Lab 7 1x4a2d
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Resonance Energy of Naphthalene by Bomb Calorimetry
Written By: Elizabeth Swisher Partners: Amethyst Vozar, Michael Clear, Brittany Yang
Abstract: The main objectives of this lab are to determine the heat of combustion of naphthalene and compare it to the literature value; to calculate the theoretical heat of combustion of solid naphthalene using bond energies, the heat of sublimation of naphthalene, and the heat of vaporization of water; and to relate the heat of combustion of naphthalene determined from bond energies to the literature value and discuss the reason for the differences as well as what this difference tells us about the resonance energy in naphthalene. An oxygen bomb calorimeter was used in this lab to measure the standard enthalpy of combustion of naphthalene. The heat of combustion of naphthalene was experimentally found to be -5186.749
The literature value was found on the NIST Chemistry Website
and was determined to be – 5160.2
⁄
.
Introduction: The study of the heat produced or required by chemical reactions is called thermochemistry. It involves the measurement of temperature changes that results from the evolution of heat during the course of the reaction. The changes in internal energy or enthalpy for chemical reactions can be determined from such measured temperature changes. These values can then be used to gain understanding into the nature of the chemical bonding in the compounds involved in the reaction. Experimental heats of combustion are usually determined in a bomb calorimeter. The amount of heat generated from the complete combustion of the standard sample- benzoic acid- and the partial burning of the zinc fuse wire is calculated from their known heats of combustion. The total
2
heat generated by the sample and the fuse is equivalent to
. Knowing this, equation one
was found to be: ( ) Where
[(
)
(
)]
is heat of combustion of the sample or fuse, in J/g, m is mass of the sample
combusted,
is difference between the initial mass of fuse and final mass of fuse after
combustion, and
is difference between final and initial temperature of water. The value
found for the heat capacity of the calorimeter is now a known number and can be used in the calculations for naphthalene because the same calorimeter was used, so the heat capacity will be the same. By using equation 2 and this known value the heat of combustion of naphthalene can be found:
Where
is the heat of capacity for the calorimeter found in equation 1. By multiplying this
value by the molar mass of naphthalene, the molar heat combustion of naphthalene can be found. This is an important conversion because when finding the experimental enthalpy of combustion shown in equation 3, the molar heat combustion is needed: ( )
This value can be compared to the literature value of the enthalpy of combustion to check to see if the experiment was done successfully. The heat of combustion can finally be found theoretically. Using the chemical equation of the combustion of naphthalene and Hess’s Law as shown below by equation 4 and 5, respectively, can do this:
3
(
)
(
)
Experimental Description: A benzoic acid (C7H6O2) tablet was weighed and was 0.9808g. The tablet was then placed into the combustion pan and a 10 cm zinc wire was measured and placed into the eyelet of each electrode and firmly touched the benzoic acid tablet. The wire could not touch the metal pan because the fuse would short out before the sample would be able to combust. Next, 1 mL of distilled water was put below the combustion pan in the bomb to saturate the atmosphere in the bomb with water vapor, so that the water produced by the combustion will all be in the liquid state. The bomb was then closed. The bomb was then purged with oxygen gas multiple times to make certain that all other atmospheric gases were out of the bomb and only oxygen gas was left. The oxygen- purged bomb was then placed in the calorimeter and filled with 2 mL of deionized water. The exact same amount of water needed to be used for both the benzoic acid and naphthalene experiment because the heat capacity of the calorimeter needs to be constant to be able to find the heat of combustion for naphthalene (C10H8). The lid was put on the colorimeter and the belt for the stirrer was attached to the stirrer motor. This procedure was done exactly the same for naphthalene, except for making a naphthalene tablet using a tablet press that resulted in a tablet weighing 0.6033g.
4
Figure 1. Shows a cross-sectional image of the oxygen bomb calorimeter used. Some important parts pointed out are 1- the thermometer, 12- the stirrer, 8 - the belt for the stirrer, 6- the motor for the belt, and 15- the bomb. Obviously, all parts of the apparatus are essential, but the list just mentioned is of utmost importance.
Discussion of Results: The bomb calorimeter produced a list of values for temperature and its corresponding time associated with the combustions. Comparing the two chemicals being combusted, it would be assumed that the resonance energy of naphthalene (61 kcal/mol) would be greater than benzene (36 kcal/mol) because the more rings a compound has, the higher its resonance energy should be.
5
Figure 2. Shows the temperature vs. time graph for the combustion on benzoic acid in the oxygen bomb calorimeter. The equations above the pre- and post- ignition line titles at the bottom of the graph correspond with the slope of the line it is above. The t60% also marked on the graph is the point 60% of the ignition is done.
Figure 3. Shows the temperature vs. time graph for the combustion on naphthalene in the oxygen bomb calorimeter. The equations above the pre- and post- ignition line titles at the
6
bottom of the graph correspond with the slope of the line it is above. The t60% also marked on the graph is the point 60% of the ignition is done. Both of these graphs show the pre- ignition part of the experiment highlighted in red, where the combustion has not yet happened. The ignition portion of the graph highlighted in blue shows the time in which the combustion was happening. Last, the post ignition portion of the graph is highlighted in green, which is the part in which the combustion has ended which is known by observing that the temperature stops increasing and levels out. The instantaneous values for the temperatures associated with the pre- and post- ignition periods can be found by using the slope equations mentioned below each graph. The t60 value was plugged in for x to find the instantaneous temperatures for each graph. These solutions are summarized below.
Table 1. Shows the instantaneous values of the initial and final temperatures of ignition of the benzoic acid and naphthalene combustions. Their corresponding uncertainties are listed after the found temperatures, expressed by . Benzoic Acid
Naphthalene
Instantaneous
Instantaneous
Instantaneous
Instantaneous
Ti
Tf
Ti
Tf
26.178
28.0025
25.4925
27.5933
7
Table 2. Shows the value found for the heat capacity for the calorimeter, the heat of combustion of naphthalene and the molar heat of combustion of naphthalene. Their corresponding uncertainties are listed after the found values, expressed by . Unc. Of
Unc. Of
0.092114 -10,479.660 J/K
0.091933 J/K
-40.429 kJ/g
kJ/g
-5181.785 kJ/mol
0.092114 kJ/g
Table 3. Shows the enthalpy of combustion for gaseous naphthalene calculated using Hess’s law, the experimental enthalpy of combustion of naphthalene found by using equation 3, the enthalpy of combustion given by the NIST Chemistry Webbook, the enthalpy of combustion for solid naphthalene also using Hess’s law, and finally the absolute value of the difference of the theoretical combustion of solid naphthalene and the literature value for the enthalpy of combustion. The corresponding uncertainty for the experimental is also shown. Unc. Of
Absolute Value of
( )
( )
-
( )
-5352 kJ/mol
-5186.749 kJ/mol
1.0071 kJ/mol
-5160.2 kJ/mol
-5442.28 kJ/mol
262.08 kJ/mol
Finding the difference between the theoretical and literature value for the enthalpy of combustion is important to be able to see how well the experiment went and to make sure the information we received form the experiment is reliable. The uncertainty in temperature is a major, if not the major source of error in the final results of the experimental
. The short calculation below shows how much the
would change if the
temperature was increased by 0.1K:
8
( )
(
)
By this small change, the
(
)(
)(
)
decreased by 9.965 kJ/mol. There is a huge error in the change
of temperature so it is imperative to run the experiment as perfect as possible. Conclusion: The heat of combustion of naphthalene was experimentally found to be -5186.749
The literature value was found on the NIST Chemistry Website ⁄
and was determined to be – 5160.2
. This is important to compare these two
values to make sure the data retrieved from the experiment is reliable. Also, the bond energies were used to find the theoretical heat of combustion of solid naphthalene using the heat of vaporization of water, which was found to be -5442.28 kJ/mol.
Sample Calculations: ( )
[(
( )
(
) (
( )
)(
)(
)] (
)(
)
)
9
(
(
)
)
(
)
(
)
(
)
( )
(
( )
)
( )
( )
( )
()
( )
( )
( )
(
)(
( )
( )
Absolute Value of the difference of
)(
)
()
()
( )
-
:
( )
10
Error Analysis: Instantaneous Temperatures:
11
Acknowledgements: Thank to Alec for helping us with this lab even though he had a baby on the way! Congrats!!
Literature Cited: 1. Dr. Bratoljub H. Milosavljevic Lab Packet for Chem 457 Experimental Physical Chemistry Spring, 2014. 2. http://webbook.nist.gov 3. http://en.wikipedia.org/wiki/Naphthalene 4. http://en.wikipedia.org/wiki/Benzoic_acid 5. http://en.wikipedia.org/wiki/Calorimeter
12
Written By: Elizabeth Swisher Partners: Amethyst Vozar, Michael Clear, Brittany Yang
Abstract: The main objectives of this lab are to determine the heat of combustion of naphthalene and compare it to the literature value; to calculate the theoretical heat of combustion of solid naphthalene using bond energies, the heat of sublimation of naphthalene, and the heat of vaporization of water; and to relate the heat of combustion of naphthalene determined from bond energies to the literature value and discuss the reason for the differences as well as what this difference tells us about the resonance energy in naphthalene. An oxygen bomb calorimeter was used in this lab to measure the standard enthalpy of combustion of naphthalene. The heat of combustion of naphthalene was experimentally found to be -5186.749
The literature value was found on the NIST Chemistry Website
and was determined to be – 5160.2
⁄
.
Introduction: The study of the heat produced or required by chemical reactions is called thermochemistry. It involves the measurement of temperature changes that results from the evolution of heat during the course of the reaction. The changes in internal energy or enthalpy for chemical reactions can be determined from such measured temperature changes. These values can then be used to gain understanding into the nature of the chemical bonding in the compounds involved in the reaction. Experimental heats of combustion are usually determined in a bomb calorimeter. The amount of heat generated from the complete combustion of the standard sample- benzoic acid- and the partial burning of the zinc fuse wire is calculated from their known heats of combustion. The total
2
heat generated by the sample and the fuse is equivalent to
. Knowing this, equation one
was found to be: ( ) Where
[(
)
(
)]
is heat of combustion of the sample or fuse, in J/g, m is mass of the sample
combusted,
is difference between the initial mass of fuse and final mass of fuse after
combustion, and
is difference between final and initial temperature of water. The value
found for the heat capacity of the calorimeter is now a known number and can be used in the calculations for naphthalene because the same calorimeter was used, so the heat capacity will be the same. By using equation 2 and this known value the heat of combustion of naphthalene can be found:
Where
is the heat of capacity for the calorimeter found in equation 1. By multiplying this
value by the molar mass of naphthalene, the molar heat combustion of naphthalene can be found. This is an important conversion because when finding the experimental enthalpy of combustion shown in equation 3, the molar heat combustion is needed: ( )
This value can be compared to the literature value of the enthalpy of combustion to check to see if the experiment was done successfully. The heat of combustion can finally be found theoretically. Using the chemical equation of the combustion of naphthalene and Hess’s Law as shown below by equation 4 and 5, respectively, can do this:
3
(
)
(
)
Experimental Description: A benzoic acid (C7H6O2) tablet was weighed and was 0.9808g. The tablet was then placed into the combustion pan and a 10 cm zinc wire was measured and placed into the eyelet of each electrode and firmly touched the benzoic acid tablet. The wire could not touch the metal pan because the fuse would short out before the sample would be able to combust. Next, 1 mL of distilled water was put below the combustion pan in the bomb to saturate the atmosphere in the bomb with water vapor, so that the water produced by the combustion will all be in the liquid state. The bomb was then closed. The bomb was then purged with oxygen gas multiple times to make certain that all other atmospheric gases were out of the bomb and only oxygen gas was left. The oxygen- purged bomb was then placed in the calorimeter and filled with 2 mL of deionized water. The exact same amount of water needed to be used for both the benzoic acid and naphthalene experiment because the heat capacity of the calorimeter needs to be constant to be able to find the heat of combustion for naphthalene (C10H8). The lid was put on the colorimeter and the belt for the stirrer was attached to the stirrer motor. This procedure was done exactly the same for naphthalene, except for making a naphthalene tablet using a tablet press that resulted in a tablet weighing 0.6033g.
4
Figure 1. Shows a cross-sectional image of the oxygen bomb calorimeter used. Some important parts pointed out are 1- the thermometer, 12- the stirrer, 8 - the belt for the stirrer, 6- the motor for the belt, and 15- the bomb. Obviously, all parts of the apparatus are essential, but the list just mentioned is of utmost importance.
Discussion of Results: The bomb calorimeter produced a list of values for temperature and its corresponding time associated with the combustions. Comparing the two chemicals being combusted, it would be assumed that the resonance energy of naphthalene (61 kcal/mol) would be greater than benzene (36 kcal/mol) because the more rings a compound has, the higher its resonance energy should be.
5
Figure 2. Shows the temperature vs. time graph for the combustion on benzoic acid in the oxygen bomb calorimeter. The equations above the pre- and post- ignition line titles at the bottom of the graph correspond with the slope of the line it is above. The t60% also marked on the graph is the point 60% of the ignition is done.
Figure 3. Shows the temperature vs. time graph for the combustion on naphthalene in the oxygen bomb calorimeter. The equations above the pre- and post- ignition line titles at the
6
bottom of the graph correspond with the slope of the line it is above. The t60% also marked on the graph is the point 60% of the ignition is done. Both of these graphs show the pre- ignition part of the experiment highlighted in red, where the combustion has not yet happened. The ignition portion of the graph highlighted in blue shows the time in which the combustion was happening. Last, the post ignition portion of the graph is highlighted in green, which is the part in which the combustion has ended which is known by observing that the temperature stops increasing and levels out. The instantaneous values for the temperatures associated with the pre- and post- ignition periods can be found by using the slope equations mentioned below each graph. The t60 value was plugged in for x to find the instantaneous temperatures for each graph. These solutions are summarized below.
Table 1. Shows the instantaneous values of the initial and final temperatures of ignition of the benzoic acid and naphthalene combustions. Their corresponding uncertainties are listed after the found temperatures, expressed by . Benzoic Acid
Naphthalene
Instantaneous
Instantaneous
Instantaneous
Instantaneous
Ti
Tf
Ti
Tf
26.178
28.0025
25.4925
27.5933
7
Table 2. Shows the value found for the heat capacity for the calorimeter, the heat of combustion of naphthalene and the molar heat of combustion of naphthalene. Their corresponding uncertainties are listed after the found values, expressed by . Unc. Of
Unc. Of
0.092114 -10,479.660 J/K
0.091933 J/K
-40.429 kJ/g
kJ/g
-5181.785 kJ/mol
0.092114 kJ/g
Table 3. Shows the enthalpy of combustion for gaseous naphthalene calculated using Hess’s law, the experimental enthalpy of combustion of naphthalene found by using equation 3, the enthalpy of combustion given by the NIST Chemistry Webbook, the enthalpy of combustion for solid naphthalene also using Hess’s law, and finally the absolute value of the difference of the theoretical combustion of solid naphthalene and the literature value for the enthalpy of combustion. The corresponding uncertainty for the experimental is also shown. Unc. Of
Absolute Value of
( )
( )
-
( )
-5352 kJ/mol
-5186.749 kJ/mol
1.0071 kJ/mol
-5160.2 kJ/mol
-5442.28 kJ/mol
262.08 kJ/mol
Finding the difference between the theoretical and literature value for the enthalpy of combustion is important to be able to see how well the experiment went and to make sure the information we received form the experiment is reliable. The uncertainty in temperature is a major, if not the major source of error in the final results of the experimental
. The short calculation below shows how much the
would change if the
temperature was increased by 0.1K:
8
( )
(
)
By this small change, the
(
)(
)(
)
decreased by 9.965 kJ/mol. There is a huge error in the change
of temperature so it is imperative to run the experiment as perfect as possible. Conclusion: The heat of combustion of naphthalene was experimentally found to be -5186.749
The literature value was found on the NIST Chemistry Website ⁄
and was determined to be – 5160.2
. This is important to compare these two
values to make sure the data retrieved from the experiment is reliable. Also, the bond energies were used to find the theoretical heat of combustion of solid naphthalene using the heat of vaporization of water, which was found to be -5442.28 kJ/mol.
Sample Calculations: ( )
[(
( )
(
) (
( )
)(
)(
)] (
)(
)
)
9
(
(
)
)
(
)
(
)
(
)
( )
(
( )
)
( )
( )
( )
()
( )
( )
( )
(
)(
( )
( )
Absolute Value of the difference of
)(
)
()
()
( )
-
:
( )
10
Error Analysis: Instantaneous Temperatures:
11
Acknowledgements: Thank to Alec for helping us with this lab even though he had a baby on the way! Congrats!!
Literature Cited: 1. Dr. Bratoljub H. Milosavljevic Lab Packet for Chem 457 Experimental Physical Chemistry Spring, 2014. 2. http://webbook.nist.gov 3. http://en.wikipedia.org/wiki/Naphthalene 4. http://en.wikipedia.org/wiki/Benzoic_acid 5. http://en.wikipedia.org/wiki/Calorimeter
12
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