Diprotic Titration: Multi-Step Chemical Reactions

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Diprotic Titration: Multi-Step Chemical Reactions
012-10738 r1.04
Diprotic Titration: Multi-Step Chemical Reactions
Introduction
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.
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.
Note: You may want to take a
snapshot of the first page of
this lab as a cover page for
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
H+ can be removed
• Acids are substances that have a hydrogen
ion that can be donated.
H+ can be added
to form H2O
• 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.
HNO3(aq) + H2O(l)
Acid
Base
NO3-(aq) + H3O+(aq)
Conjugate
base
Conjugate
acid
Diprotic Titration: Multi-Step Chemical Reactions
Self-Check
1. 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
...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
polyprotic
diprotic
2
H2SO3
polyprotic
triprotic
3
H3PO4
Diprotic Titration: Multi-Step Chemical Reactions
Self-Check
Specific Type of Acid
2. Nitric acid (HNO3) is best described as a
___________ acid.
a) triprotic
b) diprotic
c) polyprotic
d) monoprotic
Number of H+ Ions
to Donate
monoprotic
1
diprotic
2
triprotic
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-Check
3. 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
Equivalence
point
Diprotic Titration: Multi-Step Chemical Reactions
Self-Check
4. 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 SAFE
Always wash hands
to remove residue
before leaving
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
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
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
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:
a. 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:
a. Repeat the process used in calibration point
1 using a pH 10 buffer solution.
b. Tap OK to exit the calibration screen and
then tap OK again to return to the lab.
Diprotic Titration: Multi-Step Chemical Reactions
Setup
3. Connect the drop counter to the data collection
system.
4. Assemble the titration apparatus.
Buret clamp
Ring stand
Buret
pH sensor
Right-angle clamp
Drop counter
Micro stir bar
Beaker, 250-mL
Magnetic stirrer
Q1: Will a pH sensor that is
not calibrated give
precise results? Why
must the pH sensor be
calibrated?
Diprotic Titration: Multi-Step Chemical Reactions
Setup
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.
Off position
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?
Diprotic Titration: Multi-Step Chemical Reactions
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.
Q3:How can the green
light on the drop
counter help you
determine if the HCl is
falling in
distinguishable drops?
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
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.
Q5:Record the volume of
Na2CO3 added to the
beaker in the text box
below.
Diprotic Titration: Multi-Step Chemical Reactions
Collect Data
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.
Continue to the next page.
Diprotic Titration: Multi-Step Chemical Reactions
Q6:What reaction step is Q7:What begins to happen in
taking place at the
the beaker when the pH
beginning of the
of the solution is lower
titration between the
than 8? Explain these
starting pH (~12) and a
observations.
pH value of 8?
Diprotic Titration: Multi-Step Chemical Reactions
Collect Data
4. When the pH of the
solution stabilizes at a pH
less than 2, close the
stopcock.
Continue to the next page.
Diprotic Titration: Multi-Step Chemical Reactions
Collect Data
5. Tap
to stop data
collection.
6. Record the final volume of
HCl in the burette.
Diprotic Titration: Multi-Step Chemical Reactions
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.*
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.
Diprotic Titration: Multi-Step Chemical Reactions
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)
*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.
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 Analysis
5. 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
.
Diprotic Titration: Multi-Step Chemical Reactions
Data Analysis
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 + H2O
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
Diprotic Titration: Multi-Step Chemical Reactions
Data Analysis
7. Calculate the average molarity of Na2CO3
solution using the value from each
equivalence point. Show your work.
Equivalence
point
Equivalence
point
Diprotic Titration: Multi-Step Chemical Reactions
Analysis
1. When did the bubbles produced by the formation of
carbon dioxide gas start to become visible?
Diprotic Titration: Multi-Step Chemical Reactions
Analysis
2. 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
Analysis
3. 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
Analysis
4. 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
Analysis
5. 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
Analysis
6. Why did the bubbling stop after the second
equivalence point?
THINK!
Diprotic Titration: Multi-Step Chemical Reactions
Synthesis
1. 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
Synthesis
2. 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
Synthesis
3. Calculate the number of liters of carbon dioxide produced. Assume standard
temperature and pressure. Show your work.
Diprotic Titration: Multi-Step Chemical Reactions
Multiple Choice
1. 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 Choice
2. 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 Choice
3. 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
Multiple Choice
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
1
diprotic
2
triprotic
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
Congratulations!
You have completed the lab.
Please remember to follow your teacher's instructions for cleaning-up and submitting
your lab.
Chemistry Template
References
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.html
2.BEAKER http://freeclipartnow.com/science/flasks-tubes/beaker.jpg.html
3.CORROSIVE WARNING http://commons.wikimedia.org/wiki/Image:DIN_48442_Warnung_vor_Aetzenden_Stoffen_D-W004.svg
4.BE SAFE http://freeclipartnow.com/signs-symbols/warnings/safety-hands.jpg.html
5.VINEGAR (DISTILLED WATER) http://freeclipartnow.com/household/chores/cleaners/vinegar.jpg.html
6.BURETTE http://commons.wikimedia.org/wiki/File:Burette.svg
7.BEAKER http://www.freeclipartnow.com/science/flasks-tubes/beaker-2.jpg.html
8.HYDROXIDE ION http://commons.wikimedia.org/wiki/File:Hydroxide-3D-vdW.png
9.THINK SIGN http://www.freeclipartnow.com/education/signs/think.jpg.html
10.CALCULATE CLIP ART http://www.freeclipartnow.com/education/supplies/ruler-and-calculator.jpg.html

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