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

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?


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


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


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


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


...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)


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


...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


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


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


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


...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


...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


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.


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


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.


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.


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


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.


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.


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.


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.


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.


4. When the pH of the solution stabilizes at a pH less than 2, close the stopcock.

Collect Data

Continue to the next page.


Collect Data5. Tap to stop data

collection.

6. Record the final volume of HCl in the burette.


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.*


*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)


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.


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.


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.


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


Analysis1. When did the bubbles produced by the formation of

carbon dioxide gas start to become visible?


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)


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?


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?


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?


Analysis6. Why did the bubbling stop after the second

equivalence point? THINK!


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.


Synthesis2. Calculate the number of moles of carbon dioxide

that was produced. Show your work.

What was the limiting reagent?


Synthesis3. Calculate the number of liters of carbon dioxide produced. Assume standard

temperature and pressure. Show your work.


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


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


Multiple Choice3. How many equivalence points are there on a

titration of a diprotic acid?

a) 0

b) 1

c) 2

d) 3


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


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


Multiple Choice???

You have completed the lab.Congratulations!

Please remember to follow your teacher's instructions for cleaning-up and submitting your lab.


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

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