EXPERIMENT. Acid-Base Chemistry

Transcription

1 EXPERIMENT Acid-Base Chemistry Hands-On Labs, Inc. Version Review the safety materials and wear goggles when working with chemicals. Read the entire exercise before you begin. Take time to organize the materials you will need and set aside a safe work space in which to complete the exercise. Experiment Summary: You will classify solutions as neutral, acidic, or basic using a ph indicator. You will determine if a neutralization reaction has occurred. 1 Hands-On Labs, Inc.

2 Learning Objectives Upon completion of this laboratory, you will be able to: Describe and identify acids, bases, and neutral substances. Define ph and ph indicators and explain how the ph scale relates to the acidity of a substance. Define conjugate acid and conjugate base, and describe how they are formed as a product of the breakdown of acids and bases. Compare and contrast the chemical reactions of strong and weak acids and bases, and the relative ph of each type of reaction. Classify solutions as neutral, acidic, or basic using a ph indicator. Determine if an acid or a base neutralization reaction has occurred. Classify specific household products as acids, bases, or neutral. Time Allocation: 2.5 hours 2 Hands-On Labs, Inc.

3 Materials Student Supplied Materials Quantity Item Description 2 Aluminum pie pans 1 Camera, digital or smartphone 5 Disposable cups 5 Household liquid items: cleaning products, glass cleaner, soft drinks, or liquid food items. 1 Pair of scissors 1 Pencil 1 Roll of paper towels 1 Sheet of white paper 1 Source of tap water 32 Toothpicks 1 Trash can HOL Supplied Materials Quantity Item Description 1 Pair of safety goggles 3 Pairs of gloves 2 ph test strips, wide range 1 Test tube cleaning brush 1 Well plate Well plate Hands-On Labs, Inc.

4 1 Experiment Bag: Acid-Base Chemistry: 1- Acetic acid CH 3 COOH, 0.8M, 5% - 2 ml in pipet 1- Ammonium hydroxide NH 4 OH, 1 M - 3 ml in pipet, also known as aqueous ammonia 2- Bromothymol blue, 0.04% - 2 ml in pipet 1- Disodium hydrogen orthophosphate Na 2 HPO 4, 0.2 M - 2 ml in pipet 1- Hydrochloric acid HCl, 1.0 M - 3 ml in pipet 1- Hydrogen nitrate HNO 3, 1.0 M - 2 ml in pipet 1- Hydrogen sulfate H 2 SO 4, 1.0 M - 2 ml in pipet 1- Limewater, calcium hydroxide Ca(OH) 2, saturated - 2 ml in pipet 1- Phosphoric acid H 3 PO 4, 1.0 M - 2 ml in pipet 1- Sodium acetate CH 3 COONa, 0.5 M - 2 ml in pipet 1- Sodium bicarbonate NaHCO 3, 1.0 M - 2 ml in pipet 1- Sodium bisulfite NaHSO 3, 0.1 M - 2 ml in pipet 1- Sodium carbonate Na 2 CO 3, 1.0 M - 3mL in pipet 1- Sodium dihydrogen phosphate NaH 2 PO 4, 0.5 M - 2 ml in pipet 1- Sodium hydrogen sulfate NaHSO 4, 0.1 M - 2 ml in pipet 1- Sodium hydroxide NaOH, 0.5 M - 3 ml in pipet 1- Trisodium phosphate Na 3 PO 4, 0.1 M - 2 ml in pipet Note: To fully and accurately complete all lab exercises, you will need access to: 1. A computer to upload digital camera images. 2. Basic photo editing software, such as Microsoft Word or PowerPoint, to add labels, leader lines, or text to digital photos. 3. Subject-specific textbook or appropriate reference resources from lecture content or other suggested resources. Note: The packaging and/or materials in this LabPaq kit may differ slightly from that which is listed above. For an exact listing of materials, refer to the Contents List included in your LabPaq kit. 4 Hands-On Labs, Inc.

5 Background Acids and Bases Acids are substances that are characterized by their ability to donate protons, or hydrogen ions (H + ), to other substances in a solution. Acids can be strong or weak. Concentrated hydrochloric acid (HCl), which may corrode metals and burn human skin, is an example of a strong acid. Weak acids do not ionize completely and are found in everyday substances such as chlorogenic acid (C 16 H 18 O 9 ), which is found in coffee. See Figure 1A. Acidity is a measure of the amount of dissolved hydrogen ions, [H + ], in a solution. The greater number of hydrogen ions in a solution, the more acidic the solution. Strong acids donate hydrogen ions in water, to a much greater extent than weak acids. Bases are substances that are characterized by their ability to accept protons or produce hydroxide ions (OH - ) in an aqueous solution. As for acids, the difference in strength of bases directly relates to the degree in which hydroxide ions are formed by the base in water. Like strong acids, strong bases are also dangerous chemicals. An example of a strong base is concentrated sodium hydroxide (NaOH) which can degrade the proteins on human skin and cause burns. Weak bases are found in many everyday items such as hand soap. See Figure 1B. When feeling the consistency of a base, it feels slippery because it degrades the fatty acids and oils in the skin on contact. However, due to the possibility that an acid or a base could be very dangerous to human tissue, never experiment with an acid or a base by tasting or touching it. Figure 1. A. Chlorogenic acid (C 16 H 18 O 9 ) is a weak acid found in coffee. Acids have a sour taste. Volkov B. Hand soap is an example of a weak base. Bechelli 5 Hands-On Labs, Inc.

6 ph Scale and Indicators To determine if a substance is an acid or a base, a ph scale may be used. ph is a measure of the concentration of hydrogen ions [H + ] in a substance. The equation for ph is: ph = log [H + ] The equation can be used to calculate the ph for a known concentration of H+ ions: H+ = 1.4 x 10-5 M ph = -log (1.4 x 10-5 ) ph = 4.85 The ph of a solution can also be used to determine the concentration of H+ ions present: ph = 8.5 H+ = H+ = 3.2 x 10-9 M The logarithm for ph is based on a scale of 10. As ph is a negative logarithm, the ph of a substance increases as the concentration of hydrogen ions [H + ] decreases. For example, a ph of 2 is 10 2 times, or 100 times, more acidic than a ph of 4. Figure 2 shows the relationship between ph and the concentration of [H + ] in a solution. Acids and bases are present in every aspect of our lives. Car batteries contain sulfuric acid (H 2 SO 4 ), with a ph near zero (0), whereas household bleach (sodium hypochlorite NaClO) has a ph of about Other common acids include: ascorbic acid (H 2 C 6 H 6 O 6 ) Vitamin C; carbonic acid (H 2 CO 3 ) - used in soft drinks; and citric acid (C 6 H 8 O 7 ) found in citrus fruits such as lemons and oranges. Common bases include: aluminum hydroxide (Al[OH] 3 ) - found in antacids and deodorants; sodium bicarbonate (NaHCO 3 ) baking soda, which can be used for baking, extinguishing fires and as an antacid; and calcium hydroxide (Ca[OH] 2 ) - found in mortar and plaster. Many baking recipes call for lemon juice and baking soda what happens chemically when these interact, and what is the outcome in baking? 6 Hands-On Labs, Inc.

7 Figure 2. The relationship between ph and the concentration of [H + ] in a solution. On the ph scale, a ph greater than 0 and less than 7 is acidic, a ph of 7.0 is neutral, and a ph greater than 7.0 is basic. In addition to the concentration of [H + ] ions, bases can be measured by the presence of ion called a hydroxide ion (abbreviated as OH ). For bases, the ph value increases as the amount of hydroxide ions in the solution increases. A neutral substance is a substance that is neither an acid nor a base. A common way to measure the ph value of a substance is through the use of a ph indicator. A ph indicator changes color at a specific ph or over the course of a ph range. For example, the Bromothymol blue (BTB) indicator is yellow in a ph below 6.0. At a ph of , the indicator is a bluish-green, and above 7.6, BTB is blue. Therefore, BTB is a useful indicator to determine whether a substance is an acid (yellow), whether the substance is around neutral ( ), or whether the substance is a base (blue). See Figure Hands-On Labs, Inc.

8 Figure 3. Bromothymol blue (BTB) as an indicator. The well on the left shows how BTB reacts to an acid, the well in the middle shows how BTB reacts to a neutral substance, and the well on the right shows how BTB reacts to a base. Conjugate Bases and Conjugate Acids Acids dissociate in water to produce a hydronium ion (H 3 O + ) and a conjugate base (A - ). See Equation 1. Another way to think of a conjugate base is as what remains of the acid after the proton has been lost. The conjugate acid is formed when the proton is accepted by the base. In the presence of an acid, water will behave as a base (as it is functioning as a proton acceptor) and will accept the proton donated by the acid to create a hydronium ion (H 3 O + ). In Equation 1, the conjugate acid is H 3 O +. Equation 1. The dissociation of an acid in water. The strength of an acid is dependent on its ability to fully dissociate in water. A strong acid fully dissociates into hydronium ions (H 3 O + ) and a conjugate base (A - ). Thus, the equilibrium position for the strong acid lies mostly to the right in Equation 1. For example, hydrochloric acid (HCl) is a strong acid. The stronger the tendency of an acid to dissociate in water, the more the hydrogen ions [H + ] that end up in the solution. Therefore, a strong acid has a low ph. See Equation 2. Equation 2. A strong acid, HCl. Equilibrium lies overwhelmingly to the right. Weaker acids form solutions with higher ph values than stronger acids of the same concentration. A stronger acid has a weaker conjugate base, with water having a higher affinity for the proton than the weaker conjugate base. On the other hand, a weaker acid has a stronger conjugate base, with the stronger conjugate base having a higher affinity for the proton than water. Therefore, less of the weaker acid dissociates in water, which results in less [H+] in the solution and a higher ph. 8 Hands-On Labs, Inc.

9 Bases, like acids, also dissociate in water to form a hydroxide ion (OH - ) and a conjugate acid (BH + ). See Equation 3. In the presence of a base, water behaves as an acid, because it will donate a proton to form the conjugate acid. In Equation 3, the conjugate acid is BH +. The conjugate base is formed from the acid (in this case, water) when the proton is donated. In Equation 3, the conjugate base is OH -. Equation 3. The dissociation of a base (B) in water. Just as for an acid, the strength of a base is dependent on its ability to fully dissociate in water. A strong base fully dissociates into hydroxide ions (OH - ) and a conjugate acid (BH + ), while a weak base only partially dissociates. The equilibrium position for the strong base lies mostly to the right in Equation 3. For example, sodium hydroxide (NaOH) is a strong base. The stronger the tendency of a base to dissociate in water, the more hydroxide ions (OH - ) end up in the solution. Therefore, a strong base has a high ph. See Equation 4. Equation 4. A strong base, NaOH. Equilibrium lies overwhelmingly to the right. Weaker bases form solutions with a lower ph than stronger bases at the same concentration. A stronger base has a weaker conjugate acid, with less ability to donate protons than water, resulting in a higher concentration of hydroxide ions. Weaker bases have stronger conjugate acids that are more likely to donate protons than water is, resulting in lower hydroxide ion concentrations and lower ph. All acids, weak and strong, produce H + ions in water, while all bases produce OH - ions in water. Therefore, when an acid and a base are present in the same solution, these ions combine to form water molecules, through a process called neutralization. See Equation 5. Neutralization reactions result in a solution with a ph between that of the original acid and base solutions, depending upon the strength and molarity of those solutions. If equal numbers of moles of strong acid and strong base are combined, the resulting solution is neutral, or ph 7. Equation 5. Mixing a hydrogen ion and a hydroxide ion creates water, a ph neutral substance. In the following exercises, you will determine the ph of a variety of chemicals using ph paper and Bromothymol blue as an indicator. In addition, you will mix the acids and bases in a 1:1 ratio and determine the ph of the new solutions. 9 Hands-On Labs, Inc.

10 Exercise 1: Using ph Paper and the Indicator Bromothymol Blue (BTB) In this exercise, you will determine the approximate ph for 16 different chemicals using ph paper and BTB. Note: It is important to read the entire procedures before beginning the exercise. 1. Gather gloves, a sheet of paper, a pencil, 2 disposable pie pans, paper towels, scissors, the 24-well plate, the 96-well plate, wide-range ph paper, a trash container, and the Acid/Base Chemistry experiment bag. Note: The 24-well plate will be used to store and organize the chemicals used in both Exercise 1 and Exercise 2. A different, 96-well plate will be used to perform the experiments. The same well plates will be used for both activities, without cleaning in between, so it is very important to place materials in only the designated wells. 2. Create labels for the 24-well plate on the sheet of paper as follows: a. Fold the paper in half. b. On half of the sheet of paper, place the 24-well plate on the paper and use a pencil to draw around the 24-well plate. c. Use the pencil to mark where the wells are positioned in each row and column, marking each side of the wells on the paper drawing. See Figure 4. Figure 4. Mark where each well row and well column starts and ends on the paper below the 24-well plate Hands-On Labs, Inc.

11 d. Use the edge of the well plate to draw a straight line, connecting the positions of where each well row and well column begins and ends. See Figure 5. Figure 5. Draw a straight line using the edge of the well plate to indicate where each well begins and ends. e. Label the drawing, starting with the first well square in the upper left portion of the grid. Label the wells 11A-11H and 12A-12H as shown in Figure 6. Figure 6. Label the grid 11A-11H and 12A-12H. Note that these labels correspond to the wells on the 96-well plate Hands-On Labs, Inc.

12 f. On the other half of the paper, use a pencil to draw around the 24-well plate again. Note: This drawing will be used for Exercise 2, but it is easier to do this before you cut open the chemicals. g. Use the pencil to mark where the wells are positioned in each row and column, marking every side of the wells on the paper drawing as you did previously. Refer to Figure 5. h. Use the edge of the well plate to draw a straight line across the tracing of the well plate, connecting the positions of where each row and column begins and ends as done before. Refer to Figure 5. Label the drawing, starting with the first well in the upper left portion of the grid. This time, label the paper grid A1-A8 and BA-BH. See Figure 7. Figure 7. Label the second grid A1-A8 and BA-BH. Note that these labels correspond to the rows and columns of the 96-well plate used in Exercise Turn the paper so the drawing labeled 11A-11H and 12A-12H is facing upward and place the folded paper in a disposable pie pan. Put the 24-well plate on the piece of paper over the markings. See Figure Hands-On Labs, Inc.

13 Figure 8. Place the 24-well plate on the paper over the drawing labeled 11A-11H and 12A-12H. 4. Open the Acid/Base Chemistry bag and arrange the chemicals from the bag in the 24-well plate, with bulbs face down, according to the names of chemicals that correspond to each ID Number, as listed in Data Table 1 in your Lab Report Assistant. For example, the pipet labeled Acetic Acid (CH 3 COOH) will go over the well labeled 11A. See Figure 9. Figure 9. Place the chemicals, bulb face up, in the well plate according to the corresponding ID Numbers in Data Table 1. For example, acetic acid goes in the well that is marked 11A Hands-On Labs, Inc.

14 5. Place the 2 pipets containing BTB in 2 unlabeled wells of the 24-well plate, bulb facing downward. See Figure 10 for complete setup. Figure 10. Complete setup after step Before moving on to the experiment, make an educated guess (acid or base) for each chemical by studying the chemical formula, shown in Data Table 1. Record your guess in the column of Data Table 1 labeled, Hypothesis: Acid or Base. 7. Retrieve the wide-range ph paper, and cut each piece of paper in half lengthwise and then again in half widthwise. See Figure 11. Place the cut pieces of ph paper in a pile in a dry location, such as on a sheet of paper, to be used for Exercises 1, 2, and 3. Figure 11. Cut each sheet of the ph paper in half lengthwise and widthwise. The final pieces are shown on the right, four pieces from the original piece Hands-On Labs, Inc.

15 8. Put on your pair of gloves and safety goggles. Safety Warning! You will be working with very strong acids and bases. Do not continue this exercise without wearing protective equipment. You should also wear old clothing that will cover your arms and legs. 9. Wet a paper towel and place it on the unused disposable pie pan. 10. Use a pair of scissors to carefully cut off the tip of the first chemical pipet. Cut the pipet over a trash container so the trash container will catch the tips of the pipet. 11. Replace the pipet, bulb facing downward, back into the 24-well plate in its proper location. 12. Use the wet paper towel after cutting the pipet to carefully wipe any residue left on the scissors from the chemical. 13. Repeat steps 10-12, carefully cutting each pipet, one by one, replacing the pipet back in its prospective location in the 24-well plate, and wiping off the scissors. 14. Keep the used paper towel on the pie pan, but also lay a new, dry piece of paper towel flat in the same pie pan. 15. Place the 96-well plate in the pie pan over the new paper towel. 16. Put 2 drops of a chemical in the 96-well plate. Use the row and column that corresponds to the chemical s ID Number. For example, 2 drops of acetic acid will be placed in 11A; column 11, row A of the 96-well plate. Do this for each chemical. See Figure 12. Note: If an air bubble is caught in the tip of the pipet, expel the first drop onto the wet paper towel located in the pie plate. You want each drop that goes into the well plate to be a full drop. Figure 12. Add 2 drops of each chemical in the row/column that corresponds to the chemical s ID Number from Data Table 1. For example, place 2 drops of acetic acid in well 11A Hands-On Labs, Inc.

16 17. Use 1 piece of cut ph paper for one well of the 96-well plate that contains a chemical to determine the approximate ph of the chemical. a. Dip the paper into the well and pull it out immediately. It is important to quickly remove the ph paper from the well to avoid dissolving the chemicals from the ph paper into the chemicals in the well. b. Immediately compare the color of the paper to the scale that was provided with the ph paper. c. Record the ph in Data Table 1 in the column labeled ph. d. Repeat for each chemical, and only use 1 sectioned piece of ph paper for 1 well. 18. Put 1 drop of BTB in each well of columns 11 and Record the color of each well in Data Table 1 in the column labeled BTB color. 20. Do not clean up any chemicals or any materials from this exercise. You will use them all in the next exercise. Leave all chemical pipets in the 24-well plate. Do not rinse the 96-well plate as it may contaminate the other portions of it. Questions A. When you created hypotheses to guess whether each chemical was an acid or base, what specifically did you look for in the chemical formulas? B. What is the value of using ph paper as an indicator? C. What is the value of using BTB as an indicator? When might BTB be a better choice than ph paper? D. Write the chemical equation of mixing hydrogen nitrate (HNO 3 ) with water. E. Write the chemical equation of mixing ammonium hydroxide (NH 4 OH) with water. F. What are the conjugate acids and conjugate bases of questions D and E? 16 Hands-On Labs, Inc.

17 Exercise 2: Acid-Base Reactions In this exercise, you will use the ph paper and BTB indicator to determine the amount of neutralization that has occurred from mixing the acids with the bases. 1. Ensure gloves and safety goggles are worn throughout this exercise. 2. Remove the 24-well plate (with chemicals still stored in the well plate) from the disposable pie plate and turn the sheet of paper over, so the side of the paper that has the labels A1-A8 and BA-BH is facing upward. 3. Place the 24-well plate back on the pie plate, directly over the labels. See Figure 13. Figure 13. Place the 24-well plate back on the pie plate over the labels A1-A8 and BA-BH. Note that the chemical pipets were omitted from the well plate in the photo for clarity. 4. Organize the chemicals in the 24-well plate so the 8 acids are over ID Numbers A1-A8 and 8 bases are over ID Numbers BA-BH, as shown in Data Table 2 in your Lab Report Assistant. 5. Put 2 drops of an acid in the 96-well plate in the column that corresponds to the ID number in Data Table 2. For example, acetic acid (CH 3 COOH) is in A1, so put 2 drops of acetic acid in each row (A-H) of column 1 of the 96-well plate. For A2, put 2 drops of hydrogen nitrate in each row of column 2 of the 96-well plate. Continue for the remaining 63 wells. Note: If an air bubble is caught in the tip of the pipet, expel the first drop onto the wet paper towel located in the pie plate. You want each drop that goes into the well plate to be a full drop Hands-On Labs, Inc.

18 6. Put 2 drops of a base in the 96-well plate in the row that corresponds to the ID letter in Data Table 2. For example, sodium acetate (CH 3 COONa) is BA, so put 2 drops of sodium acetate in each column (1-8) of that row of the 96-well plate. Note: When Step 6 is completed, well A1 will contain 2 drops of acetic acid and 2 drops of sodium acetate, well A2 will contain 2 drops of hydrogen nitrate and 2 drops of sodium acetate, etc. 7. Repeat Step 6 for the remaining 56 wells of the 96-well plate, referencing Data Table 2 as necessary, to ensure that the correct base is added to the appropriate well. 8. Break or cut 32 toothpicks in half. 9. Use ½ of a toothpick to stir well A1 in the 96-well plate that contains both the acid and base. Only use 1 toothpick per well. Discard the toothpick in a trash container after it has been used. 10. Repeat Step 9 for the remaining 63 wells. 11. Use 1 cut piece of ph paper for a well of the 96-well plate that contains an acid and base mixture to determine the approximate ph of the chemical. a. Dip the paper into the well and immediately remove it. b. Immediately compare the color of the paper to the scale that was provided with the ph paper. c. Determine and record the ph in Data Table 2 in the row and column that corresponds to the row and column in the 96-well plate, next to ph. See example in Data Table 2 of a mixture of acetic acid (CH 3 COOH) and sodium acetate (CH 3 COONa). 12. Repeat Step 11 for the remaining 63 wells. Use a new sectioned piece of ph paper for each well. 13. Now, put 1 drop of BTB in each of the 64 wells containing the acid/base solutions. 14. Record the color of each well in Data Table 2, next to BTB color, in the row and column that corresponds to the row and column in the 96-well plate. See example in Data Table 2 for well A1; the mixture of acetic acid (CH 3 COOH) and sodium acetate (CH 3 COONa). 15. Use the digital camera to take a photo of the well plate so that you will be able to use the photograph as a reference when answering the questions. Make sure that the photograph is well referenced, so you will be able to identify each well in the plate. 16. Resize and insert the image along with your answer for Question A. Refer to the appendix entitled Resizing an Image for guidance. 17. Once all data has been recorded, with gloved hands, rinse both well plates with copious amounts of running water. Tap the well plates on paper towels to remove liquid and repeat the rinse as needed. Dispose of the 96 well plate in the trash, but return the clean 24 well plate to the kit for future use Hands-On Labs, Inc.

19 Questions A. Visually analyze the wells colored with BTB, and remember that acids are organized by columns and bases are organized by rows. What specific acids or bases are weak or strong acids or bases? Explain why. B. Why did some of the chemicals neutralize and others did not? C. Did any of the mixtures get more acidic than the original acid or less acidic than the original base? Why do you think this happened or did not happen? 19 Hands-On Labs, Inc.

20 Exercise 3: Testing the ph of Household Products In this exercise, you will use ph paper to classify five household products. Safety Warning! Do not mix the household products. 1. Put on your safety glasses and gloves, keep them on for the remainder of this experiment. 2. Locate 5 household products to be used for ph classification. Possible substances to test include home cleaning products, soft drinks, or liquid food items. 3. Record the name of 1 chosen household substance in Data Table 3 in your Lab Report Assistant. 4. Predict if the household substance is an acid, base, or neutral. Record the prediction in Data Table Pour a small amount of the household substance in a clean cup. 6. Use 1 cut piece of ph paper to determine the approximate ph of the chemical. a. Dip the paper into the cup containing the household substance and immediately remove it. b. Immediately compare the color of the paper to the scale that was provided with the ph paper. c. Determine the ph of the substance and record the ph in Data Table 3, under the ph Value column. 7. Use the ph to categorize the substance as an acid, base, or neutral. Record in Data Table 3, under the conclusion column. 8. Repeat Steps 3-7 for the remaining 4 household substances. Use only 1 sectioned piece of ph paper for each substance. Use a clean cup for each substance. 9. Clean all items and return to the lab kit for future use. 10. When you are finished uploading photos and data into your Lab Report Assistant, save and zip your file to send to your instructor. Refer to the appendix entitled Saving Correctly, and the appendix entitled Zipping Files, for guidance with saving the Lab Report Assistant in the correct format. Questions A. What properties did you use to determine whether the substances you tested were acid or base? B. Did any of your findings surprise you? Why or why not? 20 Hands-On Labs, Inc.