Download Decomposition Reactions

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12. Chemical Decomposition
And Now for Some
Fractions
Student Instruction Sheet
Challenge
Determine the percent of H2O2 in store-bought hydrogen peroxide solution.
Equipment and Materials
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computer with USB port
(2) PASPORT USB interface
USB 4-Port Hub (optional)
PASPORT Absolute Pressure Sensor
PASPORT Temperature Sensor
DataStudio software
rubber stopper, two-hole
Erlenmeyer flask, 125-mL
graduated cylinder, 100-mL
• Balance
• manganese dioxide (MnO2), 0.2 g
• hydrogen peroxide (H2O2) solution from a
grocery store, 100.0 mL
• protective gear
• glycerin, 1 drop
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Student Instruction Sheet
•
Student Response Sheet
If your computer does not have two USB ports, you will
need a powered USB 4-port Hub.
Note:
Safety Precautions
Remember, follow the directions for using the equipment.
Wear safety glasses and follow standard laboratory
safety procedures.
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Background
In order to better study and understand the incredible variety of possible
chemical reactions, chemists classify reactions in a number of different ways.
One common classification scheme recognizes four major types of chemical
reactions:
1. combination or synthesis reactions
2. decomposition reactions
3. substitution or single replacement reactions
4. double displacement or double replacement reactions
Starting from this classification scheme, reactions can be further classified
according to the type of chemistry that occurs – acid-base neutralization, or
oxidation-reduction reactions, for example. In this lab activity, you will
investigate one example of a decomposition reaction. This type of reaction occurs
when a substance chemically breaks down to form new products. The general
formula for a decomposition reaction is:
AB → A + B
The reaction you will investigate in this experiment is the decomposition of
hydrogen peroxide. You may be familiar with this compound; hydrogen peroxide
solution can be commonly purchased in any pharmacy and it is found in most home
medicine cabinets. In fact, the reaction you will be studying takes place in the
bottle: hydrogen peroxide decomposes over time to form two products, liquid
water and gaseous oxygen. Normally this process occurs slowly, but hydrogen
peroxide solution is typically packaged in a dark-brown bottle to further limit the
rate of the decomposition reaction. In the exploration that follows, you will add a
catalyst, manganese dioxide, to the hydrogen peroxide in order to speed up the
decomposition reaction rate. You will be able to monitor the reaction’s progress
by using a pressure sensor to measure the oxygen gas that is produced as a result
of the reaction. By using your knowledge of chemical reactions, stoichiometry,
and the Ideal Gas Law, you will be able to calculate the percent concentration of
hydrogen peroxide in the store-bought solution.
You will treat the oxygen produced in this reaction as an ideal gas, defined as one
in which all collisions between atoms or molecules are perfectly elastic and in
which there are no intermolecular attractive forces. Try to visualize an ideal gas
as a collection of perfectly hard spheres that collide but that otherwise do not
interact with each other. The Ideal Gas Law describes a predictable relationship
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between the pressure, volume, and temperature of such a gas:
PV = nRT
In the Ideal Gas Law, P is absolute pressure (measured in kPa), V is volume (in L),
n is the number of moles, R is the universal gas constant (R = 8.3145 L kPa / mol
K), and T is absolute temperature (Kelvin).
Using the Ideal Gas Law will let you calculate the number of moles of gas inside
your flask before and after the reaction takes place. You will then be able to use
the stoichiometry of the reaction to predict the percent concentration of
hydrogen peroxide in the store-bought solution you will be testing.
Predict
Before beginning the eLab, complete the prediction and vocabulary portions of
the Student Response Sheet.
Explore
Computer Setup
1. Plug the USB interfaces into the computer’s USB ports or USB Hub.
2. Plug the Temperature Sensor
into one of the USB
interfaces. This will
automatically launch the
PASPORTAL window.
To computer
3. Plug the Absolute Pressure
Sensor into the other USB
interface.
To computer
4. Choose the appropriate
DataStudio configuration file entitled
12 Chemical Decomp CF.ds.
and proceed with the following instructions.
Configuration files automatically launch the appropriate
display(s), sampling rate(s), etc.
Note:
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Equipment Setup
1. Put a drop of glycerin on the barb end
of a quick-release coupler that comes
with the Absolute Pressure Sensor.
Insert the end of the coupler into
one end of a short piece (about 15.0
cm) of plastic tubing that also comes with the sensor.
2. Use a second drop of glycerin on the barb end of the other type of plastic
connector that comes with the sensor, and insert this connector into the
other end of the plastic tubing. Fit this side of the connector into one of the
holes in the two-hole rubber stopper.
3. Align the quick-release connector
on the other end of the plastic
tubing with the pressure port of
the Absolute Pressure Sensor.
Push the connector onto the port,
and then turn the connector clockwise until it clicks (about one-eighth turn).
4. Insert the Temperature Sensor into the second hole of the two-hole stopper.
If the sensor fits loosely in the hole, place some rubber tubing around the
probe. Ensure that the tip of the probe will not contact the liquid or foam—it
must measure the temperature of the gas inside the flask.
5. Use the balance and a piece of weighing paper to measure 0.20 grams of
maganese dioxide (MnO2).
6. Use the balance to find the mass of the empty Erlenmeyer flask. Record the
mass on your Student Response Sheet.
7. Pour 10.0 mL of hydrogen peroxide
solution into the 125-mL Erlenmeyer
flask. Determine the mass of the
flask containing the solution, and
record this value on the Student
Response Sheet.
Note: Do not insert the
Temperature Sensor too far
into the Erlenmeyer flask. The
stainless-steel probe should not get wet during the reaction.
You want to measure the temperature of the gas inside the
flask, not the liquid.
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8. Seat the rubber stopper holding the 2 sensors firmly in place into the neck of
the Erlenmeyer flask.
Record Data
1. Click the Start (
) button to begin recording the pressure and
temperature conditions inside the flask.
2. In order to start the reaction you will need to add the 0.20 g of manganese
dioxide (MnO2) to the flask as quickly and efficiently as possible. To do so,
fold or curve the weighing paper to form a funnel, remove the stopper from
the flask, and pour the MnO2 powder in quickly. Immediately replace the
stopper in the flask.
3. Hold down the stopper until the pressure stabilizes.
4. Click the Stop (
) button to end data collection.
Analyze
Record calculations in your Data Table on the Student Response Sheet as you
complete your analysis.
1. From the graph, find the initial pressure, final pressure, initial temperature,
and final temperature. Record them on the Student Response Sheet.
2. Save your DataStudio (on the File menu, click Save Activity As...) to the
location specified by your teacher.
3. Answer all the questions on the Student Response Sheet.
4. Follow your teacher’s instructions regarding cleaning up your work space.
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Student Response Sheet
Name: ________________________________
Date: ________________________________
And Now for Some Fractions
Vocabulary
Use available resources to find the definitions of the following terms:
catalyst: _____________________________________________________
___________________________________________________________
decomposition reaction: _________________________________________
___________________________________________________________
hydrogen peroxide (H2O2): _______________________________________
___________________________________________________________
ideal gas: ____________________________________________________
___________________________________________________________
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Predict
1. As described in the Background section of the Student Instruction Sheet,
hydrogen peroxide decomposes over time to form two products, liquid water
and gaseous oxygen. Write and balance the correct chemical equation for this
process in the space below.
___________________________________________________________
___________________________________________________________
___________________________________________________________
2. Based on the equation you wrote above, how do you expect the temperature
and pressure conditions inside the flask to change as the decomposition
reaction proceeds?
___________________________________________________________
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3. If a sealed air-tight container of an unknown gas contained n number of moles
of gas, and you doubled the number of moles while keeping the temperature
and volume constant, what would happen to the pressure? Explain your answer.
___________________________________________________________
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Data
a) Mass of empty flask
b) Mass of flask with H202 solution
c) Mass of H2O2 solution [(b) – (a)]
d) Initial temperature
e) Final temperature
f) Initial pressure
g) Final pressure
h) Calculated initial moles of gas
i) Calculated final moles of gas
j) Moles of O2 produced [(i) – (h)]
Analyze
1. Considering the volume of liquid used in this experiment, determine the volume
that is occupied by gas inside the flask.
Hint: This volume will be needed for additional
calculations below.
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2. For the experiment’s starting conditions, use the Ideal Gas Law to calculate
the initial number of moles of gas in the flask.
Hint: Remember to use correct units for all quantities.
___________________________________________________________
___________________________________________________________
___________________________________________________________
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___________________________________________________________
3. Use the Ideal Gas Law and the final temperature and pressure values to
calculate the number of moles of gas in the flask at the conclusion of the
experiment.
___________________________________________________________
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___________________________________________________________
4. Determine the number of moles of oxygen gas produced by the reaction, and
enter this value into the data table.
___________________________________________________________
___________________________________________________________
___________________________________________________________
5. Based on your analysis of the amount of oxygen gas produced, use the balanced
chemical equation for the decomposition of hydrogen peroxide to determine
the number of moles of hydrogen peroxide consumed.
Hint: Refer to question #1 from the Predict section
above.
___________________________________________________________
___________________________________________________________
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6. Determine the percent concentration of hydrogen peroxide in your original
sample solution:
a. Convert the amount of hydrogen peroxide (number of moles) to mass
(grams).
___________________________________________________________________________
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b. Divide the calculated mass of hydrogen peroxide present by the mass of
the original H202 solution. Multiply this ratio by 100 to obtain a calculated
value for the percentage of hydrogen peroxide in the solution.
___________________________________________________________________________
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Synthesize
1. According to the bottle’s label, what is the percent concentration of the
store-bought hydrogen peroxide solution you used?
___________________________________________________________________________
___________________________________________________________
___________________________________________________________
How does this value compare to your calculated percent concentration?
___________________________________________________________
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2. If your calculated value is different from the percentage listed on the bottle’s
label, what sources of error might account for the difference?
___________________________________________________________
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3. What was the purpose of the manganese dioxide (MnO2) used in the
experiment? Did it react with another substance? If so, what were the new
products formed?
___________________________________________________________
___________________________________________________________
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