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Experiment Group
Thermochemistry: Materials and Temperature Control
Sophisticated materials-testing labs work on the same calorimeter principles covered in general chemistry.
(Courtesy of John Gadja, Construction Technology Laboratories.)
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Purpose This module is designed to explore the role of heat capacity as a factor in choosing a
material with special heat requirements. Heat is a measure of energy transfer into or out of a system and is closely related to temperature. When a system absorbs heat, there is a rise in its temperature; when a system gives off or loses heat, its temperature decreases. In this process, heat is directly related to the temperature change of the system. Thus, if we understand a materials ability to absorb heat, we should be able to control the associated temperature changes. Different substances respond differently to heat; some substances require only a little heat to achieve a temperature change, whereas other substances require considerable heat to raise their temperature even one degree. Heat capacity is a measure of this propertyit measures the ability of a substance to hold or store heat. Materials with large heat capacities are able to store more heat than materials with smaller heat capacities. Heat capacity is one of the properties that engineers consider when they design systems that must control temperature changes.
Schedule of Experiment 1: Skill-Building Lab: Heat Capacity and the Fireproof Safe the Labs Construct a calorimeter and determine its heat capacity (group work).
Determine the specific heat for copper (group work). Devise a method for measuring the specific heat for other substances, including
glass beads, cork, and concrete (group work). Choose the best interstitial material for a fireproof safe by comparing the calculated temperature of the safe (after half an hour in the fire) for all the materials measured (individual work).
Experiment 2: Application Lab: The Control of Temperature in Chemical Reactions Measure the heat evolved in a chemical reaction (individual work). Devise and test a method (using the materials from Lab 1) that will decrease the
temperature change for this reaction (group work).
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Use uncertainty analysis to determine the approximate range of acceptable values
for the heat of reaction (individual work). Compare H with E for this reaction (group work).
Scenario Your company specializes in the design and manufacture of materials used in fireproofing. Although there are many uses for such materials, two of your primary sales targets are companies that produce fireproof containers and those that produce fireresistant building materials. Fire-resistant or fireproof materials do not necessarily protect their interior contents from fire damage indefinitely, but they should protect long enough to allow the real possibility of rescue. In the case of fireproof safes, you need to consider materials that will keep the intense heat of a fire from damaging whatever is inside the safe. The requirements of fire-resistant building materials are similar but more demanding. Fireproof walls and floors must delay the spread of fire to other parts of the building and must also keep the structural support system intact as long as possible, to give occupants and firefighters time to leave the building. Your chief responsibility lies in the research and development of appropriate materials for these two markets. Your first assignment is to develop a workable blueprint for the manufacture of small, fire-resistant boxes. What do you need to consider? To answer this, you must first bear in mind the purpose of making a safe fireproof. Obviously, you dont want the materials inside to melt or burn. This means you must control the flow of heat between the fire on the outside and the contents of the safe. Most fireproof safes are constructed by putting a smaller box inside a larger box. The filler (or interstitial) material between the two boxes must prevent the heat of the outside fire from damaging the contents inside the safe. The ultimate question, then, is to identify a reasonable interstitial material. You will have to examine the major factors that govern the amount of heat that can pass through the filler material, as well as other manufacturing considerations such as the cost of such materials. You must also develop a method or technique that can be used to measure the flow of heat into or out of a system. Then you must test all the materials available to your lab group in order to determine an appropriate filler material. You will base your decision on the information you obtained from the calorimetry experiments in the skill-building lab, from your analysis of the factors important to this problem, and by comparing the temperature inside the safe (using several different filler materials) after 30 minutes in a fire. In this application, the filler materials must keep generated heat outside the system. As preparation for this task, you will construct and use a calorimeter to measure the specific heats of various materials. This will familiarize you with the calorimeter setup and give you practice making the measurements necessary for heat calculations. It will also provide an experience of heat measurement different from Experiment 2 in this experiment group. In Experiment 2, you will measure the heat evolved in a chemical reaction that takes place in aqueous solution. The second experiment mimics the heat produced in runaway chemical fires such as the combustion of airplane fuel in the World Trade Center disaster. Although the WTC buildings were initially weakened by the impacts of the planes, it was the resulting fires that caused the structures to collapse. City fire codes require fireproofing in high-rise buildings that will contain a fire for approximately two hours. This generally allows time to evacuate occupants, salvage possessions, or quench the fire. In the case of the WTC, however, evidence ultimately showed that the impacts had knocked some of the fireproofing material off of the steel supports. Your second task is to find a material that will absorb enough heat to fireproof structural materials. You will apply what you have learned about heat capacity to identify a substance capable of decreasing the heat of a mock runaway reaction.
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EXPERIMENT 1 Skill-Building Lab: Heat Capacity and the Fireproof Safe
Pre-Laboratory Assignment NAME: ____________________________________________________________ Complete these exercises after reading the experiment but before coming to the laboratory to do it. 1. One of the factors engineers must consider is the validity of any assumptions made while they perform their jobs. In this experiment, several temperature assumptions are made. List two of these assumptions and explain why you think they are reasonable assumptions to make. Due Before Lab Begins
2. Two common mistakes made in this experiment are (1) loss of heat when the hot substance is transferred to the cold water and (2) failure to attain a constant final temperature. Discuss the steps you will take to avoid these mistakes.
3. During an experiment, you perform many different procedures and the final results of your experiment depend largely on how well you carry out these procedures as well as on the design of your lab apparatus. Decide whether the following conditions will have a large or a small effect on your experimental results. Give a reason for your answer. (a) The calorimeter lid has a large hole in it.
(b) Hot water is transferred along with the metal into the calorimeter water.
(c) The initial temperature of the cold water is 28 C.
(d) You forget to put the lid on the calorimeter.
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Thermochemistry: Materials and Temperature Control
(e) You use an alcohol thermometer instead of a mercury thermometer.
(f) You record the final temperature before thermal equilibrium is established.
4. Suppose a hot (89.5 C) piece of copper metal (cs 0.385 J g 1 K 1) with a mass of 2.55 g is put into 50.0 g of water (cs 4.184 J g 1 K 1). Calculate the final temperature of the water if its initial temperature is 25.7 C.
5. A fireproof safe is made of two thin metal boxes, one inside the other, with equal spacing between each pair of adjacent surfaces. Calculate the volume between the two boxes given the following dimensions. Include units. Smaller box: Larger box: 25 dm 32 dm 20 dm 27 dm 15 dm 22 dm
6. Predict which material will be best for the fireproof safe. Give your reasons.
7. Suppose you spill the boiling water (used in this experiment to heat irregular solid substances) on your arm. What first aid procedure would you use to control the damage to the burned arm?
8. What is the general method used in your lab to heat water? List two safety precautions you must heed during the heating process.
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EXPERIMENT 1 Skill-Building Lab: Heat Capacity and the Fireproof Safe
Background
Heat capacity measures the amount of heat needed to raise the temperature of a substance one degree. In other words, heat capacity measures the ratio of heat to temperature change, C q/ T, where C is the heat capacity, q is the heat, and T Tfinal Tinitial Tf Ti. When we examine the relationship given by this formula, we see that the magnitude of C depends on the magnitude of the ratio q/ T. C will be large when q/ T is large; C will be small when q/ T is small. It is logical that T must be kept small to protect materials from fire damage, that is, the large amounts of heat produced by fires. This implies a large value of the ratio q/ T and suggests that the research and development personnel might do well to consider candidate materials with large heat capacities. The heat produced in various types of fires is known or can be measured, and T is determined by the nature of the materials that must be protected from fire damage. Heat capacity is an important property to industries whose livelihood depends on the ability to control heat transfer. The heat capacity of an object depends on the size (mass) of the object. It is an extensive property. For example, 2000 kg of water has a greater heat capacity than 20 kg of water. In other words, 2000 kg of water can store more heat than 20 kg of water. To directly compare the heat-storing capability of two or more substances, we use specific heat capacity (cs), which is the heat capacity per gram of a substance, cs q/(m T). The specific heat capacity is used in the calculation of heat, q mcs(Tf Ti) mcs T
where m is the mass of the substance in grams and cs has units of J g 1 K 1. For mole amounts, we use the molar heat capacity (cp). Molar heat capacity is the amount of heat needed to raise the temperature of one mole of substance one degree. Then the molar heat capacity is cp q/(n T) and heat is calculated as q ncp T, where n number of moles of a substance and cp is the molar heat capacity with units of J mol 1 K 1. When expressing the absolute temperature of a system, we usually must convert to the Kelvin scale. However, when measuring temperature change, we may use either the Kelvin or the Celsius scales. The Kelvin and Celsius temperature scales are different, but, because the size of the degree unit in these two scales is the same, T has the same numeric value in both temperature scales. A temperature change from 32.4 to 26.2 C gives T 6.2 C. The corresponding change on the Kelvin scale is from 305.6 to 299.4 K; T 6.2 K. In this experiment, you will learn how to determine the heat capacity of a metal (copper) that may be useful in a safe. You will then examine the heat capacities of several other substances that might be good filler materials for a safe: glass beads, cork, and concrete. The shapes of some of these materials may require you to explore other methods of heating them to attain the initial high temperature. Not all of these materials will fit inside a test tube. Although we might consider direct submersion into the hot water, this introduces some degree of error if any of the hot water is transferred to the cold water container along with the substance. In addition, direct submersion is a poor choice for either the cork, which is porous and absorbs water, or the concrete, which is a mixture and tends to fall apart in the hot water bath. A heatresistant plastic wrap or bag would be a good alternative to the test tube we will use
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Thermochemistry: Materials and Temperature Control
for metal pellets. The plastic material allows close thermal contact between the solid and the hot water while preventing the solid from becoming wet. By the end of this experiment, you will be asked to make a choice of filler materials based on your experimental results for the specific heat capacity of the candidate materials. The choice may depend on a variety of properties that you didnt test. Are there other factors you should consider in your choice of material? Suppose the only important property to consider is thermal conductivity. Thermal conductivity measures the transfer of heat by conduction across a solid. How do our materials compare in their ability to conduct heat? Instead of doing the experiment, we will look up these values in The CRC Handbook of Chemistry and Physics. The thermal conductivity of cork is 0.04 cal s 1 cm 2. The thermal conductivity values of concrete, copper, and glass are 1.4 cal s 1 cm 2, 400 cal s 1 cm 2 , and 28 cal s 1 cm 2, respectively. If you consider only thermal conductivity for the safe, which one of these materials do you think would be the best to use? At the end of this experiment, you will be able to check on the validity of your guess. A typical design for a fireproof safe is shown in Figure J-1. The inner box has the dimensions height 0.100 m, depth 0.120 m, and width 0.240 m. The outer box has the dimensions height 0.134 m, depth 0.154 m, and width 0.274 m, making the space between adjacent surfaces 0.017 m. A material is to be selected for the interstitial space between the two metal boxes such that the following design specifications are met. Design Specifications 1. The interior of the safe is not to exceed 177 C when exposed to a fire of temperature 954 C for half an hour. 2. The safe is to be portable, so that an average person can carry it. Minimum weight is desired. 3. The safe is to be a consumer product. Cost is to be minimized. Using an equation we will give you later, you can calculate the inner temperature of the safe after half an hour of exposure to the fire temperature for each interstitial material. The temperature must remain less than 177 C for the safe to be classified as fireproof. You will have to calculate the mass of each material needed to fill the interstitial space in the safe. You can do this by calculating the volume between the inner and outer boxes and by measuring the density of each substance used in this lab. Then, mass equals the product of density and volume. For this calculation, we will assume that the geometry of the fireproof safe is fixed for all materials. You can determine the cost of each material needed to fill the interstitial space if you know the cost per unit mass. Finally, you will choose one of the materials you tested as best suited for the fireproof safe. Figure J-1
Height
Depth Width
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Calorimetry
Heat capacity values can be experimentally determined using calorimetry methods. Calorimetry experiments use initial and final temperature measurements to determine the heat generated or absorbed by some change. The change may be as simple as the heat transfer between a hot piece of metal and cold water, or it may involve measuring the heat of a chemical reaction. To determine T in a calorimetry experiment, it is necessary to control the heat transfer by isolating the system of interest in a well-insulated container called a calorimeter. In thermodynamics, we define the universe as consisting of two parts: (1) a particular system of interest (usually just called the system) and (2) everything else outside the system (usually called the surroundings). Energy can be transferred between the system and its surroundings, but the total amount of energy distributed between the two remains constant. In other words, Euniv Esys Esurr 0. For this experiment, the surroundings will be the calorimeter; the system, then, is the contents of the calorimeter. We are only concerned with thermal energy monitored by the variable heat, q, so quniv 0 qsys qsurr. Determining the Heat Capacity of the Calorimeter Calorimeters can be constructed using a variety of designs. The Styrofoam cup containing your hot morning coffee is a type of calorimeter designed to minimize the transfer of heat from the coffee to the surrounding air. The calorimeter you will use in this experiment will have a lid so that essentially all of the heat transferred stays inside the calorimeter. Because the calorimeter is the only component of the surroundings, we assume that qsurr qcal. The amount of heat actually absorbed by the calorimeter can be determined experimentally by measuring the heat transfer that occurs when hot water is added to cold water inside the calorimeter. Theoretically, the heat lost by the hot water should equal the heat gained by the cold water. This is the law of conservation of energy. Any discrepancy in the two values is due to the heat absorbed by the calorimeter, which includes the thermometer and stirring bar. The amount of heat absorbed by the calorimeter is related to the heat capacity of the calorimeter and is expected to be the same for similar systems. The heat capacity of the calorimeter is symbolized as Ccal. Ccal has units of J K 1. The overall system is adiabatic (quniv 0), so the calorimeter constant is calculated by quniv 0 qsys qsurr qsurr qcal qsys qcw qhw quniv 0 qcw qhw qcal qcw ccwmcw tcw qhw chwmhw thw qcal (qcw qhw) C tcal heat change for the cold water heat change for the hot water heat change for the calorimeter t of initial contents of calorimeter.
where qcw qhw qcal tcal
The specific heat of water is ccw c hw 4.184 J g 1 K 1. To determine qcal, you need to evaluate qcw and qhw. For the same calorimeter, Ccal may be considered constant when used with comparable masses of water. However, T will most likely vary from experiment to experiment, so qcal must be recalculated for each experiment.
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Thermochemistry: Materials and Temperature Control
Determining the Specific Heat Capacity of Substances In this experiment, you will also measure the temperature of cold water before and after some hot metal is added to it. When two substances having different temperatures are placed into close physical contact with one another, heat will flow from the hotter substance into the colder substance until both substances arrive at the same temperature. They will attain thermal equilibrium. A simple example of this is the immersion of a hot metal into a beaker of cold water. The transfer of heat from the metal to the surrounding water and beaker can be followed by monitoring the temperature change of the water. The water temperature measured when the metalwater system has reached thermal equilibrium is the final temperature for both substances. Measurement of this temperature change allows one to calculate the heat transferred from the metal to the cold water and the calorimeter. Heat is calculated as the product of mass, specific heat capacity, and temperature change: q mcs t. As the system approaches thermal equilibrium, the heat given off by the metal and the heat absorbed by the surrounding water and beaker are equal but are given opposite signs by convention. In thermodynamics, whatever leaves the system is considered a negative quantity while that which is absorbed by or added to the system is considered to be positive in sign. In this experiment, heat leaves the metal and is absorbed by the cold water and the calorimeter. Because your overall system is adiabatic, we can write quniv 0 qsys qsurr qcw qmetal qcal 0 qmetal (mcs t)metal
qcw (mcs
qsys qsurr qmetal qcal qcw qcal t)cw C tcal
Note that in this case, the values for t will be different for the metal and the cold water because these are different substances. Substances attain thermal equilibrium when their temperatures become equal after some period of close physical contact. A solid substance can attain thermal equilibrium with a second substance (such as water) in many different ways. In the first part of this experiment, the solid substance is a metal and is not soluble in water. To bring the metal to the temperature of the hot water, you need an experimental design that will keep the metal dry and allow you to transfer the heated metal quickly (and safely) to the beaker containing cold water. Large pieces of metal could reasonably be contained in a heat-resistant zip-lock plastic bag. Small pellets of metal could be heated inside a test tube immersed in hot water. With the passage of 7 to 10 min, it is reasonable to assume that the heat from the water bath has been transferred to all of the metal. This is generally a valid assumption, and it has the additional advantage of ensuring that the final temperature measured is due solely to the transfer of heat from the metal to the cool water and not from any drops of water from the hot water bath. By initially keeping the metal sample dry, no water from the hot water bath is transferred into the calorimeter beaker along with the heated metal. The setup for this experiment is straightforward. It involves two containers of water, one that is used to heat a known mass of the metal to some measured, elevated temperature, and a second, cooler sample of water in the calorimeter into which the hot metal will be put. The amount of water in the first beaker is unimportant because it is used merely to heat the metal. It is crucial, however, to measure the mass of water in the second beaker because this mass enters into the calculation of heat. The initial temperature of the hot metal will be assumed to be the same as that of the hot, boiling water. The initial temperature of the cold water will be measured before the
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Skill-Building Lab: Heat Capacity and the Fireproof Safe
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addition of the hot metal. The final temperature of both the metal and the cold water will be the samethe highest water temperature attained after the addition of the hot metal to the cold water. With these measurements, you can calculate the specific heat of the metal. Throughout the experiment and the calculations, remember three important points: 1. The major assumption is that no heat flows out of the calorimeter. 2. Neither the heat capacity nor the specific heat of any component changes either with temperature or with time. 3. Heat absorbed is a positive quantity; heat given off is a negative quantity. Or, if something gives off heat, then it absorbs a negative amount of heat. Calorimetry experiments are relatively simple to perform, but you should be careful to avoid some common errors. One source of error is in measurement. In calorimetry, we measure mass and temperature. As the calculation of heat (q) depends strongly on the quality of these two measurements, you should take great care to obtain the most accurate measurements possible. The more difficult of the two measurements is temperature or, more precisely, the final temperature. After adding the hot substance to the cold water, you should continue taking temperature readings every 15 s until there is no change in temperature for three successive readings. Try not to be impatient; stopping too soon will greatly affect your final results. A second source of error lies in not performing the steps in the procedure quickly and cleanly. In this experiment, you have a colder substance (generally water) and a warmer substance (for example, water, metal pellets, a chunk of concrete). You need the initial temperature of each; then you need a final temperature after the warmer substance has been added to the colder water. If you add warm water to cold water, the transfer and final temperature measurement are straightforward to obtain. However, you may have to use some imagination in the heating and transfer of some of the other materials. Is the material you are heating actually at the temperature of the hot water? Have you allowed it to come to thermal equilibrium? How much heat are you losing in the transfer from the hot water bath to the cold water in the calorimeter? Is it negligible? How can you decrease the heat loss? Questions such as these should be discussed with your lab partner so that you may optimize the quality of your lab results.
CAUTION: This experiment necessitates the use of hot water. The two most common methods of heating water in the lab use either a bunsen burner or a hot plate. If your lab uses open flames, such as those produced by a bunsen burner, be careful that no part of your body (including your hair) or your clothing comes into contact with the fire. You must wear goggles at all times in the lab. You must also be careful that other people in your lab are not working with flammable chemicals. Flammable chemicals must not be opened or used in the vicinity of an open flame. If your lab uses hot plates for heating purposes, follow the directions of your lab instructor about the location and usage of the hot plates. Most hot plates are equipped with a light that is on when the hot plate is on. Hot plates may appear the same to you whether they are turned on (and hot) or turned off (and cold). Observe proper precautions when working in the vicinity of the hot plate.
Procedure
Part I: Formation of Groups For group discussion and data analysis purposes, this experiment is best done in groups of two. Your group should collaborate on the construction of the calorimeter
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Thermochemistry: Materials and Temperature Control
and all measurements needed to calculate specific heat values. You will construct and use a calorimeter, and then determine the calorimeter constant and the specific heat capacity of copper. In Parts IV, V, and VI you will divide the work to be done between you. Engineers do many experiments during the course of a day, a week, or a year. They must be prepared to discuss, analyze, or report on any one of these experiments at any time. Part of their job is to carefully record the laboratory setup, conditions, assumptions, and all data that might help them fulfill these responsibilities. Measurements, changes in procedure, accidental spillage, and all other details that may influence experimental results should be recorded in your notebook so that everyone in the group can know happened. what Take time to prepare your notebook; indicate the method used, sketch a picture, and keep track of problems and difficulties so that you can avoid them in the future. Begin now by preparing a table that specifies the measurements you will contribute to the group. Your notebook should include the masses and temperatures needed for the calculation of the calorimeter constant and the specific heat capacity of different materials. Part II: Construction of a Calorimeter (Group Work) There are several kinds of calorimeters you may use in this experiment. One is a simple Styrofoam cup with a foil cover. It has the advantage that it is easy to construct and use. However, the light weight of the Styrofoam means that you must be especially careful to avoid tipping it over. To use Styrofoam cups, obtain two cups to nest one inside the other and a 3 3 in. square of foil to form a cover for the nested cups. Carefully mold the foil to the top of the cup assembly so that it can be removed and replaced easily. Make a small hole in the center of the foil just large enough to accommodate a thermometer. The nested Styrofoam cups can be stabilized by resting them inside a glass beaker. A sturdier calorimeter may be made by placing a 250-mL beaker inside a larger beaker or inside a special metal can. The 250-mL beaker is used as the inner cup of the calorimeter. If the outer part of the calorimeter is a larger beaker, you can use paper toweling as insulation between the two beakers. There are also metal cans available to use as the outer part of the calorimeter. In this case, the 250-mL beaker is supported by a metal ring that fits inside the can. When properly assembled, the beaker is suspended by the insulating metal ring so that no part of the beaker touches the can. Further insulation can be provided by wrapping the inner beaker with paper towels. A lid can be fashioned from a 5 5 in. square of cardboard. Cut a small hole in the center of the cardboard for the thermometer. Wrap a rubber band around the thermometer to position it so that it does not touch the bottom of the calorimeter beaker. Yet another calorimeter is a small thermos. In fact, any insulated container makes an acceptable calorimeter if it minimizes the transfer of heat between the inside reaction container and the atmosphere. Follow the direction of your laboratory instructor in your choice of calorimeter materials. Part III: Measurement of the Heat Capacity of the Calorimeter (Group Work) Weigh out two samples of deionized water. An appropriate amount of water for this experiment is about 50 g. The mass can be any value within 10% of this. Record all masses to a precision of 0.01 g. Put one water sample into the calorimeter beaker and measure its initial temperature. Measure the thermometer readings at eye level and carefully estimate the temperature to within 0.2 C. Heat the second sample of water until its temperature is about 10 to 15 higher than that of the cold water.
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Record the initial temperature of the warm water. When you are ready, quickly pour the warm water into the calorimeter beaker with the cooler water, replace the lid, and monitor the temperature as it changes. Record the temperature readings every 15 s for 3 to 5 min. Note the highest temperature attained by the water mixture in the calorimeter. The system is assumed to be at thermal equilibrium when the change of temperature over time becomes very small. There is only one final temperature; it is the same for both the hot and the cold water. Repeat for a total of two trials. During the laboratory period it is important that you detect any mistakes in procedure or technique before going on to the next part of the experiment. Calculate the heat capacity of your calorimeter for each trial. The two values should be within 5% of each other. If not, repeat the determination until the values are consistent. Use an averaged value of Ccal for the remainder of the calculations. Part IV: Obtain the Specific Heat Capacity of Copper (Group Work) Preparation of the Calorimeter Water Place about 100 g of deionized water into the calorimeter beaker. (Record the precise mass in your laboratory notebook.) Record the temperature of the water to 0.2 C. This temperature is the initial temperature of the water in your calculations. Heating the Sample Half-fill a test tube with an accurately measured mass of copper metal pellets. Heat the test tube and metal contents to thermal equilibrium in a hot water bath, being careful to keep the metal dry. However, to guarantee that all of the metal has reached thermal equilibrium, be careful to keep the level of the water surrounding the test tube above the contents of the test tube. Allow about 7 to 10 min for thermal equilibrium to be established. Then record the temperature of the water bath to the same degree of precision as you did for the cooler water in the calorimeter. This will be the initial temperature of the metal in your calculations. Pour the hot metal into the water in the calorimeter. To do this transfer quickly but efficiently, work together to take off and replace the calorimeter lid. Carefully mix the contents of the calorimeter while you watch the thermometer. Note and record the highest temperature attained (to 0.2 C). This number is the final temperature of the water in the calorimeter, the calorimeter itself, and the metal, as all three are in thermal contact. Perform two experimental trials. Part V: Determining the Specific Heat Capacity of Cork, Glass, and Concrete (Group Work) Plan a procedure for the measurement of specific heat with samples of cork, glass, and concrete, or other materials supplied by your instructor. A different group member should set up and direct each of these. Do two trials for each substance. These substances require a full 15 min in the hot water bath for thermal equilibrium to be reached. Decreasing this time will result in very poor calculated values for specific heat capacity. Part VI: Determining Density It is also necessary to determine the density of each substance, as density is one of the parameters included in the calculation of the inside temperature of a safe surrounded by a raging fire. Determine the densities of all the materials for which you determined specific heat capacity in this experiment.
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Thermochemistry: Materials and Temperature Control
1. Copper: Report your two measured values together with at least four other values obtained by other members of your class. Find the average value for the specific heat of copper. Calculate the percentage of error between your averaged result and the reported value found in the CRC Handbook of Chemistry and Physics or your textbook. 2. Include in your lab report a calculation of the uncertainty propagated through the calculation of specific heat capacity for copper. 3. Calculate the specific heat capacity for all the substances measured in Parts V and VI. Report the averaged value obtained from your two trials for each substance. 4. Calculate the temperature inside a fireproof safe (Tsafe) during a fire, using each of the candidates for filler material. An estimation of Tsafe can be obtained from Tsafe Tfire (Tfire Tsafe, initial)e
hAs csm
t
where Tfire 954 C, Tsafe, initial room temperature, and h is the heat transfer coefficient and is assumed constant for the three materials. [It is calculated to be 6 J (s1 m2 K1).] As represents the exterior surface area of the safe in square meters, t represents time in seconds, and cs and m refer to the specific heat and mass (in grams) of the interstitial material of the safe. 5. Prepare a table that includes the following data for all measured substances: specific heat capacity, density, thermal conductivity, mass to fill interstitial space in safe, and Tsafe. Group Discussion Discuss the following together and include your answers in the final report of each group member. 1. The equation used to calculate the inside safe temperature contains cs as a variable. How does the material with the highest cs value rate when the inside temperature is calculated? (Refer to your table of heat capacity values for these substances.) 2. Do your experimental findings support the statement, Substances with large specific heat capacity values are good heat-storing substances? Why or why not? 3. Compare the half-hour safe temperature calculated using each of the materials. The inside temperature cannot exceed 177 C for the safe to be considered fireproof. The substances that meet or come closest to meeting this specification are possible filler materials. List them. 4. Which property do you think is the most important in determining a good interstitial material for a fireproof safe? Why? 5. Which material is best suited as the filler material for the fireproof safe? Defend your choice. Did you include cost in your decision? 6. What happened to the thermal conductivity comparison you made (that is, how did that parameter enter into your choice for the best interstitial material)? Did your intuition direct you to choose the lowest conductivity material (that is, the cork)? Did your experiments and calculations support your intuition? 7. As a final check on your results, check the weight of a commercially available fireproof safe. Would you say that the manufacturer came to a conclusion similar to yours for the interstitial material of the safe?
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EXPERIMENT 2 Application Lab: The Control of Temperature in Chemical Reactions
Pre-Laboratory Assignment Due Before Lab Begins
NAME: __________________________________________________________________________________ Complete these exercises after reading the experiment but before coming to the laboratory to do it. 1. Which of the substances from the skill-building lab had the highest heat capacity? Which had the lowest heat capacity? Which of these substances is better at storing heat? Defend your answer by calculating the heat absorbed by 5.00 g of each of these substances, assuming T 30.00 C. Use your experimental values for specific heat capacity.
2. List some possible sources of error in a calorimetry experiment and detail how your group might have had better results for the substances you measured in the skill-building lab.
3. Explain what is meant by the term adiabatic. Name two assumptions concerning heat transfer that are validated because the system is adiabatic.
4. Calculate the change in internal energy ( E) that occurs in the combustion of 5.00 g of methanol, CH3OH, at a temperature of 298 K. Methanol burns according to CH3OH (g) 3 O (g) CO2 (g) 2 2 2 H2O (g) H 676.5 kJ
5. Calculate the moles of H2O2 in 50.00 g of a 3% by mass solution of H2O2.
6. Because it is important for you to measure temperatures accurately in this experiment, you may be using mercury thermometers. Why must you not use the thermometer as a stirrer in your calorimeter?
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EXPERIMENT 2 Application Lab: The Control of Temperature in Chemical Reactions
Background
The first law of thermodynamics tells us that the total amount of energy in the universe is constant. We usually think of this as the law of conservation of energy. The interesting thing about energy is that it can take many different formsheat, light, and electricity are all ways in which energy is manifested. Because energy can be transferred from substance to substance using many different vehicles, it can be difficult to keep track of the energy involved in chemical processes. In a chemical reaction, the system of interest is the reaction itself and the surroundings are usually taken to mean the immediate surroundings, including the container and the surrounding air; in short, anything that touches the system. Ordinary chemical reactions allow us to measure some interesting thermodynamic quantities. For example, the internal energy of the system changes with each bond that is broken or formed. The change in internal energy ( E) is the sum of two terms, heat and work: E q w
For experiments performed under ordinary laboratory conditions, the pressure is assumed to be constant. This is shown in measurements of heat by a subscript p, signifying heat at constant pressure (qp). Heat at constant pressure is the same as the change in enthalpy, H: qp H
For systems under normal laboratory conditions of pressure, then, we can measure H quite simply by measuring qp. The change in internal energy ( E) is E qp w H w H P V
Here the work term simplifies to the volume expansion of a gas against the constant pressure of the atmosphere. For chemical reactions that involve no expansion of gases, w 0 and E H qp
For chemical reactions that generate a number of moles of gas that differs from that originally present in the reactants, however, the work term must be evaluated. If we assume that any gas generated by the chemical reaction behaves as an ideal gas, then V can be evaluated by making appropriate substitutions using the ideal gas equation, V Then P V P nRT P RT n nRT P
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Thermochemistry: Materials and Temperature Control
where T and R (8.314 J mol 1 K 1) are constant, and n number of moles of gas (product) number of moles of gas (reactants). This substitution allows a calculation of E given H, the temperature, and the chemical equation for the reaction. Heat Heat is a means by which energy can be transferred between a system and its surroundings. Some reactions require the addition of heat. These are called endothermic reactions. As endothermic reactions absorb heat, qrxn Hrxn 0. Reactions that liberate heat to their surroundings are called exothermic reactions. In an exothermic reaction, qrxn Hrxn 0. Although heat and temperature are not the same thing, there is a direct correlation between them. As heat flows into an object, the temperature generally increases; as heat flows out of an object, the temperature decreases. Hence, heat transfer can be monitored and measured by watching the temperature. Thermochemistry is a branch of chemistry that studies heat transfer. For heat transfer to occur there must be thermal contact between the substances in the system. To measure the heat transferred during a chemical reaction, it is imperative that we prevent the heat from escaping or diffusing into the surrounding atmosphere before a proper measurement can be obtained. This is accomplished by using a calorimeter. In this experiment, you will put a weighed amount of H2O2 solution into a calorimeter and measure the heat that is given off by the decomposition reaction of H2O2 occurring in the solution. You can assume that all of the heat transferred in the reaction stays inside the calorimeter (that is, assume adiabatic conditions). The H2O2 solution contains a very small amount of H2O2 (15% by mass) and a predominant amount of H2O (9995% by mass). The heat liberated by the decomposition of H2O2 is absorbed by the solution water and by the calorimeter. The Decomposition Reaction We often use a dilute solution of hydrogen peroxide, H2O2, to thoroughly cleanse cuts and abrasions. It is a familiar first aid remedy. When H2O2 contacts your skin, it bubbles vigorously, cleansing away surface grit. Warmth is a secondary sensation in the cleansing process, because the reaction is exothermic. The bubbling action is due to the rapid decomposition of the hydrogen peroxide, 2 H2O2 (aq) 2 H2O (l) O2 (g) (1)
This decomposition reaction is the one that you will be examining today. The isolated decomposition of H2O2 occurs very slowly at room temperature. However, the reaction can occur very quickly in the presence of a suitable catalyst. A catalyst is a substance that can alter the rate of a chemical reaction without itself being permanently changed. To better understand how a catalyst may work, lets look at another system. The decomposition of hydrogen peroxide can also be catalyzed by iron(II) ion. In acid solution, iron(II...