PUTTING IT ALL TOGETHER
11. Chemical Synthesis
Putting It All Together
Student Instruction Sheet
Challenge
Investigate a type of synthesis reaction, the formation of magnesium oxide, by
analyzing a series of equivalent reactions and determining the heat of combustion
for magnesium.
Equipment and Materials
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Computer with USB port
PASPORT USB interface
PASPORT Temperature Sensor
DataStudio software
Three-Finger Clamp
stirring rod
Balance
protective gear
Small Tripod Base & Rod
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beaker, 250-mL
1.0 M hydrochloric acid (HCl), 200.0 mL
magnesium oxide (MgO), 1.0 g
graduated cylinder, 100-mL
magnesium ribbon (Mg), 0.5 g
foam cup
weighing paper
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Student Instruction Sheet
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Student Response Sheet
Safety Precautions
Wear safety glasses and gloves and follow standard
laboratory safety procedures.
If any skin accidentally comes in contact with the
hydrochloric acid solution, immediately rinse the area
thoroughly with tap water.
Keep water away from electrical outlets, the computer,
the keyboard, and the PASPORT equipment.
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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
In this activity, you will study one example of a combination or synthesis reaction.
As the name suggests, a reaction of this type occurs when two or more
substances combine chemically to form a new product. The general formula for a
synthesis reaction is:
A + BC → ABC
The synthesis reaction you will be investigating follows the above general
formula:
A
1
Mg ( s ) + --O2 ( g ) → MgO ( s ) + Energy
2
Reaction A is the combustion or “burning” of magnesium.
You have probably witnessed an example of reaction A if you have ever seen a
photograph taken using flash powder. The reaction is very exothermic, producing
considerable heat and light, which creates the flash. Iron rusting, a common
combustion reaction that happens much more slowly, is also a synthesis reaction
similar to the one above.
To understand any chemical process, it is important to be able to quantify the
amount of energy change that occurs as a result of the reaction. This energy is
called the heat of reaction and is represented by the symbol ΔHrxn. By
convention, ΔHrxn for any exothermic reaction is given a negative sign, indicating
the reaction loses energy. ΔHrxn for any endothermic reaction is given a positive
sign, because the reaction gains energy from the surroundings. Looking at
equation A above, you can see that the formation of magnesium oxide results in
energy being released to the surrounding—but how much energy? The heat of
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reaction for such a combustion reaction is also called, logically, its heat of
combustion (in symbol form, ΔHC). Measuring this heat energy is often done
through calorimetry—for example, you could “burn” a strip of magnesium metal
and use the energy given off to heat a known quantity of water in a calorimeter
cup. This is a traditional method in chemistry for measuring the heat of a
reaction.
If a reaction happens too rapidly to make use of the calorimeter, such as the
synthesis of magnesium oxide in reaction A above, more indirect methods can be
used to calculate ΔHC. In this exploration you will rely on measuring the energy
released or absorbed during simpler chemical reactions to indirectly calculate
ΔHC for magnesium. As long as the other reactions lead to the same overall
outcome, then the energy of combustion can be calculated by mathematically
adding the amounts of energy released or absorbed by each of the other
reactions. Reactions must obey the law of conservation of energy just as they
must satisfy the law of conservation of mass!
Below are three reactions that are equivalent to the combustion of magnesium.
You will be asked to confirm that equations B, C, and D
are equivalent to equation A. See the Student Response Sheet
Note:
.
prediction section
B
MgCl 2 ( aq ) + H 2 O ( l ) + Energy → MgO ( s ) + 2HCl ( aq )
(Magnesium chloride and water react to form magnesium oxide and hydrochloric
acid.)
C
Mg ( s ) + 2HCl ( aq ) → MgCl 2 ( aq ) + H 2 ( g ) + Energy
(Magnesium metal and hydrochloric acid form magnesium chloride and hydrogen
gas.)
D
1
H 2 ( g ) + --O2 ( g ) → H 2 O ( l ) + Energy
2
(Hydrogen and oxygen form water.)
You will carry out two of the above reactions in the laboratory and measure the
resulting energy change each produces. Then, with some additional data provided
for reaction D, you will be asked to calculate the heat of combustion for
magnesium as represented by equation A.
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Predict
Before beginning the eLab, complete the prediction and vocabulary portions of
the Student Response Sheet.
Explore
Computer Setup
1. Plug the USB interface into the computer’s USB port.
To computer
2. Plug the Temperature Sensor
into the USB interface. This
will automatically launch the
PASPORTAL window.
3. Choose the appropriate
DataStudio configuration file
entitled
11 Chemical Synthesis CF.ds
and proceed with the following instructions.
Configuration files automatically launch the appropriate
display(s), sampling rate(s), etc.
Note:
Equipment Setup
1. Tare your balance to the mass of the weighing paper. Measure approximately
1.0 g of magnesium oxide (MgO) onto the piece of weighing paper. Record the
exact mass in your data table, and set the MgO powder aside for use in Part 1.
2. Use a second piece of weighing paper to measure 0.5 g of magnesium ribbon.
Record the exact mass in your data table, and set the Mg ribbon aside for use
in Part 2.
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11. Chemical Synthesis
3. Use a base and support rod
and a clamp to support a
Temperature Sensor as
shown.
PUTTING IT ALL TOGETHER
clamp
Temperature
Sensor
foam cup
4. Place a foam cup into a
250-mL beaker as shown in
the diagram. Measure and
pour 100 mL of 1.0 M
hydrochloric acid into the
foam cup.
beaker
Record Data
Part 1
React MgO with HCl (the reverse of reaction B)
1. Lower the Temperature Sensor into the acid solution, and click the Start
(
) button to begin recording the temperature.
2. After approximately 5 seconds, add the white magnesium oxide powder to the
acid in the foam cup. Observe the change in temperature on the Graph
Display.
3. Use the stirring rod to stir the contents of the cup until a maximum
temperature has been reached and the temperature starts to drop.
4. Click the Stop (
) button to end data collection.
5. Remove the Temperature Sensor from the cup and rinse the end of the probe.
6. Discard the solution as directed and rinse the cup.
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Part 2
React Mg ribbon with HCl (reaction C)
1. Measure and pour another 100.0 mL of 1.0 M hydrochloric acid into the foam
cup. Put it back into the beaker as it was in Part 1.
2. Lower the Temperature Sensor into the solution.
3. Click the Start button to begin recording the temperature.
4. After approximately five seconds, add the magnesium ribbon to the acid in the
foam cup. Observe the change in temperature on the Graph Display.
5. Use the stirring rod to stir the contents of the cup until a maximum
temperature has been reached and the temperature starts to drop.
6. Click the Stop button to end data collection.
7. Remove the Temperature Sensor from the cup and rinse the end of the
sensor.
8. Discard the solution as directed by your instructor and rinse the cup.
Analyze
1. The minimum and maximum temperatures for Run #1 and Run #2 are displayed
in the Legend box. Record these values on the Student Response Sheet.
2. Save your DataStudio file (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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11. Chemical Synthesis
Student Response Sheet
Name: ___________________________________
Date: ___________________________________
Putting It All Together
Vocabulary
Use available resources to find the definitions of the following terms:
combination (synthesis) reaction: ___________________________________
___________________________________________________________
combustion: ___________________________________________________
___________________________________________________________
heat of combustion (ΔHC): ________________________________________
___________________________________________________________
heat of reaction (ΔHrxn): _________________________________________
___________________________________________________________
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Predict
1. Add equations B, C, and D below by canceling like terms on the left and the
right. Write the net equation that results.
B
MgCl2 ( aq ) + H 2 O ( l ) + Energy → MgO ( s ) + 2HCl ( aq )
C
Mg ( s ) + 2HCl ( aq ) → MgCl 2 ( aq ) + H 2 ( g ) + Energy
D
1
H 2 ( g ) + --O 2 ( g ) → H 2 O ( l ) + Energy
2
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2. How are the heats of reaction for equations B, C, and D (ΔHrxnB, ΔHrxnC, and
ΔHrxnD) related to the heat of reaction for the net equation you wrote above?
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3. Consider a general synthesis reaction of the following type:
A + BC → ABC + Energy
a. Write an equation for the reverse of this reaction:
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b. How is ΔHrxn of the forward reaction related to ΔHrxn of the reverse
reaction? Which reaction is exothermic, which is endothermic, and how can
you tell?
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St uden t Response Sheet
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11. Chemical Synthesis
Data
Part 1 Reaction
(MgO + HCl)
Part 2 Reaction
(Mg + HCl)
Volume of 1.00 M HCl
Mass of solid (powder or ribbon)
Initial temperature, T1
Final temperature, T2
Change in temperature, ΔT
Analyze
1. Calculate the change in temperature, ΔT, for both reactions and complete the
data table above.
2. How do the chemical reactions you performed in the laboratory compare to
those represented by equations B, C, and D from the “Predict” section above?
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3. Calculate the energy released in each laboratory reaction using the formula:
Energy = C P × m × ΔT
Assume that CP = 4.18 J/g°C, and m = 100.0 g of HCl solution. Convert joules to
kJ in your final answer, and show your work in the space provided below.
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4. The heat of reaction, ΔHrxn, is commonly reported in units of kJ/mol. For the
reactions you carried out in part 1 and part 2, follow the steps outlined below
to calculate the amount of energy released per mole.
a. First determine the number of moles of magnesium oxide (MgO) and
magnesium ribbon (Mg) used.
Hint: Molar mass of MgO is 40.3 g/mol, and molar mass of Mg is 24.3 g/mol
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b. Divide the amount of energy calculated in question #3 above by the number
of moles to calculate the amount of energy released per mole (ΔHrxn) in
each reaction.
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Synthesize
1. Recall that by convention, ΔHrxn is designated with a (+) sign for endothermic
reactions and a (–) sign for exothermic reactions. Look carefully at the
calculations you made above and fill in the table below. (Note that the heat of
reaction for the formation of water has already been provided for you.)
Eqn
B
Reactants
Results
Heat of Reaction
MgO ( s ) + 2 HCl ( aq )
ΔHrxnB =
MgCl 2 ( aq ) + H 2 ( g )
ΔHrxnC =
MgCl 2 ( aq ) + H 2 O ( l )
+ Energy →
C
Mg ( s ) + 2HCl ( aq ) →
+ Energy
D
300
1
H 2 ( g ) + --O 2 ( g ) →
2
H 2 O ( l ) + Energy
St uden t Response Sheet
PS-2808
ΔHrxnD = –285.8 kJ/mol
PUTTING IT ALL TOGETHER
11. Chemical Synthesis
2. Based on the heats of reaction calculated above, calculate the heat of reaction
for the MgO synthesis reaction (ΔHrxnA). Show your work in the space
provided and record your answer in the table below.
refer to your answers from the “Predict” and “Analyze” sections above, and be
careful of positive and negative signs.
Hint:
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Eqn
A
Reactants
1
Mg ( s ) + --O2 ( g ) →
2
Results
Heat of Reaction
MgO ( s ) + Energy
ΔHrxnA = -580 kJ/mol
3. Propose an explanation for why the method you have used in this exploration
for calculating the heat of reaction is preferable to using calorimetry.
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4. Why can reaction A, the combustion of magnesium, be classified as a synthesis
reaction?
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5. Based on your data, is the combustion of magnesium endothermic or
exothermic? Explain.
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