The Bomb Calorimetry Experiment

TOJET: The Turkish Online Journal of Educational Technology – December 2016, Special Issue for INTE 2016

Teaching An Undergraduate Thermochemistry Lab Session In A Safe And Easy Way: The Bomb Calorimetry Experiment Samir EL HAJJAJI School of Science and Engineering Al Akhawayn University in Ifrane, Moroccos [email protected]

Abdelkrim OUARDAOUI School of Science and Engineering Al Akhawayn University in Ifrane, Moroccos [email protected]

ABSTRACT This paper reports a hands-on, laboratory-based experience to tackle non-chemistry major engineering students’ lack of motivation and understanding in thermochemistry. This approach consisted in devising and setting up a safe and feasible laboratory experiment dealing with bomb calorimetry, in such a way that any student can grasp the basic thermochemistry concepts introduced. Students’ feedback as well as laboratory reports were appropriately collected and assessed to verify that the intended outcomes were achieved. Results were very promising as most of students declared that they managed and they observed an increase in their interest and motivation towards thermochemistry. INTRODUCTION Topics in introductory thermodynamics, and thermochemistry in particular, are taught to most of our undergraduate science students in the framework of a General Chemistry course. The student population studied in this work is composed of undergraduate, non-chemistry major, engineering students.1 These students are enrolled at Al Akhawayn University in Ifrane, a Moroccan university following a liberal arts education curriculum based on the American system.2 As part of their curriculum, these students are required to take a onesemester General Chemistry I course followed by another semester of General Chemistry II that can be taken as science elective course. In both semesters aspects of thermochemistry are covered. Over the years it became evident that the motivation and interest of students for the subjects were not satisfactory. In this regard, many studies have been carried out and determined factors impacting teaching and learning of physical chemistry3-6 and electrochemistry7-8 at the university level. Towns et al.3 concluded in their review that professors should take into consideration chemistry, physics, engineering, and mathematics when performing research on their student’s performance in, and understanding of, thermodynamics. Finally, studies led independently by Sözbilir and Ouardaoui et al. identified several factors to be major problems experienced by students in either physical chemistry9 or environmental chemistry,10 two of which drew our attention and we focused upon: the abstract nature of concepts and the lack of student motivation. THE STUDY In a dual attempt to promote student’s interest for the subject and augment their understanding of thermochemistry topics studied in class, we took a hands-on approach by introducing a laboratory experiment dealing with calorimetry. Indeed, abstract concepts such as the first law, enthalpy, heat capacity and an adiabatic system could thanks to this experiment be made concrete to students, thus enabling a better integration of the material covered in lectures. The experiment is titled “bomb calorimetry” and introduces various concepts covered in the classroom one week beforehand. These concepts are those of enthalpy of combustion, the first law of thermodynamics, an adiabatic system, and the heat capacity of a calorimeter among others. The purpose of the experiment is to determine experimentally the enthalpy of combustion of a refined vegetable oil, by using a Parr 1341 oxygen bomb calorimeter. The experiment is made up of two parts; the first part consists in determining the heat capacity of the calorimeter, Ccal. This is done by placing a 1.00 g pellet of calorific grade benzoic acid in the sample cup of the

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bomb head. Next, a 10cm long fuse wire is attached to the two electrode terminals and bent sufficiently to be in near contact with the surface of the pellet. Then, the head is assembled into the bomb vessel to which was added 1mL of distilled water. Finally, it is sealed hand tight before being purged and loaded with medical grade oxygen at a pressure of about 25 atm. The bomb vessel is then gently placed in a bucket containing 2000.00g of water.! The jacket top is installed on top of the bucket, the stirrer is turned on and a digital thermometer with a precision of 0.1°C is immersed in the water bath. The system is allowed to equilibrate for a period of 5-10 minutes, until a stable temperature is reached. The bomb was fired after connecting to a 19V voltage source to the two external electrode leads which were plugged into the terminals on top of the bomb. For safety purposes, from this point, no one, students or professor alike, should stand over the calorimeter or approach the calorimeter within 1.5 meters until the end of the experiment. A rise in temperature, ∆T1, in the range of 2-3°C, should ensue from the firing of the bomb within 20 seconds. The temperature is monitored and recorded to the nearest 0.1°C in the lab report at 30 seconds intervals with a digital thermometer until five successive readings are exactly the same. Once the reaction completed, the bomb is gently removed from the bucket and combustion gases are vented over a period of one minute through the exhaust valve into the fume hood. Students are then required to determine the rise in temperature ∆T1 by plotting on graph paper the temperature recorded as a function of time. The typical graph obtained is shown in Graph 1.

25,0 Temperature (°C)

24,5 24,0 ΔT = 2.6°C

23,5 23,0 22,5 22,0 Time (min)

Graph 1: Temperature vs. time graph for the combustion of benzoic acid. A good estimate of ∆T1 is obtained by calculating the difference between the highest and lowest temperatures attained. The heat capacity of the calorimeter can then be calculated by using the formula:11 C cal (kJ / °C ) = −ΔHcombbenzoic acid (kJ / g )) ×

m sample ( g ) ΔT1 (°C )

The second part of the experiment consists in reproducing the exact same procedure carried out in the first part, but with a sample of 1.00 g of refined vegetable oil instead of benzoic acid. A new rise in temperature ∆T2 is determined which allows students to calculate the molar enthalpy of combustion of the oil sample by using the following formula:11 ΔHcomboil (kJ / mol ) = −C cal (kJ / °C ) ×

ΔT2 (°C ) & m oil ( g ) # !! $$ M % oil ( g / mol ) "

To keep the experiment simple and feasible with basic equipment, many approximations were made. The first one concerns ∆T since in a more rigorous procedure, a correction would have to be applied because of the fact that the calorimeter is not perfectly adiabatic. Similarly, other corrections would concern the heat of formation of HNO3 and H2SO4 and the heat of combustion of the fuse wire. Moreover, only one trial is performed in each part of the experiment while it is recommended to perform 7 trials to get a representative mean value. However time constraints associated to the lab session (110 min) along with a willingness to keep the experiment within students’s grasp, we deliberately omitted to apply the abovementioned corrections.


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Strongly Agree

Agree

Disagree

Strongly disagree

No comments

FINDINGS To assess the students’ feedback on the newly devised calorimetry experiment, a survey questionnaire was administered to a sample of 31 students taking the General chemistry I course in the Spring 2016 semester. Students’ written reports were collected at the end of the laboratory session. They were also kindly asked to fill in our online-based survey questionnaire12 the same week. Reports and filled questionnaires13 were then gathered and analysed. Lectures were taught by two Chemistry Professors. However, the data from the hands-on experiment were gathered by only one Professor who teaches the laboratory sessions as well. The students surveyed belonged to one of two lecture groups and to one of three laboratory sections. The collected data from the survey are given in Table 1 below:

13

13

2

0

3

This experiment has helped me to clear some ambiguities on thermochemistry concepts encountered in the lecture

6

21

1

0

3

3

I feel I have made some progress in my understanding of thermochemistry

9

18

1

0

3

4

My motivation to study thermochemistry has increased

5

13

7

1

5

5

I developed self-confidence and more interest in thermochemistry

5

19

3

1

3

6

I can see how the first law of thermodynamics applies to this bomb calorimetry experiment

17

12

2

0

0

7

I experienced some difficulties in calculating the heat of combustion

1

2

14

12

2

8

I understand now what an adiabatic calorimeter is

20

9

2

0

0

9

I know now how the heat of combustion of a material can be measured with an oxygen bomb calorimeter

15

15

1

0

0

10

I think this experiment is very unsafe and the student is put in jeopardy when performing it

1

5

12

12

1

Entry

Measurement

1

I found this experiment interesting

2

Table 1:

Results collected after probing 31 students who recently performed the bomb calorimetry experiment.

The analysis of the survey questionnaires and students written reports clearly demonstrates that students benefited greatly from their newly devised laboratory experiment. Firstly, the great majority of respondents found the experiment interesting (entry 1) and developed thanks to it an interest for thermochemistry (entry 5). Similarly, most students found the experiment helpful in disambiguating concepts covered in class (entry 2). Likewise, a large portion of the respondents believed to have made progress and to be more self-confident in the subject (entries 3 and 5). As for motivation, only around one fourth of students reported to have no increase of motivation to study thermochemistry after performing the experiment (entry 4).


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97%

77%

94%

84%

94%

87%

77%

90%

84%

100%

87%

The results equally indicate that concepts such as the first law of thermodynamics (entry 6) and an adiabatic system (entry 8) were well comprehended by students. Unexpectedly, only 10% of the students conceded experiencing difficulties in calculating the heat of combustion (entry 7); this encouraging result is a sign that the associated lecture material was well integrated by students prior to perform the lab experiment. The result obtained in entry 9 indicates that an overwhelming 97% of respondents responded they knew now how a bomb calorimeter can be used to determine a heat of combustion. Graph 2 summarizes the results shown in Table 1.

58%

80%

60% 50%

19% 3% Entry 9

6% Entry 8

10%

6% Entry 6

Entry 5

Entry 4

Entry 3

Entry 2

0%

Entry 1

10%

Entry 7

13%

3%

3%

20%

6%

30%

Entry 10

40%

26%

Percentage (%)

70%

Entry in Table 1 Strongly agree + Agree Disagree + Strongly disagree No comments

Graph 2: Summary of results shown in Table 1. It is worth mentioning that nearly four out of five respondents deemed the experiment to be fairly safe (entry 10); this is a very significant asset since students’ safety in the laboratory is of paramount importance for professors as well as universities as a whole. As a matter of fact, this result was very comforting given that the entire experiment has been designed from the outset with safety in mind as a top priority. This is emphasized by the utilization of a digital thermometer instead of a glass calorimetric mercury thermometer. Indeed, with experience we quickly realized that using the high-precision mercury thermometer, originally supplied with the calorimeter, is highly risky for the user since it forces him/her to keep his/her head right above the calorimeter in order to monitor the temperature. Under those conditions, should an incident such as an explosion caused by a faulty valve occur, the consequences would be catastrophic. Note that such an explosion, which resulted in material damage but no casualty during a lab session, has already been reported at the University of California, Davis in 2012.14 The advantage of using the probe of a digital thermometer instead of an analogic one is that it creates a safe distance of 1.50 meter between the bomb and the user. CONCLUSION To conclude, this study has successfully proved that non-chemistry engineering students’ lack of motivation and understanding in thermochemistry can be boosted by taking appropriate actions. Our action consisted in exposing them to thermochemistry concepts initially addressed in the lecture, in a laboratory setting. Requiring them to perform this simple yet highly pedagogical bomb calorimetry experiment enabled them to overcome


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many of their weaknesses. According to their own feedback as well as their professor’s evaluation of written reports, they increased their motivation, understanding, self-confidence, interest and knowledge related to thermochemistry. The experimental procedure of the bomb calorimetry experiment described herein was conceived with a student-centred mindset by focusing on providing a good level of clarity and simplicity. This approach turned out to be rewarding for both students and their professors. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

12. 13. 14.

Areas of concentration offered to undergraduate engineering students at Al Akhawayn University in Ifrane: general engineering, engineering and management science and computer science. www.aui.ma K. Bain, A. Moon, M. R. Mack, M. H. Towns, «A review of research on the teaching and learning of thermodynamics at the university level», Chem. Educ. Res. Pract., 2014, 15, 320-335. K. E. Hahn, W. F. Polik, «Factors influencing success in physical chemistry», J. Chem. Educ., 2004, 81(4), 567–572. G. Nicoll, J. S. Francisco, «An investigation of the factors influencing student performance in physical chemistry», J. Chem. Educ., 2001, 78(1), 99–102. M. E. Derrick, F. W. Derrick, «Predictors of success in physical chemistry», J. Chem. Educ., 2002, 79(8), 1013–1016. A. Metioui, A. Legrouri, A. Ouardaoui, «Historical Development of Electrochemical Cell and Student Conceptions about its Functioning», Proceedings of The 22nd Conference On Chemistry Education, Rome, Italy, 15-20 July 2012, 378-380. A. Metioui, A. Legrouri, A. Ouardaoui, «From "ANIMAL" Electricity to "METALLIC" Electricity and the Beginning of the Electrochemistry: The Didactical View», Proceedings of The 22nd Conference On Chemistry Education, Rome, Italy, 15-20 July 2012, 237-240. M. Sözbilir, «What makes physical chemistry difficult?: perceptions of Turkish chemistry undergraduates and lecturers», J. Chem. Educ., 2004, 81(4), 573–578. A. Legrouri, A. Ouardaoui, K. Sendide, A. Metioui, «Increasing motivation of non-science students for environmental chemistry through field studies», Proceedings of the International Conference on Recent Trends in Science Education, Smolenice, Slovakia republic, 15-17 October 2012, 20-24. (a) P. W. Atkins, J. de Paula, «Physical Chemistry, 7th ed.», Oxford University Press, Oxford, UK, 2002. (b) R. J. Silbey, R. A. Alberty, M. G. Bawendi, «Physical Chemistry, 4th ed.», Wiley, New York, 2004. (c) R. Chang, «Physical chemistry for the chemical and biological sciences, 3rd ed.», University Science Books, Sausalito, California, 2000. Link to the online-based survey questionnaire: https://docs.google.com/forms/d/1a6URd8hcmRHQJIQ6ZsNvqtMH1vdRdXhB09dw7yxPOVU/viewfor m?fbzx=-2274070907725030453 Link to students’ responses to the online-based survey questionnaire: https://docs.google.com/spreadsheets/d/1AgFkWMByHw4uVLsKsxTj6RerFbloB0f1MuQr3ygZRE/pubhtml?gid=0&single=true http://cenblog.org/the-safety-zone/2012/02/to-avoid-an-exploding-calorimeter-maintain-it/

APPENDICES •

Online survey questionnaire submitted to students: https://docs.google.com/forms/d/1a6URd8hcmRHQJIQ6ZsNvqtMH1vdRdXhB09dw7yxPOVU/viewf orm?fbzx=-2274070907725030453

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Students’ responses to the online survey questionnaire submitted to students: https://docs.google.com/spreadsheets/d/1AgFkWMByHw4uVLsKsxTj6RerFbloB0f1MuQr3ygZRE/pubhtml?gid=0&single=true

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“Bomb calorimetry” experiment


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