Diprotic Titration: Multi-Step Chemical Reactions 012-10738 r1.04.
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Transcript of Diprotic Titration: Multi-Step Chemical Reactions 012-10738 r1.04.
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.
IntroductionJournals and Snapshots
Note: You may want to take asnapshot of the first page ofthis lab as a cover page for your Journal.
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.
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?
Diprotic Titration: Multi-Step Chemical Reactions
Background• Acids are substances that have a hydrogen
ion that can be donated.
• Bases are complements to acids in that bases accept hydrogen ions.
• When an acid gives up a hydrogen ion, the anionic portion left behind is called the conjugate base.
• When a base accepts a hydrogen ion, the newly created species is called the conjugate acid.
H+ can be removed
H+ can be added
to form H2O
HNO3(aq) + H2O(l) NO3-(aq) + H3O+(aq)
Acid Base Conjugatebase
Conjugateacid
Diprotic Titration: Multi-Step Chemical Reactions
Self-Check1. When an acid gives up a hydrogen ion, the anionic
portion left behind is called the _______________.
a) conjugate acid
b) conjugate base
c) diprotic acid
d) diprotic base
Diprotic Titration: Multi-Step Chemical Reactions
General Type of Acid
...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.
Specific Type of Acid
Number of H+ Ions to Donate
Example
monoprotic
polyprotic
polyprotic
monoprotic
diprotic
triprotic
1
2
3
HCl
H2SO3
H3PO4
Diprotic Titration: Multi-Step Chemical Reactions
Self-Check2. Nitric acid (HNO3) is best described as a
___________ acid.
a) triprotic
b) diprotic
c) polyprotic
d) monoprotic
Specific Type of Acid Number of H+ Ions to Donate
monoprotic
diprotic
triprotic
1
2
3
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)
Diprotic Titration: Multi-Step Chemical Reactions
Self-Check3. When do you expect to observe bubbles (formation
of a gas) during the reaction between sodium carbonate and hydrochloric acid?
a) during step one only
b) during step two only
c) during both step one and step two
d) no bubbles will form
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
Equivalencepoint
Diprotic Titration: Multi-Step Chemical Reactions
Self-Check4. How many equivalence points will there be on a
titration curve for a diprotic acid?
a) 1
b) 2
c) 3
d) 4
Diprotic titration curve
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 SAFEAlways wash hands to remove residue
before leaving
Diprotic Titration: Multi-Step Chemical Reactions
Materials and EquipmentCollect 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
Diprotic Titration: Multi-Step Chemical Reactions
...Materials and EquipmentAlso 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
Diprotic Titration: Multi-Step Chemical Reactions
...Materials and EquipmentAlso 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
waste Distilled Water
Diprotic Titration: Multi-Step Chemical Reactions
Sequencing Challenge
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.
C. Setup the titration equipment and calibrate the pH sensor.
D. Measure and record the starting volumes of sodium carbonate and HCl. Start collecting data and then release the titrant.
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.
Diprotic Titration: Multi-Step Chemical Reactions
Setup
1. Connect the pH sensor to the data collection system.
2. Calibrate the pH sensor:
a. Pour approximately 25 mL of the pH 4 buffer solution in a 50-mL beaker.
b. Pour approximately 25 mL of the pH 10 buffer solution in a second 50-mL beaker.
c. Remove the pH sensor from its electrode storage bottle and rinse it with distilled water to thoroughly clean the sensor.
d. Read the instructions on how to calibrate the pH sensor on the next page.
Buffer solutions and pH sensor
Diprotic Titration: Multi-Step Chemical Reactions
To Calibrate the pH Sensor: Note: During the calibration process you will not be able to return to this page.
1. Open the Calibrate Sensor screens: a. Tap b. Tap CALIBRATE SENSOR
2. Ensure that the correct measurements are selected:c. Sensor: (name of sensor)
Measurement: pH Calibration Type: 2 point
b. Tap NEXT
3. Calibration Point 1: a. Place the pH probe in a pH 4 buffer
solution. b. Enter 4.0 as the pH in the Standard
Value box under Calibration Point 1. c. Tap Read From Sensor under
Calibration Point 1. d. Rinse the pH probe thoroughly using
distilled water.
4. Calibration Point 2:e. Repeat the process used in calibration point
1 using a pH 10 buffer solution. f. Tap OK to exit the calibration screen and
then tap OK again to return to the lab.
Diprotic Titration: Multi-Step Chemical Reactions
Q1: Will a pH sensor that is not calibrated give precise results? Why must the pH sensor be calibrated?
Setup3. Connect the drop counter to the data collection
system.
4. Assemble the titration apparatus.
Diprotic Titration: Multi-Step Chemical Reactions
Buret clamp
Ring stand
Right-angle clamp
Drop counter
Beaker, 250-mL
Buret
pH sensor
Micro stir bar
Magnetic stirrer
5. Use a disposable pipette to rinse the inside of the burette with several milliliters of the standardized hydrochloric acid (HCl) solution.
6. Drain the HCl from the burette into the waste container.
7. Repeat this process two more times.
8. 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).
9. Drain a small amount of titrant through the drop counter into the waste container.
Q2:Why is it necessary to drain a small amount of titrant through the drop counter before you begin a titration?
Setup
Off position
Diprotic Titration: Multi-Step Chemical Reactions
Q3:How can the green light on the drop counter help you determine if the HCl is falling in distinguishable drops?
Setup
10.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.
11.Close the stopcock and then remove the waste container.
Diprotic Titration: Multi-Step Chemical Reactions
Setup
12.Read the initial volume of the titrant in the burette to 0.01 mL.
13.Using the 100-mL graduated cylinder measure 100.0 mL of distilled water and add it to a clean 250-mL beaker.
14.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.
Q4:Record the initial volume of HCl and the concentration of the HCl solution in the text box below.
Diprotic Titration: Multi-Step Chemical Reactions
Q5:Record the volume of Na2CO3 added to the beaker in the text box below.
Setup
15.Add the pH sensor, micro stir bar, and 250-mL beaker containing the Na2CO3 solution to the titration apparatus.
16.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.
Diprotic Titration: Multi-Step Chemical Reactions
1. Tap to start collecting data.
2. Carefully open the stopcock on the burette so that 2 to 3 drops per second are released.
3. Continue to collect data until the pH of the solution stabilizes at a pH less than 2.
Collect Data
Continue to the next page.
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.
Diprotic Titration: Multi-Step Chemical Reactions
4. When the pH of the solution stabilizes at a pH less than 2, close the stopcock.
Collect Data
Continue to the next page.
Diprotic Titration: Multi-Step Chemical Reactions
Collect Data5. Tap to stop data
collection.
6. Record the final volume of HCl in the burette.
Diprotic Titration: Multi-Step Chemical Reactions
a. Enter the final volume of HCl in the text box on the right.
b. Record the initial volume of HCl in the text box on the right.
c. Subtract the initial volume of HCl from the final volume of HCl and record the total volume in the text box on the right.
* To View a Page in the Journal: 1. Tap to open the Journal
screen. 2. Tap or to scroll
through the thumbnail size pages of the journal.
3. Tap the thumbnail image of the page to view it.
Data Analysis
1. 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.*
Diprotic Titration: Multi-Step Chemical Reactions
*To Create a Calculation:1. Tap to open the Experiment
Tools screen. 2. Tap CALCULATED DATA to open
the calculator. 3. Enter the calculation in the
space provided. 4. Tap Measurements to insert
collected data into the calculation.Note:
[drop count] = the collected datavolume 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)
Data Analysis
2. Create a calculation to convert drop count to volume (mL). *
CalcVolume = [drop count]*(volume of titrant/ final drop count)
Diprotic Titration: Multi-Step Chemical Reactions
3. Find the slope and volume at the first equivalence point (point with the greatest slope).*
*To Find an Equivalence Point:1. Tap then to open the
Graph Properties screen. 2. Tap the Measurement box for the
x-axis and tap CalcVolume. Then tap OK. Tap to scale the axes.
3. Tap then tap a data point above and below the steepest region of the graph.
4. Tap and then tap to display the slope and the coordinate of the data point.
5. Tap and until the slope is the greatest.
6. Tap to select the equivalence point.
Diprotic Titration: Multi-Step Chemical Reactions
4. Find the slope and volume at the second equivalence point (point with the greatest slope).*
*To Find an Equivalence Point:1. Tap then to open the
Graph Properties screen. 2. Tap the Measurement box for the
x-axis and tap CalcVolume. Then tap OK. Tap to scale the axes.
3. Tap then tap a data point above and below the steepest region of the graph.
4. Tap and then tap to display the slope and the coordinate of the data point.
5. Tap and until the slope is the greatest.
6. Tap to select the equivalence point.
Diprotic Titration: Multi-Step Chemical Reactions
Data Analysis5. Calculate the molar concentration of Na2CO3 at the first equivalence point:
a. 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.
b. Convert from moles of HCl to moles of Na2CO3 using the balanced chemical equation for the first reaction step: HCl + Na2CO3 NaHCO3 + NaCl
c. Use the moles of Na2CO3 and the starting volume of Na2CO3 to determine molarity of the Na2CO3.
Diprotic Titration: Multi-Step Chemical Reactions
Note: the values need for the calculations on this page can be found in your journal .
6. Calculate the molar concentration of Na2CO3 at the second equivalence point:a. Determine the number of moles of HCl added using the volume of HCl added
at the second equivalence point and the molarity of the HCl solution. b. Convert from moles of HCl to moles of Na2CO3 using the balanced chemical
equation for the overall reaction: 2 HCl + Na2CO3 CO2 + 2 NaCl + H2Oc. Use the moles of Na2CO3 and the starting volume of Na2CO3 to determine
molarity of the Na2CO3.
Data AnalysisDiprotic Titration: Multi-Step Chemical Reactions
Note: the values need for the calculations on this page can be found in your journal
Data Analysis7. Calculate the average molarity of Na2CO3
solution using the value from each equivalence point. Show your work.
Equivalence point
Equivalencepoint
Diprotic Titration: Multi-Step Chemical Reactions
Analysis1. When did the bubbles produced by the formation of
carbon dioxide gas start to become visible?
Diprotic Titration: Multi-Step Chemical Reactions
Analysis2. 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)
Diprotic Titration: Multi-Step Chemical Reactions
Analysis3. 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?
Diprotic Titration: Multi-Step Chemical Reactions
Analysis4. 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?
Diprotic Titration: Multi-Step Chemical Reactions
Analysis5. The reaction stopped bubbling at the second equivalence
point. What do you notice about the volume of acid needed to reach each equivalence point?
Diprotic Titration: Multi-Step Chemical Reactions
Analysis6. Why did the bubbling stop after the second
equivalence point? THINK!
Diprotic Titration: Multi-Step Chemical Reactions
Synthesis1. 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.
Diprotic Titration: Multi-Step Chemical Reactions
Synthesis2. Calculate the number of moles of carbon dioxide
that was produced. Show your work.
What was the limiting reagent?
Diprotic Titration: Multi-Step Chemical Reactions
Synthesis3. Calculate the number of liters of carbon dioxide produced. Assume standard
temperature and pressure. Show your work.
Diprotic Titration: Multi-Step Chemical Reactions
Multiple Choice1. In an acid-base titration, the equivalence point of
the curve shows ___________.
a) the point where there are the same number of moles of acid and base.
b) the point where the burette has run out of acid.
c) the point where pH is a maximum.
d) the point where pH is a minimum.
Equivalence point
Titration Curve
Diprotic Titration: Multi-Step Chemical Reactions
Multiple Choice2. Carbonic acid is known to be a diprotic acid.
Why?
a) Because it reacts with a base.
b) Because it has two hydrogen ions that can be donated.
c) Because it makes carbon dioxide.
d) Because it can act as an acid or a base.
H2CO3
Diprotic Titration: Multi-Step Chemical Reactions
Multiple Choice3. How many equivalence points are there on a
titration of a diprotic acid?
a) 0
b) 1
c) 2
d) 3
Diprotic Titration: Multi-Step Chemical Reactions
4. The titration curve below represents a ________ acid.
a) monoprotic
b) diprotic
c) triprotic
d) tetraprotic
Specific Type of Acid Number of H+ Ions to Donate
monoprotic
diprotic
triprotic
1
2
3
Diprotic Titration: Multi-Step Chemical Reactions
Multiple Choice
5. What gas is released when an acid reacts with sodium carbonate?
a) hydrogen gas
b) oxygen gas
c) methane gas
d) carbon dioxide gas
Diprotic Titration: Multi-Step Chemical Reactions
Multiple Choice???
You have completed the lab.Congratulations!
Please remember to follow your teacher's instructions for cleaning-up and submitting your lab.
Diprotic Titration: Multi-Step Chemical Reactions
ReferencesChemistry Template
All images were taken from PASCO documentation, public domain clip art, or Wikimedia Foundation Commons.
1.CUP WITH GAS http://www.freeclipartnow.com/food/beverages/soda/soft-drink-icon.jpg.html2.BEAKER http://freeclipartnow.com/science/flasks-tubes/beaker.jpg.html3.CORROSIVE WARNING http://commons.wikimedia.org/wiki/Image:DIN_4844-2_Warnung_vor_Aetzenden_Stoffen_D-W004.svg4.BE SAFE http://freeclipartnow.com/signs-symbols/warnings/safety-hands.jpg.html5.VINEGAR (DISTILLED WATER) http://freeclipartnow.com/household/chores/cleaners/vinegar.jpg.html6.BURETTE http://commons.wikimedia.org/wiki/File:Burette.svg7.BEAKER http://www.freeclipartnow.com/science/flasks-tubes/beaker-2.jpg.html8.HYDROXIDE ION http://commons.wikimedia.org/wiki/File:Hydroxide-3D-vdW.png9.THINK SIGN http://www.freeclipartnow.com/education/signs/think.jpg.html10.CALCULATE CLIP ART http://www.freeclipartnow.com/education/supplies/ruler-and-calculator.jpg.html