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Diprotic Titration: Multi-Step Chemical Reactions

Explore the titration of diprotic acids and learn about multi-step chemical reactions. Understand how to determine unknown concentrations and observe titration curves.

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Diprotic Titration: Multi-Step Chemical Reactions

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  1. Diprotic Titration: Multi-Step Chemical Reactions 012-10738 r1.04

  2. Diprotic Titration: Multi-Step Chemical Reactions Introduction Each page of this lab that contains the symbol should be inserted into your journal. After completing a lab page with the snapshot symbol, tap (in the upper right hand corner) to insert the page into your journal. Journals and Snapshots The Snapshot button is used to capture the screen. The Journal is where snapshots are stored and viewed. The Share button is used to export or print your journal to turn in your work. Note: You may want to take a snapshot of the first page of this lab as a cover page for your Journal.

  3. Diprotic Titration: Multi-Step Chemical Reactions Lab Challenge • A titration can tell you how much of an acid is present in a solution, but what happens if the acid has more than one hydrogen ion that it can donate? • How will the titration curve differ? Why does the titration curve change and can an unknown concentration still be determined?

  4. Diprotic Titration: Multi-Step Chemical Reactions Background H+ can be removed • Acids are substances that have a hydrogen ion that can be donated. • Bases are complements to acids in that bases accept hydrogen ions. H+ can be added to form H2O • When an acid gives up a hydrogen ion, the anionic portion left behind is called the conjugatebase. • When a base accepts a hydrogen ion, the newly created species is called the conjugate acid. HNO3(aq) + H2O(l) NO3-(aq) + H3O+(aq) Conjugate base Conjugate acid Acid Base

  5. Diprotic Titration: Multi-Step Chemical Reactions Self-Check • When an acid gives up a hydrogen ion, the anionic portion left behind is called the _______________. • conjugate acid • conjugate base • diprotic acid • diprotic base

  6. Diprotic Titration: Multi-Step Chemical Reactions ...Background • Polyprotic acids have more than one hydrogen ion they can donate. • Acids can be further classified by the number of hydrogen ions they can donate. General Type of Acid Specific Type of Acid Number of H+ Ions to Donate Example monoprotic monoprotic 1 HCl diprotic polyprotic 2 H2SO3 polyprotic triprotic 3 H3PO4

  7. Diprotic Titration: Multi-Step Chemical Reactions Self-Check Number of H+ Ions to Donate Specific Type of Acid • 2. Nitric acid (HNO3) is best described as a ___________ acid. • triprotic • diprotic • polyprotic • monoprotic 1 monoprotic 2 diprotic triprotic 3

  8. Diprotic Titration: Multi-Step Chemical Reactions ...Background • A carbonate ion (CO32-) is the conjugate base of carbonic acid (H2CO3). By making a solution of sodium carbonate, the solution contains carbonate ions that can accept hydrogen ions. • These carbonate ions can be titrated with an acid, such as hydrochloric acid. The acceptance of hydrogen ions takes place in two steps: Step 1:HCl (aq) + Na2CO3 (aq) NaHCO3(aq) + NaCl (aq) Step 2:HCl (aq) + NaHCO3 (aq) CO2(g) + H2O (l) + NaCl (aq) Overall Reaction: 2 HCl (aq) + Na2CO3 (aq) CO2(g) + H2O (l) + 2 NaCl (aq)

  9. Diprotic Titration: Multi-Step Chemical Reactions Self-Check • 3. When do you expect to observe bubbles (formation of a gas) during the reaction between sodium carbonate and hydrochloric acid? • during step one only • during step two only • during both step one and step two • no bubbles will form

  10. Diprotic Titration: Multi-Step Chemical Reactions ...Background • Each step can be seen in a titration curve. As the titration progresses, the pH can be measured with a sensor. • The point at which the curve is the steepest indicates an equivalence point (where the number of moles of acid equals the number of moles of base). • For polyprotic acids, there will be one equivalence point for each hydrogen that can be donated. Equivalence point Equivalence point

  11. Diprotic Titration: Multi-Step Chemical Reactions Self-Check • 4. How many equivalence points will there be on a titration curve for a diprotic acid? • 1 • 2 • 3 • 4 Diprotic titration curve

  12. Diprotic Titration: Multi-Step Chemical Reactions Safety • Follow all common laboratory safety procedures. • Hydrochloric acid is a strong acid. Avoid contact with the eyes and skin. • Wash hands after handling chemicals, equipment, and glassware. • Be sure that all acids and bases are neutralized before being disposed of down the drain. BE SAFE Always wash hands to remove residue before leaving

  13. Diprotic Titration: Multi-Step Chemical Reactions Materials and Equipment Collect all of these materials before beginning the lab. • Drop counter • pH sensor • Buffer solution pH 4, 25 mL • Buffer solution pH 10, 25 mL • Beakers (2), 50-mL • Micro stir bar • Magnetic stirrer • Funnel • Ring Stand

  14. Diprotic Titration: Multi-Step Chemical Reactions ...Materials and Equipment Also collect these additional materials before beginning. • Buret • Buret clamp • Right-angle clamp • Beaker, 250-mL • Transfer pipet • Graduated cylinder, 100-mL • Graduated cylinder, 50-mL • Wash bottle filled with distilled (deionized) water

  15. Diprotic Titration: Multi-Step Chemical Reactions ...Materials and Equipment Also collect these additional materials before beginning. • Waste container • Distilled (deionized) water, 200 mL • 1.0 M Hydrochloric acid solution, 50 mL • Sodium carbonate solution, 50 mL Distilled Water waste

  16. Diprotic Titration: Multi-Step Chemical Reactions Sequencing Challenge The steps to the left are part of the procedure for this lab activity. They are not in the right order. Determine the correct sequence of the steps, then take a snapshot of this page. A.When the pH stabilizes (at a pH less than 2) close the stopcock and then stop collecting data. Record the final volume. B.Use the data collected to calculate the concentration of the sodium carbonate solution. D.Measure and record the starting volumes of sodium carbonate and HCl. Start collecting data and then release the titrant. C.Setup the titration equipment and calibrate the pH sensor.

  17. Diprotic Titration: Multi-Step Chemical Reactions Setup • Connect the pH sensor to the data collection system. • Calibrate the pH sensor: • Pour approximately 25 mL of the pH 4 buffer solution in a 50-mL beaker. • Pour approximately 25 mL of the pH 10 buffer solution in a second 50-mL beaker. • Remove the pH sensor from its electrode storage bottle and rinse it with distilled water to thoroughly clean the sensor. • Read the instructions on how to calibrate the pH sensor on the next page. Buffer solutions and pH sensor

  18. Diprotic Titration: Multi-Step Chemical Reactions To Calibrate the pH Sensor: • Calibration Point 1: • Place the pH probe in a pH 4 buffer solution. • Enter 4.0 as the pH in the Standard Valuebox under Calibration Point 1. • Tap Read From Sensor underCalibration Point 1. • Rinse the pH probe thoroughly using distilled water. • 4. Calibration Point 2: • Repeat the process used in calibration point 1 using a pH 10 buffer solution. • Tap OK to exit the calibration screen and then tap OK again to return to the lab. Note: During the calibration process you will not be able to return to this page. • 1. Open the Calibrate Sensor screens: • Tap • Tap CALIBRATE SENSOR 2.Ensure that the correct measurements are selected: • Sensor: (name of sensor) • Measurement: pH • Calibration Type: 2 point • b. Tap NEXT

  19. Diprotic Titration: Multi-Step Chemical Reactions Setup Q1: Will a pH sensor that is not calibrated give precise results? Why must the pH sensor be calibrated? • Connect the drop counter to the data collection system. • Assemble the titration apparatus. Buretclamp Buret pH sensor Ring stand Right-angle clamp Drop counter Microstir bar Beaker, 250-mL Magnetic stirrer

  20. Diprotic Titration: Multi-Step Chemical Reactions Setup Q2:Why is it necessary to drain a small amount of titrant through the drop counter before you begin a titration? • Use a disposable pipette to rinse the inside of the burette with several milliliters of the standardized hydrochloric acid (HCl) solution. • Drain the HCl from the burette into the waste container. • Repeat this process two more times. • Make sure the stopcock is in the "off" position and then use a funnel to fill the burette with ~50 mL of the standardized HCl solution (titrant). • Drain a small amount of titrant through the drop counter into the waste container. Off position

  21. Diprotic Titration: Multi-Step Chemical Reactions Setup Q3:How can the green light on the drop counter help you determine if the HCl is falling in distinguishable drops? • Practice adjusting the stopcock on the burette so that the titrant goes through the drop counter in distinguishable drops that fall at about 2 to 3 drops per second. • Note:It is important that you have good control of adjusting the stopcock. If you accidently open the stopcock too far and the NaOH flows out (as opposed to drops out), you will have to start over. • Close the stopcock and then remove the waste container.

  22. Diprotic Titration: Multi-Step Chemical Reactions Setup Q4:Record the initial volume of HCl and the concentration of the HCl solution in the text box below. • Read the initial volume of the titrant in the burette to 0.01 mL. • Using the 100-mL graduated cylinder measure 100.0 mL of distilled water and add it to a clean 250-mL beaker. • Using the 50-mL graduated cylinder measure 20.0 mL of 0.5 M sodium carbonate (Na2CO3) solution and add it to the 100.0 mL of distilled water in the beaker. Record the exact volume of Na2CO3 added in the text box on the next page.

  23. Diprotic Titration: Multi-Step Chemical Reactions Setup Q5:Record the volume of Na2CO3 added to the beaker in the text box below. • Add the pH sensor, micro stir bar, and 250-mL beaker containing the Na2CO3 solution to the titration apparatus. • Turn on the magnetic stirrer and begin stirring at a slow to medium speed. Note:Make sure the bulb of the pH sensor is fully submerged.

  24. Diprotic Titration: Multi-Step Chemical Reactions Collect Data • Tap to start collecting data. • Carefully open the stopcock on the burette so that 2 to 3 drops per second are released. • Continue to collect data until the pH of the solution stabilizes at a pH less than 2. Continue to the next page.

  25. Diprotic Titration: Multi-Step Chemical Reactions Q6:What reaction step is taking place at the beginning of the titration between the starting pH (~12) and a pH value of 8? Q7:What begins to happen in the beaker when the pH of the solution is lower than 8? Explain these observations.

  26. Diprotic Titration: Multi-Step Chemical Reactions Collect Data • When the pH of the solution stabilizes at a pH less than 2, close the stopcock. Continue to the next page.

  27. Diprotic Titration: Multi-Step Chemical Reactions Collect Data • Tap to stop data collection. • Record the final volume of HCl in the burette.

  28. Diprotic Titration: Multi-Step Chemical Reactions Data Analysis • * To View a Page in the Journal: • Tap to open the Journal screen. • Tap or to scroll through the thumbnail size pages of the journal. • Tap the thumbnail image of the page to view it. • Determine the total volume of HCl used in the titration by following steps a-c below. Volume HCl = Final Volume HCl - Initial Volume of HCl Note:The initial and final volumes of HCl were recorded earlier in the journal. Refer to your journal to get these numbers.* Enter the final volume of HCl in the text box on the right. Record the initial volume of HCl in the text box on the right. Subtract the initial volume of HCl from the final volume of HCl and record the total volume in the text box on the right.

  29. Diprotic Titration: Multi-Step Chemical Reactions *To Create a Calculation: • Tap to open the Experiment Tools screen. • Tap CALCULATED DATAto open the calculator. • Enter the calculation in the space provided. • Tap Measurements to insert collected data into the calculation. Data Analysis 2. Create a calculation to convert drop count to volume (mL). * CalcVolume= [drop count]*(volume of titrant/ final drop count) Note: [drop count] = the collected data volume of titrant = total volume of NaOH used (it is in your journal) final drop count = the total number of drops added (it is displayed on the right)

  30. Diprotic Titration: Multi-Step Chemical Reactions 3. Find the slope and volume at the firstequivalence point (point with the greatest slope).* *To Find an Equivalence Point: Tap then to open the Graph Properties screen. Tap the Measurement box for the x-axis and tap CalcVolume. Then tap OK. Tap to scale the axes. Tap then tap a data point above and below the steepest region of the graph. Tap and then tap to display the slope and the coordinate of the data point. Tap and until the slope is the greatest. Tap to select the equivalence point.

  31. Diprotic Titration: Multi-Step Chemical Reactions 4. Find the slope and volume at the secondequivalence point (point with the greatest slope).* *To Find an Equivalence Point: Tap then to open the Graph Properties screen. Tap the Measurement box for the x-axis and tap CalcVolume. Then tap OK. Tap to scale the axes. Tap then tap a data point above and below the steepest region of the graph. Tap and then tap to display the slope and the coordinate of the data point. Tap and until the slope is the greatest. Tap to select the equivalence point.

  32. Diprotic Titration: Multi-Step Chemical Reactions Data Analysis • Note: the values need for the calculations on this page can be found in your journal . • Calculate the molar concentration of Na2CO3 at the first equivalence point: • Determine the number of moles of HCl added using the volume of HCl added at the first equivalence point and the molarity of the HCl solution. • Convert from moles of HCl to moles of Na2CO3 using the balanced chemical equation for the first reaction step: HCl+ Na2CO3 NaHCO3 + NaCl • Use the moles of Na2CO3 and the starting volume of Na2CO3to determine molarityof the Na2CO3.

  33. Diprotic Titration: Multi-Step Chemical Reactions Data Analysis • Note: the values need for the calculations on this page can be found in your journal • Calculate the molar concentration of Na2CO3 at the second equivalence point: • Determine the number of moles of HCl added using the volume of HCl added at the second equivalence point and the molarityof the HCl solution. • Convert from moles of HCl to moles of Na2CO3 using the balanced chemical equation for the overall reaction: 2 HCl + Na2CO3 CO2 + 2 NaCl + H2O • Use the moles of Na2CO3 and the starting volume of Na2CO3 to determine molarityof the Na2CO3.

  34. Diprotic Titration: Multi-Step Chemical Reactions Data Analysis Equivalence point • Calculate the average molarity of Na2CO3 solution using the value from each equivalence point. Show your work. Equivalence point

  35. Diprotic Titration: Multi-Step Chemical Reactions Analysis • When did the bubbles produced by the formation of carbon dioxide gas start to become visible?

  36. Diprotic Titration: Multi-Step Chemical Reactions Analysis • Write the two chemical equations as separate steps that add together to give the overall reaction: 2HCl(aq) + Na2CO3(aq) → CO2(g) + 2NaCl(aq) + H2O(l)

  37. Diprotic Titration: Multi-Step Chemical Reactions Analysis • Before the first equivalence point, the beaker contained a mixture of both carbonate ions and bicarbonate ions. Which of these two ions accept hydrogen ions easier? • How do you know?

  38. Diprotic Titration: Multi-Step Chemical Reactions Analysis • After the first equivalence point, the carbon dioxide production was rapid. Were there any carbonate ions remaining in solution? • What do you think the first equivalence point represented?

  39. Diprotic Titration: Multi-Step Chemical Reactions Analysis • The reaction stopped bubbling at the second equivalence point. What do you notice about the volume of acid needed to reach each equivalence point?

  40. Diprotic Titration: Multi-Step Chemical Reactions Analysis • Why did the bubbling stop after the second equivalence point? THINK!

  41. Diprotic Titration: Multi-Step Chemical Reactions Synthesis • Often the product of one chemical reaction can become the reactant in another chemical reaction. Give an example of how this statement is true using the reactions seen in this experiment.

  42. Diprotic Titration: Multi-Step Chemical Reactions Synthesis • Calculate the number of moles of carbon dioxide that was produced. Show your work. • What was the limiting reagent?

  43. Diprotic Titration: Multi-Step Chemical Reactions Synthesis • Calculate the number of liters of carbon dioxide produced. Assume standard temperature and pressure. Show your work.

  44. Diprotic Titration: Multi-Step Chemical Reactions Multiple Choice Equivalence point • In an acid-base titration, the equivalence point of the curve shows ___________. • the point where there are the same number of moles of acid and base. • the point where the burette has run out of acid. • the point where pH is a maximum. • the point where pH is a minimum. Titration Curve

  45. Diprotic Titration: Multi-Step Chemical Reactions Multiple Choice • Carbonic acid is known to be a diprotic acid. Why? • Because it reacts with a base. • Because it has two hydrogen ions that can be donated. • Because it makes carbon dioxide. • Because it can act as an acid or a base. H2CO3

  46. Diprotic Titration: Multi-Step Chemical Reactions Multiple Choice • How many equivalence points are there on a titration of a diprotic acid? • 0 • 1 • 2 • 3

  47. Diprotic Titration: Multi-Step Chemical Reactions Multiple Choice Number of H+ Ions to Donate Specific Type of Acid • The titration curve below represents a ________ acid. monoprotic 1 diprotic 2 triprotic 3 monoprotic diprotic triprotic tetraprotic

  48. Diprotic Titration: Multi-Step Chemical Reactions Multiple Choice ??? • What gas is released when an acid reacts with sodium carbonate? • hydrogen gas • oxygen gas • methane gas • carbon dioxide gas

  49. Diprotic Titration: Multi-Step Chemical Reactions Congratulations! You have completed the lab. Please remember to follow your teacher's instructions for cleaning-up and submitting your lab.

  50. Chemistry Template References • All images were taken from PASCO documentation, public domain clip art, or Wikimedia Foundation Commons. • CUP WITH GAS http://www.freeclipartnow.com/food/beverages/soda/soft-drink-icon.jpg.html • BEAKER http://freeclipartnow.com/science/flasks-tubes/beaker.jpg.html • CORROSIVE WARNING http://commons.wikimedia.org/wiki/Image:DIN_4844-2_Warnung_vor_Aetzenden_Stoffen_D-W004.svg • BE SAFE http://freeclipartnow.com/signs-symbols/warnings/safety-hands.jpg.html • VINEGAR (DISTILLED WATER) http://freeclipartnow.com/household/chores/cleaners/vinegar.jpg.html • BURETTE http://commons.wikimedia.org/wiki/File:Burette.svg • BEAKER http://www.freeclipartnow.com/science/flasks-tubes/beaker-2.jpg.html • HYDROXIDE ION http://commons.wikimedia.org/wiki/File:Hydroxide-3D-vdW.png • THINK SIGN http://www.freeclipartnow.com/education/signs/think.jpg.html • CALCULATE CLIP ART http://www.freeclipartnow.com/education/supplies/ruler-and-calculator.jpg.html

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