“Greening” a Familiar General Chemistry Experiment: Coffee Cup

Oct 11, 2016 - Coffee cup calorimetry, performed with calorimeters made with styrofoam coffee cups, is a familiar experiment in the general chemistry ...
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Laboratory Experiment pubs.acs.org/jchemeduc

“Greening” a Familiar General Chemistry Experiment: Coffee Cup Calorimetry to Determine the Enthalpy of Neutralization of an Acid− Base Reaction and the Specific Heat Capacity of Metals A. M. R. P. Bopegedera* and K. Nishanthi R. Perera† Department of Chemistry, The Evergreen State College, Olympia, Washington 98505, United States S Supporting Information *

ABSTRACT: Coffee cup calorimetry, performed with calorimeters made with styrofoam coffee cups, is a familiar experiment in the general chemistry laboratory. These calorimeters are inexpensive, easy to use, and provide good insulation for most thermodynamics experiments. This paper presents the successful substitution of paper coffee cups for styrofoam cups to construct homemade calorimeters. Important modifications to the experimental protocol when using paper cup calorimeters are discussed. Data collected using calorimeters constructed from the two types of cups are presented in order to demonstrate that comparable results can be obtained for the enthalpy of neutralization of an acid−base reaction and for the heat capacities of metals. These results also compare favorably with published values. Finally, an approach to using scientific literature to help general chemistry students examine whether paper or styrofoam cups provide the “greener” option is discussed. KEYWORDS: First-Year Undergraduate/General, High School/Introductory Chemistry, Laboratory Instruction, Problem Solving/Decision Making, Calorimetry/Thermochemistry, Green Chemistry, Heat Capacity, Physical Properties, Quantitative Analysis, Thermodynamics



BACKGROUND Home-made constant pressure calorimeters, inexpensively constructed using styrofoam coffee cups, are frequently used in the general chemistry laboratory to introduce thermodynamics concepts. Examples of such experiments published recently in this Journal include measuring the heat of sublimation of dry ice,1 enthalpy of decomposition of hydrogen peroxide,2 heat of formation of magnesium oxide,3 thermochemical analysis of acid−base neutralization,4 heat capacity of metals,5 measurement of heat flow,6 heat of vaporization of nitrogen,7 heat of combustion of zirconium,8 entropy change of urea dissolution,9,10 measurement of neutralization and acid dissociation of the hydrogen carbonate ion,11 and explorations of Dulong and Petit’s law.12,13 Vannatta et al. published a threepart experiment engaging students in real-world connections with the uses of calorimetry.14 An inquiry-based investigation for specific heat was published by Herrington.15 Hayes described an active learning approach to determining the stoichiometry relationship of an acid−base neutralization reaction.16 A small-scale calorimeter, constructed with a styrofoam block, and its effectiveness in measuring enthalpies of neutralization, redox, and dissolution reactions was published by Brouwer.17 As evidenced above, styrofoam cup calorimeters provide reliable data in a multitude of thermochemistry experiments in the general chemistry laboratory. Construction of these © XXXX American Chemical Society and Division of Chemical Education, Inc.

calorimeters is quick and easy. Styrofoam cups used are readily available, economical, and reusable. Therefore, these calorimeters can be used in general chemistry laboratories in every educational environment, including those with large enrollments. However, styrofoam products are increasingly disfavored by the public since they are a high volume, poorly recycled, consumer waste product that is highly visible in landfills and public spaces. Therefore, some institutions (including ours) and cities are moving toward banning them.18,19 General chemistry students are aware of these challenges. Several articles published in this Journal used calorimeters made from alternative materials. Ruekberg presented calorimeters made from Thermos brand snack jars20 as an economical, safe, and sturdy option. Bindel et. al used them to measure specific heat capacities of metals.21 Stankus et al.22 constructed calorimeters with polypropylene Tri-Pour beakers and obtained comparable results for the enthalpy of solution of sodium hydroxide. Kavanagh et al.23 used calorimeters made from glass beakers and a specially designed thermistor as a temperature probe to measure the enthalpy of an acid−base reaction. Although these are viable alternatives, the initial cost Received: March 10, 2016 Revised: September 16, 2016

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• Construct the calorimeter with three nested paper cups. Make the lid from a fourth cup (Figure 2A).

of their construction could be prohibitive for institutions with limited budgets (high schools, community colleges, and smaller colleges/universities) and those with large enrollment courses. Nevertheless, these attempts clearly demonstrate that chemistry educators are aspiring for alternatives to styrofoam calorimeters. We present here results from experiments for the determination of the enthalpy of neutralization of an acid− base reaction and heat capacities of metals using calorimeters constructed from paper cups instead of styrofoam. This switch is easy and inexpensive. We will compare data from styrofoam and paper cup calorimeters with published results, describe needed changes to the experimental protocol when using paper cup calorimeters, and suggest ways to engage general chemistry students in a discussion of whether paper cup calorimeters are a “greener” option.



EXPERIMENTAL DETAILS

Experiment 1: Heat of Neutralization of an Acid−Base Reaction

Two paper cups are used to construct the calorimeter. The cover is made from a third paper cup (Figure 1). The Figure 2. (A) Paper cup calorimeter constructed from three paper cups and (B) the computer controlled data collection system.

• Use room temperature tap water in the calorimeter. • As in Experiment 1, secure the calorimeter with a ring clamp/stand. Use a magnetic stirrer to mix calorimeter contents (Figure 2A). • Use a temperature probe with a computer controlled data collection system25 to record the temperature as a function of time (Figure 2B). • When heating the metal containing test tube in the hot water bath, ensure that it and the thermometer do not touch the surfaces of the water bath (Figure 3).

Figure 1. (A) Paper cup calorimeter. (B) Student setting up the paper cup calorimeter to measure the enthalpy of neutralization of an acid− base reaction.

experimental protocol is similar to that published in chemistry textbooks24 with the following changes: • Always use a digital thermometer to avoid missing the maximum temperature reached during the experiment. The maximum temperature drops off faster with paper cup calorimeters compared with styrofoam cup calorimeters. Therefore, if an analogue thermometer is used, the maximum temperature could be missed. • Secure the calorimeter with a ring clamp and stand (Figure 1). Do not handle the calorimeter to prevent thermal energy transfer from hands to and from the calorimeter. • Do avoid swirling by hand; use a magnetic stirrer/stir bar (Figure 1) to mix calorimeter contents.

Figure 3. Water bath for heating the metal. The test tube containing the metal and the thermometer should not touch the surfaces of the water bath.

• Place the hot water baths far enough away from calorimeters to ensure thermal isolation. Yet, place them sufficiently close enough so that the hot metal could be quickly transferred from the water bath to the calorimeter (minimizes heat loss to the environment during transfer).

Experiment 2: Heat Capacity of Metals

The experimental protocol is similar to that of Ngeh et al.6 and Bindel12 with the following modifications. B

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Table 1. Experimental Results for the Enthalpy of Neutralization of an Acid−Base Reaction Using Styrofoam and Paper Cup Calorimeters Average Values, kJ mol−1 (Standard Deviations) c

Parameters

Group 1, N = 6

Instructor, N = 1c

Group 2, N = 21c

Group 3, N = 8c

Styrofoam cup calorimeter Paper cup calorimeter Difference,a % Error for Styrofoam cups,b % Error for paper cups,b %

−60.3 (1.4) −56.9 (1.3) 5.64 6.26 0.264

−60.7 −58.8 3.13 6.96 3.61

n/a −59.6 (1.9) n/a n/a 5.02

n/a −57.3 (2.1) n/a n/a 0.97

a

The percent difference compares results from the paper and Styrofoam cup calorimeters. bThe percent error compares these results with the published value (−56.75 kJ/mol; see ref 24). cN is the number of data points.

Table 2. Specific Heat Capacity Results Using Calorimeters Made from Different Cup Configurations Experimentally Determined Specific Heat Capacity Values, J g−1 K−1, by Cup Configuration Two Styrofoam Cups

a

Metal

Published Specific Heat Capacity

Lead Copper Tin

0.125604 0.376812 0.217714

a

Two Paper Cups

Three Paper Cups

Specific Heat Capacity

Error, %

Specific Heat Capacity

Error, %

Specific Heat Capacity

Error, %

0.117 0.373 0.215

−6.85 −1.01 −1.25

0.118 0.409 0.197

−6.05 +8.54 −9.51

0.131 0.369 0.241

+4.30 −2.07 +10.7

The percent error was calculated by comparison with published values (see ref 26).

Students, working in pairs, completed both experiments (with two different metals in Experiment 2) within a 3 h lab period leaving sufficient time for data analysis in the computer laboratory. These experiments are also suitable for introductory chemistry and high school chemistry courses. The student handout describing experimental procedures are provided in the Supporting Information.



HAZARDS Wear protective gloves when handling hot metals, surfaces, and hot water. Concentrated hydrochloric acid causes severe burns. Sodium hydroxide is corrosive and hygroscopic. It causes eye and skin burns and respiratory tract and digestive tract irritation. The diluted solutions of the acid and base provided to students minimized these risks. Students were required to wear eye protection and gloves. Instructions on cleaning up acid−base spills were incorporated into the prelab lecture. Leftover acid−base solutions were neutralized and disposed down the drain. Hazardous information and CAS numbers for all chemicals are available in the Supporting Information.



Figure 4. Temperature as a function of time for measuring the heat capacity of copper using calorimeters made from two styrofoam cups, two paper cups, and three paper cups (data collected by the authors).

RESULTS AND ANALYSIS

Experiment 1: Heat of Neutralization of an Acid−Base Reaction

This experiment was first tested by a small group of students (Group 1) to compare results between styrofoam and paper cup calorimeters. The experiment was then conducted with two other groups (Groups 2 and 3) with only paper cup calorimeters (see Table 1). Complete data and analysis are available in the Supporting Information. Experiment 2: Heat Capacity of Metals

The authors measured the heat capacity of several metals using calorimeters made from three different cup configurations (see Table 2). Graphs of temperature as a function of time using these different cup configurations for measuring the heat capacity of copper are given in Figure 4. Figure 5 represents comparable data collected by two student teams for the determination of the heat capacity of copper. Complete sets of

Figure 5. Temperature as a function of time for measuring the heat capacity of copper using a calorimeter made from three paper cups collected by two student groups.

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Performance of product in landfills

Biodegradable and compostable.

Benzene is a suspected carcinogen.38 Ethylene is highly flammable.39 Intermediates of the chemical process (ethylbenzene and styrene) are flammable and carcinogenic. Ethylbenzene is mutagenic.40,41 Styrene forms explosive mixtures with air.41 Susceptible to UV degradation but releases aldehydes and ketones42 to the environment. Photodegraded material is susceptible to biodegradation.42

Cost of recycling is less than the virgin material.30 Consumes water.30 High.28 Recycled material can be incinerated for its high energy value (46 MJ/kg compared with 44 MJ/kg for heating oil),34 but environmentally undesirable products are formed during incineration.35 Styrofoam is nontoxic.37

Durable and reusable until cup breaks.

Styrofoam Cup Attributes D

Paper Cup Attributes

IS PAPER A “GREENER” OPTION THAN STYROFOAM? A show of hands indicated that all students assumed paper cups to be the greener option although students could not compare with any degree of confidence the raw materials used for construction or the durability/reusability, recyclability, energy consumption in manufacture, toxicity, and landfill performance of the two types of cups. Because paper is recycled on campus and in homes, students assumed that paper cups are made from recycled paper. Students had little knowledge of styrofoam other than it is not recycled on campus or through residential recycling although styrofoam cups bear a recycle symbol. Using literature materials provided by the instructor,27−42 students worked in groups to generate the following table (Table 3) to

Factors Compared

Table 3. Comparison of Paper and Styrofoam Cups



Raw materials used and Wood pulp (virgin paper) or paper pulp (recycled paper).27 Renewable. their renewability Durability/reusability Durable and reusable until cup breaks. of product Recyclability of product Curbside recycling in homes and places of employment. During recycling, material is first cleaned of ink and glue.30 Expected recovery rate was 50% at the end of the 20th century.31 Recycled paper is used for lower grade products (newsprint, egg cartons, cups, etc.).27 Cost of recycling is less than that of the virgin material.30 Consumes water (which is processed and reused).30 Energy consumption in High. Often self-generated by combusting residues of raw manufacture materials.27 Toxicity of product and None.36 raw materials

DISCUSSION Results for the heat of neutralization of an acid−base reaction from the two types of calorimeters used were comparable (Table 1). These results also compared favorably with the published values. Therefore, we recommend using calorimeters constructed from two paper cups and a lid from a third cup (as shown in Figure 1) instead of styrofoam cups for this experiment. Measuring the heat capacity of metals was inherently more challenging, regardless of the type of calorimeter used, since some thermal energy is invariably lost to the environment when transferring the hot metal to the calorimeter. Data collected by the authors graphed in Figure 4 shows that the styrofoam cup calorimeter provides the best insulation (almost no heat loss). The calorimeter made from two paper cups showed the highest rate of heat dissipation. Figure 4 and Table 2 are evidence that the three paper cup calorimeter provides comparable results with those from the styrofoam cup calorimeter. Figure 5 depicts the variability in students’ lab skills, often observed at the general chemistry level. Data collected by Student Group 1 shows a higher rate of heat dissipation compared with data from Group 2. If a computer controlled data collection system was not used, it would have been easy to miss the maximum temperature reached in the calorimeter (especially by Student Group 1). Initially, ice water was used in the calorimeter to obtain a larger temperature change upon addition of the hot metal. However, as also observed by Barth et al.,5 this introduced error since the temperature of the ice water continuously increased before the metal was added due to heat transfer from the environment. Small metal shot or pellets (with a larger surface area) were used to ensure that the entire metal submerged in the water once inside the calorimeter. This allowed for efficient heat transfer from the metal to the water. After collection, temperature/time data were exported to spreadsheet software for data analysis. Then, all student data were gathered into a class spreadsheet so students could analyze multiple data sets. Learning to transfer data between different software and using spreadsheets for data analysis are useful and transferable skills. Allocating time for students to complete most of their analysis during the laboratory period provided them the opportunity to ask questions and helped improve the overall quality of students’ lab reports.

Benzene, ethylene, and pentane (as a blowing agent). Derived from petrochemicals.28,29 Nonrenewable.



Recycled pellets have the same properties as the virgin material.30

student data and analysis are available in the Supporting Information.

Consumers must deliver32 to the few collection centers in USA.33 Material is compressed to achieve high density to make recycling economically viable.30 Only 3% was recycled in 1988.32

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Notes

build their knowledge on the manufacture of these cups and to initiate a comparison. Students experienced that both types of cups compared favorably in reusability, especially since they reused the outer (dry) cups of the calorimeters and only changed the inner (wet) cup from one experiment to the next. Additionally, wet cups were collected and dried, and all cups were stored and reused in future laboratories. In 1991, Hocking argued that Styrofoam cups are better for the environment than paper cups made from wood.43 Although these industries have since changed significantly, students learned to appreciate the complexity of these choices by reading this article. Students were pleased to learn of the efforts to increase postconsumer recycling of styrofoam due to consumer and regulatory pressures.32,44 Students located a styrofoam collection center near campus33 and were inspired to set up an on-campus collection site. Other such efforts at universities have been reported.32 Students learned that one of the biggest challenges in recycling styrofoam is its high volume-to-weight ratio. Seo et al. compared multiple compacting processes that reduce this ratio,45 and RecycleTech Inc. has developed equipment to compress recycled styrofoam to make it more transportable and convertible to consumer goods.46 Students were intrigued that styrofoam waste is recycled into useful consumer products (packaging materials, furniture, concrete, etc.).43 Starting from waste styrofoam, Yang et al. synthesized adhesives,47 Siyal et al. made a functional polymer,48 Bekri-Abbes et al. made an adsorbent,49 and Wang et al. made an adsorbent for humic acid removal.50 Students were inspired by the efforts made toward making recycled styrofoam more environmentally benign such as converting it to a biodegradable plastic51 or styrene using a barium oxide catalyst.52 As we did with our students, instructors can use this experiment to introduce many of the Green Chemistry Principles53 to general chemistry students, such as waste prevention, using less hazardous materials and renewable resources, conservation of energy, and return of safe substances to the environment for a safer planet within the first-year chemistry curriculum.



The authors declare no competing financial interest.



ACKNOWLEDGMENTS Photographs of students in the graphical abstract and in Figure 1B were taken by Dani Winder for The Evergreen State College.



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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00189. Lab documentation for Experiments 1 and 2 (PDF, DOCX) Hazardous information and CAS numbers (PDF, DOCX) Data from Student Group 3 (XLSX) Student Data for heat capacity of metals and analysis (XLSX) Paper cup calorimeter data (XLSX) Sample data analysis (PDF, DOCX)



REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address † South Dakota School of Mines and Technology, Rapid City, SD 57701.

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