm her diagnosis, she orders several tests, including one of the acidity of the patient s blood. The results from this test will be expressed in units of ph, not molar concentration. In this section, you will learn how the ph scale is used to indicate the acidity of a solution and why the ph scale is used. Hydrogen Ions from Water As you already know, water molecules are highly polar and are in continuous motion, even at room temperature. Occasionally, the collisions between water molecules are energetic enough to transfer a hydrogen ion from one water molecule to another. A water molecule that loses a hydrogen ion becomes a negatively charged hydroxide ion (OH ). A water molecule that gains a hydrogen ion becomes a positively charged hydronium ion (H 3 O ). The reaction in which water molecules produce ions is called the selfionization of water. This reaction can be written as a simple dissociation. H 2 O1l 2 m H + 1 aq 2 + OH - 1aq 2 Hydrogen ion Hydroxide ion In water or aqueous solution, hydrogen ions (H ) are always joined to water molecules as hydronium ions (H 3 O ). Hydrogen ions in aqueous solution have several names. Some chemists call them protons. Others prefer to call them hydrogen ions or hydronium ions. In this textbook, either H or H 3 O is used to represent hydrogen ions in aqueous solution. Figure 19.7 shows how two water molecules react to form one hydronium ion and one hydroxide ion. 2 INSTRUCT Have students study the photograph and read the text that opens the section. Ask, What scale is commonly used to express the acidity of a solution? (ph scale) Hydrogen Ions from Water Use Visuals L1 Figure 19.7 Ask students to examine the figure. Illustrate the reaction on the board: Show how two water molecules can react to yield a hydronium ion and a hydroxide ion. Ask, Which element is donating a pair of electons? (oxygen) Which element is accepting a pair of electrons? (hydrogen) Figure 19.7 The selfionization of water. A proton (hydrogen ion) transfers from one water molecule to another water molecule. The result is one hydronium ion (H 3 O ) and one hydroxide ion (OH ). 594 Chapter 19 Section Resources H O H Water molecule Print Guided Reading and Study Workbook, Section 19.2 Core Teaching Resources, Section 19.2 Review Transparencies, T215 T222 Laboratory Manual, Lab 40 Small-Scale Chemistry Laboratory Manual, Lab 30 H O H Water molecule H O H H Technology Interactive Textbook with ChemASAP, Problem-Solving 19.10, 19.12, 19.14, 19.15, Assessment 19.2 GoOnline, Section 19.2 Hydronium ion H O H 2 O H 2 O H 3 O OH Hydroxide ion 594 Chapter 19

2 The self-ionization of water occurs to a very small extent. In pure water at 25 C, the equilibrium concentration of hydrogen ions ([H ]) and the equilibrium concentration of hydroxide ions ([OH ]) are each only M. This means that the concentrations of H and OH are equal in pure water. Any aqueous solution in which [H ] and [OH ] are equal is described as a neutral solution. Ion Product Constant for Water In any aqueous solution, when [H ] increases, [OH ] decreases. When [H ] decreases, [OH ] increases. Le Châtelier s principle, which you learned about in Chapter 18, applies here. If additional ions (either hydrogen ions or hydroxide ions) are added to a solution, the equilibrium shifts. The concentration of the other type of ion decreases. More water molecules are formed in the process. H (aq) OH (aq) H 2 O(l) For aqueous solutions, the product of the hydrogen-ion concentration and the hydroxide-ion concentration equals [H ] [OH ] This equation is true for all dilute aqueous solutions at 25 C. As you will see, the concentrations of H and OH may change when substances are added to water. However, the product of [H ] and [OH ] is always The product of the concentrations of the hydrogen ions and hydroxide ions in water is called the ion-product constant for water (K w ). K w [H ] [OH ] Figure 19.8 Acids and bases have many uses in the home and in industry. a Unrefined hydrochloric acid, commonly called muriatic acid, is used to clean stone buildings and swimming pools. b Sodium hydroxide, or lye, is commonly used as a drain cleaner. Predicting How will each chemical affect the hydrogenion and hydroxide-ion concentration of an aqueous solution? Ion Product Constant for Water CLASS Activity Using a ph Meter L1 Purpose Students observe how a ph meter measures the PH of solutions. Materials several test tubes; 0.1M HCl; 0.1M NaOH; household products such as lemon juice, vinegar, shampoo; and liquid detergent; ph meter Procedure Set up test tubes with acids and bases of varying strength. In addition to HCl and NaOH, include household products such as lemon juice, vinegar, shampoo, and liquid detergent. Use a ph meter to measure the ph of each solution. Explain that the reading on the ph meter depends upon the concentration of hydrogen ions in the solutions. Not all solutions are neutral. When some substances dissolve in water, they release hydrogen ions. For example, when hydrogen chloride dissolves in water, it forms hydrochloric acid. HCl1 g 2 H 2 O H + 1aq 2 + Cl - 1aq 2 In such a solution, the hydrogen-ion concentration is greater than the hydroxide-ion concentration. The hydroxide ions are present from the selfionization of water. An acidic solution is one in which [H ] is greater than [OH ]. The [H ] of an acidic solution is greater than M. When sodium hydroxide dissolves in water, it forms hydroxide ions in solution. a In such a solution, the hydrogen-ion concentration is less than the hydroxide-ion concentration. Remember, the hydrogen ions are present from the self-ionization of water. A basic solution is one in which [H ] is less than [OH ]. The [H ] of a basic solution is less than M. Basic solutions are also known as alkaline solutions. Some uses for acids and bases are shown in Figure Checkpoint NaOH1 s2 H 2 O Na + 1aq 2 + OH - 1 aq 2 What is a basic solution? b Section 19.2 Hydrogen Ions and Acidity 595 Differentiated Instruction English Learners Students who do not have a good background in scientific notation may have trouble understanding the relationship between hydrogen- and hydroxide-ion concentrations. Draw diagrams to help students visualize the shift in equilibrium that occurs when one or the other kind of ion is added or removed. Answers to... Figure 19.8 Hydrochloric acid will increase [H + ] and decrease [OH ]; sodium hydroxide will increase [OH ] and decrease [H + ]. Checkpoint A basic solution is one in which [H + ] is less than [OH ] Acids, Bases, and Salts 595

3 Section 19.2 (continued) Sample Problem 19.1 Answers 9. a. basic b. basic c. acidic d. neutral M; basic Plus The [OH - ] of a carbonated soft drink is M. What is the [H + ] of this solution? Is the solution acidic or basic? ( M; acidic) For a math refresher and practice, direct students to scientific notation, page R56. The ph Concept Discuss Pure water self-ionizes to form hydrogen and hydroxide ions, yet it does not conduct electric current well. Ask, Why does this phenomenon occur? (Pure water is a poor conductor because the concentrations of the ions are low.) For help with scientific notation, go to page R56. Problem-Solving Solve Problem 10 with the help of an interactive guided tutorial. withchemasap SAMPLE PROBLEM 19.1 Finding the [OH ] of a Solution Colas are slightly acidic. If the [H ] in a solution is M, is the solution acidic, basic, or neutral? What is the [OH ] of this solution? Analyze List the knowns and the unknowns. Knowns [H ] M Ion-product constant for water: K w [H ] [OH ] Unknowns solution acidic, basic, or neutral? [OH ]?M Calculate Solve for the unknowns. [H ] is M. Because this is greater than M, the solution is acidic. By definition K w [H ] [OH ]. Therefore, 3OH H + 4 Substituting the known numerical values, compute [OH ] as follows. 3OH M Evaluate Do the results make sense? If [H ] is greater than M, the [OH ] must be less than M. At M, [OH ] is less than M. 9. Classify each solution as acidic, basic, or neutral. a. [H ] M b. [OH ] M c. [H ] M d. [OH ] M The ph Concept Expressing hydrogen-ion concentration in molarity is cumbersome. A more widely used system for expressing [H ] is the ph scale, proposed in 1909 by the Danish scientist Søren Sørensen ( ). On the ph scale, which ranges from 0 to 14, neutral solutions have a ph of 7. A ph of 0 is strongly acidic. A solution with a ph of 14 is strongly basic. Calculating ph The ph of a solution is the negative logarithm of the hydrogen-ion concentration. The ph may be represented mathematically using the following equation. ph log[h ] K w 10. If the hydroxide-ion concentration of an aqueous solution is M, what is the [H ] in the solution? Is the solution acidic, basic, or neutral? 596 Chapter 19 Facts and Figures Ions in Water Explain to the students that all aqueous systems contain both hydrogen and hydroxide ions due to self-ionization of water. At 25 C, about 1 molecule of water out of will dissociate. This amounts to 1 g of hydrogen ions and 17 g of hydroxide ions in L of water. No matter how small the concentrations, both ions are always present in a water solution. The relationship between the concentrations of H + and OH - in aqueous solutions at constant temperature is similar to the relationship between the pressure and volume of a gas at constant temperature. In both cases, the relationship is inverse. The product of the two quantities is a constant as one quantity increases, the other decreases. 596 Chapter 19

4 In a neutral solution, the [H ] M. The ph of a neutral solution is 7. ph -log log 1 + log You can calculate the ph of a solution using the log function key on a calculator. (You can review finding the logarithm of a number on a calculator in the on page R78.) Figure 19.9 shows how the hydrogen-ion concentration of a solution is used to classify the solution as neutral acidic, or basic. A solution in which [H ] is greater than M has a ph less than 7.0 and is acidic. The ph of pure water or a neutral aqueous solution is 7.0. A solution with a ph greater than 7 is basic and has a [H ] of less than M. Acidic solution: ph < 7.0 [H ] greater than M Neutral solution: ph 7.0 [H ] equals M Basic solution: ph > 7.0 [H ] less than M Concentration [H 3 O ] and [OH ] in Acidic, Neutral, and Basic Solutions Greater than M M Less than M [H 3 O ] [OH ] [H 3 O ] [OH ] [H 3 O ] [OH ] Acidic Solution Neutral Solution Basic Solution The ph values of several common aqueous solutions are listed in Table The table also summarizes the relationship among [H ], [OH ], and ph. You may notice that ph can sometimes be read from the value of [H ]. If [H ] is written in scientific notation and has a coefficient of 1, then the ph of the solution equals the exponent, with the sign changed from minus to plus. For example, a solution with [H ] M has a ph of 2.0 and a solution with [H ] M has a ph of If the ph is an integer number, it is also possible to directly write the value of [H ]. A solution with a ph of 9.0 has a [H ] M. A ph of 4 indicates a [H ] of M. For: Links on ph Visit: Web Code: cdn 1192 Figure 19.9 The hydrogen-ion concentrate of a solution is used to classify the solution as acidic, neutral, or basic. INTERPRETING GRAPHS a. Identify What is [H 3 O ] in a neutral solution? b. Describe How does [H 3 O ] compare with [OH ] in an acidic solution? c. Compare and Contrast In terms of ion concentrations, how are basic solutions different from acidic solutions? Download a worksheet on ph for students to complete, and find additional teacher support from NSTA SciLinks. Use Visuals L1 Figure 19.9 Have students study the figure and the text that follows. Explain that the ph scale shows the relationship between ph and the hydrogenion concentration. Write the expression for the ion-product constant for water on the board, and remind students about the relationship between the concentrations of hydronium ion and hydroxide ion in an aqueous solution. Point out that the product of these concentrations in aqueous solutions is always at 25 C. Arbitrarily assign concentration values to each of the hydronium-ion bars in Figure 19.9 for the acidic and basic solutions (for the neutral solution, [H + ] is M); then have students calculate the ph of those solutions. Ask, What is [OH - ] for these solutions? (Have students refer to Table 19.5 to find examples of aqueous systems with these [H + ] and [OH - ].) Interpreting Graphs a. [H 3 O + ] = M b. [H 3 O + ] = [OH - ] c. basic: [OH - ] > [H 3 O + ] acidic: [H 3 O + ] > [OH - ] Section 19.2 Hydrogen Ions and Acidity 597 Differentiated Instruction Gifted and Talented L3 Have students identify occupations in which people need to measure ph. Interested students may select a local person in an occupation from the list and ask to interview that person about his or her use of ph measurement. Have students write up their interviews as if they were preparing them for a newspaper article. Acids, Bases, and Salts 597

5 Section 19.2 (continued) Discuss Introduce ph as a simpler way to express hydrogen-ion concentration. Point out that it is easier to say the ph of a solution is 3.00 than to say that the hydrogen ion concentration is equal to moles per liter. For students who have no concept of logarithms, explain that ph is found by taking the negative of the power (exponent) of 10 that expresses the hydrogen ion concentration. Show students this example: [H] = = M; ph = log 10 ( ) = Most ph values are positive numbers, but negative values are also possible. Assuming 100% ionization, for example, the ph of a 10M HCl solution is Discuss Have students compare the ph and poh of a solution by posing a series of questions such as the following: In an acidic solution with a ph of 3.25, are there any OH - ions? (Yes, there are always some OH - ions in an aqueous solution.) How do you know this? (The product of the hydrogen ion concentration and the hydroxide ion concentration must always equal K w.) What is the poh of this solution? (ph + poh = 14; poh = = 10.75) Table 19.5 Relationship among [H ], [OH ], and ph Increasing acidity Neutral Increasing basicity [H ] (mol/l) [OH ] (mol/l) ph Aqueous system Calculating poh The poh of a solution equals the negative logarithm of the hydroxide-ion concentration. poh log[oh ] M HCI 0.1M HCI Gastric juice Lemon juice Tomato juice Black coffee Milk Pure water Blood Sodium bicarbonate, sea water Milk of magnesia Household ammonia Washing soda 0.1M NaOH 1M NaOH A neutral solution has a poh of 7. A solution with a poh less than 7 is basic. A solution with a poh greater than 7 is acidic. A simple relationship between ph and poh allows you to find either one when the other is known. ph + poh 14 ph 14 - poh poh 14 - ph ph and Significant Figures For ph calculations, you should express the hydrogen-ion concentration in scientific notation. For example, a hydrogen-ion concentration of M, rewritten as M in scientific notation, has two significant figures. The ph of this solution is 3.00, with the two numbers to the right of the decimal point representing the two significant figures in the concentration. A solution with a ph of 3.00 is acidic, as shown in Figure ph Figure The ph scale shows the relationship between ph and the hydrogen-ion concentration. Interpreting Diagrams What happens to [H ] as ph increases? Increasing acidity Neutral Increasing basicity or 1 [H ] 598 Chapter Chapter 19

6 Most ph values are not whole numbers. For example, milk of magnesia has a ph of Using the definition of ph, this means that [H ] must equal M. Thus [H ] must be less than M (ph 10.0), but greater than M (ph 11.0). If [H ] is written in scientific notation but its coefficient is not 1, then you use a calculator with a log function key to calculate ph. SAMPLE PROBLEM 19.2 Calculating ph from [H ] What is the ph of a solution with a hydrogen-ion concentration of M? Analyze List the knowns and the unknown. Knowns [H ] M phlog[h ] 11. Find the ph of each solution. a. [H ] M b. [H ] M Unknown ph? Calculate Solve for the unknown. ph -log 3H + 4 -log log log Using a calculator, log ( ) ph (9.38) Evaluate Do the results make sense? The calculated ph is between 9 ([H ] M) and 10 ([H ] M). The ph is rounded to two decimal places because the hydrogen-ion concentration has two significant figures. 12. What are the ph values of the following solutions, based on their hydrogen-ion concentrations? a. [H ] M b. [H ] 0.045M You can calculate the hydrogen-ion concentration of a solution if you know the ph. For example, if the solution has a ph of 3.00, then [H ] M. When the ph is not a whole number, you will need a calculator with a y x function key to calculate the hydrogen-ion concentration. For example, if the ph is 3.70, the hydrogen-ion concentration is greater than M (ph 4.0) and less than M (ph 3.0). To get an accurate value, use a calculator as shown in the following example. Logarithms The common logarithm (log) of a number (N) is the exponent (x) to which the base 10 must be raised to yield the number. If N 10 x, then log N x. The use of logarithms allows a large range of values to be conveniently expressed as small, nonexponential numbers. For example, log and log Although there is a range of three orders of magnitude ( or 1000) between the numbers 10 3 and 10 6, the range of the log values is only 3. The concentration of hydrogen ions in most aqueous solutions, although small, can vary over many orders of magnitude. Scientists use ph, a logarithmic scale which ranges between 1 and 14, to more conveniently express the hydrogen ion concentration of a solution. For help calculating logarithms, go to page R78. Problem-Solving Solve Problem 12 with the help of an interactive guided tutorial. withchemasap Sample Problem 19.2 Answers 11. a. ph = log[h + ] = log ( ) = 4.0 b. ph = log[h + ] = log ( ) = a. ph = log[h + ] = log ( ) = b. ph = log[h + ] = log ( ) = 1.35 Plus What are the ph values of the following three solutions, based on their hydrogen ion concentrations? a. [H + ] = M (11.00) b. [H + ] = M (8.00) c. [H + ] = M (5.0) Logarithms The CHE feature will help students who are struggling with logarithms. In Section 19.2, focus on solutions of strong acids and bases. Save calculating dissociation constants for Section For a math refresher and practice, direct students to logarithms, page R78. Section 19.2 Hydrogen Ions and Acidity 599 Answers to... Figure [H + ] decreases Acids, Bases, and Salts 599

7 Section 19.2 (continued) Sample Problem 19.3 Answers 13. a. log[h + ] = 5.00; log[h + ] = 5.00; [H + ] = M b. log[h + ] = 12.83; log[h + ] = 12.83; [H + ] = M 14. a. log[h + ] = 4.00; log[h + ] = 4.00; [H + ] = M b. log[h + ] = 11.55; log[h + ] = 11.55; [H + ] = M Plus What are the hydroxide-ion concentrations for solutions with the following ph values? a ( M) b ( M) c ( M) A soft drink has a ph of What is the hydrogen-ion concentration in the drink? ( M) Measuring ph TEACHER Demo ph Indicators Purpose Students observe how ph indicators react to the acidity of their environment. Materials 0.1% solutions of thymol blue, methyl red, bromthymol blue, and phenolphthalein; test tubes; aqueous buffers spanning ph 4 to ph 10 (Either create a range of buffers spanning ph 4 to ph 10 or higher, or purchase standard buffers with different ph values. ) Procedure Explain that many natural and synthetic pigments are weak acids that change color with varying ph. Transfer a small volume of each buffer to a test tube. Label the tubes with their respective ph values. Add 5 drops of indicator to each test tube. Set up a separate rack of tubes for each indicator and ask students to infer the ph range for which each indicator is most suitable. Students should note the colors and ph values of the solutions in tubes. In addition, add 5 drops of the indicators to two or three tubes containing a solution with an unknown ph and have students estimate its ph value using the reference For help calculating logarithms, go to page R78. Problem-Solving Solve Problem 14 with the help of an interactive guided tutorial. withchemasap 600 Chapter 19 tubes. Ask, Why would an investigator prefer to use a ph meter to measure ph? (Individual indicators are only responsive to ph changes in narrow ranges. Many different indicators are needed to span the entire ph range. A ph meter allows an SAMPLE PROBLEM 19.3 Using ph to Find [H ] The ph of an unknown solution is What is its hydrogen-ion concentration? Analyze List the knowns and the unknown. Knowns Unknown ph 6.35 [H ]?M phlog[h ] Calculate Solve for the unknown. First, rearrange the equation for the definition of ph to solve for the unknown. log[h ] ph Next, substitute the value of ph. log[h ] 6.35 Change the signs on both sides of the equation. log[h ] 6.35 Finally, determine the number that has a log of 6.35, the antilog of 6.35, using the 10 x key on the calculator. The antilog of 6.35 is Therefore, [H ] M. Evaluate Do the results make sense? The hydrogen ion concentration must be between M (ph 6) and M (ph 7). The answer is rounded to two significant figures because the ph was measured to two decimal places. 13. Calculate [H ] for each solution. a. ph 5.00 b. ph What are the hydrogen-ion concentrations for solutions with the following ph values? a b Measuring ph People need to be able to measure the ph of the solutions they use. From maintaining the correct acid-base balance in a swimming pool, to creating soil conditions ideal for plant growth, to making medical diagnoses, ph measurements have valuable applications. For preliminary ph measurements and for small-volume samples, an indicator such as the one shown in Figure is often used. For precise and continuous measurements, a ph meter is preferred. Figure Acid-base indicators respond to ph changes over a specific range. Phenolphthalein changes from colorless to pink at ph 7 9. investigator to collect quantitative values on a continuous basis throughout the entire ph range. The precision and accuracy of ph meters are superior to standard indicators. ph meters can be calibrated and used reliably at different temperatures.) 600 Chapter 19

8 If you know the [OH ] of a solution, you can find its ph. The ion-product for water defines the relationship between [H ] and [OH ]. Therefore, you can determine [H ] by dividing K w by the known [OH ]. SAMPLE PROBLEM 19.4 Calculating ph from [OH ] What is the ph of a solution if [OH ] M? Analyze List the knowns and the unknown. Knowns Unknown [OH ] M ph? K w [OH ] [H ] phlog[h ] Calculate Solve for the unknown. To calculate ph, first calculate [H ] by using the definition of K w. With the value of [H ] determined, use the definition of ph to solve for the ph. A calculator indicates that log M , therefore Evaluate Do the results make sense? A solution in which [OH ] is less than M would be acidic because Practice [HProblems ] would be greater than M. The ph is expressed to 2 decimal places because [OH ] is expressed to 2 significant figures. K w 3OH - 4 3H + 4 3H + 4 K w 3OH M M 15. Calculate the ph of each solution. a. [OH ] M b. [OH ] M ph -log 3H + 4 -log ph Calculate the ph of each solution. a. [H ] M b. [H ] M Acid-Base Indicators An indicator (HIn) is an acid or a base that undergoes dissociation in a known ph range. An indicator is a valuable tool for measuring ph because its acid form and base form have different colors in solution. The following generalized equation represents the dissociation of an indicator (HIn). HIn 1aq 2 m OH- H + 1aq 2 + In - 1aq 2 H + Acid form Base form The acid form dominates the dissociation equilibrium at low ph (high [H ]), and the base form dominates the equilibrium at high ph (high [OH ]). Problem-Solving Solve Problem 15 with the help of an interactive guided tutorial. withchemasap Section 19.2 Hydrogen Ions and Acidity 601 greater than that of a solution with a ph of 4.0. Remind students that the lower the ph value, the more acidic the solution. Sample Problem 19.4 Answers 15. a. [H + ] = K w /[OH - ] = ( )/ ( ) = M; ph = log( ) = 9.63 b. [H + ] = K w /[OH - ] = ( )/ ( ) = M; ph = log( ) = a. ph = log( ) = 4.30 b. ph = log( ) = 9.08 Plus Calculate the ph for each solution a. [OH - ] = M (3.40) b. [OH - ] = M (6.81) c. [OH - ] = M (9.94) For a math refresher and practice, direct students to logarithms, page R78. TEACHER Demo Observing ph Change Purpose Students observe a color change resulting from a change in ph. Materials Lemon juice, tea, Alka- Seltzer tablet, glass of 10% grape juice, household ammonia Procedure Without explanation, add lemon juice to tea, or add an Alka-Seltzer tablet to a glass of 10% unsweetened grape juice, or a few drops of household ammonia to another glass of juice. Have the students try to explain the changes they observe. Expected Outcome Color changes occur when lemon juice, an Alka-Seltzer tablet, or ammonia are added. The pigments in tea and grape juice are weak acids that change color with varying ph. Relate Point out to students that the ph scale can be compared to the Richter scale, which measures the strength of earthquakes. On each scale, a change of one unit represents a tenfold change in the value being measured. On the Richter scale, for example, a tremor measuring 4.0 is ten times stronger than one measuring 3.0. On the ph scale, the hydrogen-ion concentration of a solution with a ph of 3.0 is ten times Acids, Bases, and Salts 601

9 Section 19.2 (continued) Interpreting Graphs a. thymol blue b. The indicators change color over a very limited range of ph. This allows for a more accurate reading of ph change. c. bromphenol blue or bromcresol green Enrichment Question L3 Which indicator(s) would best show a change in ph from 4.6 to 4.9? (Using both bromcresol green and methyl red would give the best results.) Figure Indicators change color at a different ph. INTERPRETING GRAPHS a. Identify Which indicator changes color in a solution with a ph of 2? b. Compare and Contrast What do you notice about the range over which each indicator changes color? c. Apply Concepts Which indicator would you choose to show that a solution has changed from ph 3 to ph 5? Figure You can find acidic and basic substances in your home. a Universal indicator solution has been added to solutions of known ph in the range from 1 to 12 to produce a set of reference colors. b Universal indicator has been added to samples of vinegar, soda water, and ammonia solution. Interpreting Photographs Use the reference colors to assign ph values to vinegar, soda water, and ammonia solution. Thymol blue Bromphenol blue Bromcresol green Methyl red Alizarin Bromthymol blue Phenol red Phenolphthalein Alizarin yellow R Color Ranges of Acid-Base Indicators ph For each indicator, the change from dominating acid form to dominating base form occurs in a narrow range of approximately two ph units. Within this range, the color of the solution is a mixture of the colors of the acid and the base forms. Knowing the ph range over which this color change occurs can give you a rough estimate of the ph of a solution. At all ph values below this range, you would see only the color of the acid form. At all ph values above this range, you would see only the color of the base form. You could get a more precise estimate of the ph of the solution by repeating the experiment with indicators that have different ph ranges for their color changes. Many different indicators are needed to span the entire ph spectrum. Figure shows the ph ranges of some commonly used indicators. Indicators have certain characteristics that limit their usefulness. The listed ph values of indicators are usually given for 25 C. At other temperatures, an indicator may change color at a different ph. If the solution being tested is not colorless, the color of the indicator may be distorted. Dissolved salts in a solution may also affect the indicator s dissociation. Using indicator strips can help overcome these problems. An indicator strip is a piece of paper or plastic impregnated with an indicator. The paper is dipped into an unknown solution and compared with a color chart to measure the ph. Some indicator paper is impregnated with multiple indicators. The colors that result, which cover a wide ph range, are shown in Figure a b 602 Chapter Chapter 19

10 a c ph Meters A ph meter makes rapid, accurate ph measurements. Most chemistry laboratories have a ph meter. A ph meter connected to a computer or chart recorder can be used to make a continuous recording of ph changes. As you can see in Figure 19.15, the ph meter gives a direct readout of ph. A ph meter is often easier to use than liquid indicators or indicator strips. Measurements of ph obtained with a ph meter are typically accurate to within 0.01 ph unit of the true ph. The color and cloudiness of the unknown solution do not affect the accuracy of the ph value obtained. Hospitals use ph meters to find small but meaningful changes of ph in blood and other body fluids. Sewage, industrial effluents, and soil ph are also easily monitored with a ph meter. b d Figure Altering soil ph can affect the development of plants. a In acidic soils, hydrangeas produce blue flowers. b In basic soils, hydrangeas produce pink flowers. Evergreen plants c suffer from chlorosis, a yellowing of the foliage d if soil ph is too basic. Figure A ph meter provides a quick and accurate way to measure the ph of a solution. a Water is neutral, having a ph of 7. b The ph of vinegar, a dilute aqueous solution of ethanoic (acetic) acid, is about 3. c The ph of milk of magnesia, an aqueous suspension of magnesium hydroxide, is Applying Concepts What are some advantages of using a ph meter rather than an indicator? CLASS Activity Comparing ph Indicators and ph Meters Purpose Students compare ph indicators and ph meters. Materials chemical supply catalogs Procedure Allow students to compare the costs and capabilities of ph meters versus other quantitative acid-base indicators. Some ph meters are hand held devices, while others are larger, bench instruments, which can be connected to computer hardware and are intended for more detailed analyses. Students should compare the resolution and precision of each of the instruments and ph indicators. Ask, What are some advantages and uses of hand held devices? Why would a chemist want to have a ph meter that can be calibrated at different temperatures? (Hand-held ph meters are portable and, therefore, convenient to use in the field. Outdoor temperatures may vary widely.) a b c Section 19.2 Hydrogen Ions and Acidity 603 Answers to... Figure vinegar: 2; soda water: 4; ammonia solution: 10 to 11 Figure A ph meter is more accurate, faster, and provides continuous readings. Acids, Bases, and Salts 603

11 Section 19.2 (continued) 3 ASSESS Evaluate Understanding Have students use equations to describe the relationship between the concentrations of hydrogen and hydroxide ions in pure water and to show what happens to the equilibrium when HCl is added to water. Repeat the procedure for NaOH. (At 25 C, [H + ] [OH - ] = When acid is added, the [H + ] increases and the [OH - ] decreases. When base is added the [OH - ] increases and the [H + ] decreases.) Quick LAB Quick LAB Indicators from Natural Sources Objective Students will measure the ph of various household materials using a natural indicator and an indicator chart. Quick LAB Indicators from Natural Sources Purpose To measure the ph of various household materials by using a natural indicator to make an indicator chart. Materials knife red cabbage leaves 1-cup measure hot water 2 jars clean white cloth teaspoon tape 3 sheets of plain white paper pencil ruler 10 clear plastic cups white vinegar (CH 3 COOH) baking soda (NaHCO 3) household ammonia dropper various household items listed in Step 5 Procedure 1. Put 1 cup of finely chopped red cabbage leaves in a jar and add 1 cup of 2 2 hot water. Stir and crush the leaves with a spoon. Continue the extraction until the water is distinctly colored. 2. Strain the extract through a piece of cloth into a clean jar. This liquid is your natural indicator. 3. Tape three sheets of paper end to end. Draw a line along the center and label it at 5-cm intervals with the numbers 1 to 14. This is your ph scale. 4. Pour your indicator to about 1-cm depth into each of three plastic cups. To one cup, add several drops of vinegar, to the second add a pinch of baking soda, and to the third add several drops of ammonia. The resulting colors indicate ph values of about 3, 9, and 11, respectively. Place these colored positions on your ph scale. 5. Repeat Step 4 for household items such as table salt, borax, milk, lemon juice, laundry detergent, dish detergent, milk of magnesia, mouthwash, toothpaste, shampoo, and carbonated beverages. Analyze and Conclude 1. What was the color of the indicator at acidic, neutral, and basic conditions? 2. What chemical changes were responsible for the color changes? 3. Label the materials you tested as acidic, basic, or neutral. 4. Which group contains items used for cleaning or for personal hygiene? Prep Time 20 minutes Class Time If the period allotted to lab work is short, spread the work over two days. Safety Perform this lab in a wellventilated room. Expected Outcome Students will use a natural indicator to determine if tested materials are acidic, basic, or neutral. Analyze and Conclude 1. The initial color is purple. In an acidic solution, the indicator is red; in a neutral solution, it is blue-purple; and in a basic solution, it is green. 2. Changes in the relative number of H + and OH ions present are responsible for the color changes. 3. Answers will vary depending on the materials chosen. 4. Personal hygiene items mainly test neutral; cleaning materials, such as soap, mainly test basic. Elements Students will note that the ph affects the relative concentrations of ClO and HClO in pool water. 604 Chapter Section Assessment 17. Key Concept What is the relationship between [H ] and [OH ] in an aqueous solution? 18. Key Concept What is true about the relative concentrations of hydrogen ions and hydroxide ions in each kind of solution? a. basic b. acidic c. neutral 19. Key Concept What is true about the colors of a ph indicator? 20. Determine the ph of each solution. a. [H ] M b. [H ] M c. [OH ] M d. [OH ] M 21. What are the hydroxide-ion concentrations for solutions with the following ph values? a b c Chapter 19 If your class subscribes to the Interactive Textbook, use it to review key concepts in Section with ChemASAP Group 7A Elements Hypochlorite salts are used to disinfect swimming pools. Use page R32 of the Elements to research how the ph of swimming pool water is regulated to maintain the necessary concentration of hypochlorous acid, HOCl. Assessment 19.2 Test yourself on the concepts in Section withchemasap Section 19.2 Assessment 17. [H + ] [OH - ] = ; when [H + ] in a solution increases, the [OH - ] decreases. 18. a. Hydroxide ion concentration is greater. b. Hydrogen ion concentration is greater. c. The concentrations are equal. 19. The color of HIn (aq) must be different than the color of In - (aq) 20. a b c d a M 22. b M c M