c H2 O = J (g H 2 O)( C change)

Transcription

1 Calorimetry 1 CHM120 Introduction: Have you ever noticed the nutrition label located on the packaging of the food you buy? One of the first things listed on the label are the calories per serving. How is the calorie content of food determined? Most of the energy we get from our food is supplied by carbohydrates and fats. These food molecules are broken down in our cells in oxidation reactions. In the presence of excess oxygen, the food molecules react in a complex series of reactions to produce CO2 and H2O. We breathe in oxygen to sustain this process and exhale carbon dioxide and water vapor as reaction products. The energy released in these reactions fuels our basal metabolism (the chemical reactions constantly occurring to maintain basic body functions) and our physical activity. Because the products of these oxidation reactions are the same when food is burned in a calorimeter as when it is metabolized in the body, we can approximate the energy value of carbohydrates and fats in our bodies by doing a simple calorimetry experiment. In this experiment, the specific heat of water and its change in temperature will be used to determine the caloric content of a food sample. One calorie is the amount of energy required to raise the temperature of one gram of water one degree Celsius. A chemist s calorie (lowercase c) is different from a food Calorie (capital C). A food Calorie is actually a chemist s kilocalorie. 1 Calorie = 1 kilocalorie = 1000 calories. The density of water is 1 gram per milliliter. Outside the United States, both food and fuel values are typically reported in Joules or kilojoules (1 kj = 1000 J). To raise the temperature of one gram of water one degree Celsius requires Joules. c H2 O = J (g H 2 O)( C change) Calorimetry is the study of heat involved in a chemical reaction. A coffee-cup calorimeter is often used to measure the reaction enthalpy ΔH and the heat q involved in a chemical reaction. Several assumptions are made in a coffee-cup calorimetry experiment. It is assumed that the coffee-cup is a perfect insulator, the density of the final solution is that of water, 1.00 g/ml, and the specific heat of the solution is that of water J/g o C. [See the sample calculations for more information regarding the calculations of ΔH and q. Prior to lab read the sections of our textbook that discuss calorimetry, enthalpy, specific heat, and Hess Law.] During calorimetry, food burns and its stored energy is quickly converted into heat energy and products of combustion (carbon dioxide and water). The heat energy that is released is then transferred into the water above it in the calorimeter. The temperature change in the water is then measured and used to calculate the amount of heat energy released from the burning food. The heat energy is calculated using the equation Q = mc T where Q = heat energy, m = mass of the water, C = specific heat of the water T = change in water temperature. Laboratory Objectives: Measure delta H for two different reactions using coffee cup calorimeters Measure the specific heat capacity of a metal cylinder Measure the calorie content in several food items. This experiment contains several parts. Each part has instructions and a worksheet to enter collected data and make calculations. Each part will be done in duplicate. 1. Adapted from Sharon Anthony and Heather Mernitz from the ChemConnections module, Would You Like Fries With That? The Fuss About Fats in Our Diet by Sandra Laursen and Heather Mernitz; and Rettich, T.; Battino, R.; Karl, D. J. Heating Value of Fuels, J. Chem. Educ., 1988, 65(6),

2 Sample Calculations: A student was studying the reaction of sodium hydroxide with formic acid CHO2H in a coffee-cup calorimeter ml of 1.00 M NaOH was mixed with 50.0 ml of 1.00 M CHO2H. Prior to mixing the initial temperature of the two solutions was the same, 25.3 o C. After mixing the solution reached a maximum temperature of 31.8 o C. The chemical reaction is shown below. NaOH(aq) + CHO 2H(aq) NaCHO2(aq) + H2O(l) Assume the density of the final solution is that of water, 1.00 g/ml, the specific heat of the solution is that of water J/g o C, and that the coffee-cup is a perfect insulator. Calculate a. heat of the surroundings, qsurr, b. heat of the system, qsys, c. the reaction enthalpy, Hrxn. a. Heat of surroundings = Heat of solvent water or q(surr) = q(water) q(water) = (mass solution) x (specific heat water) x (Tfinal -Tinitial) mass solution = (total solution volume) x ( density solution) total solution volume = ( ) ml = ml density solution = 1.00 g/ml (approx. that of water) mass solution = ml x 1.00 g/ml = 100. g specific heat of water =specific heat of solution = J/(g o C) q(h2o) = 100.g x [4.184 J/(g o C)] x (31.8 o C o C) = 2,700 J q(surr) = +2,700 J = +2.7 kj (2 sig figs from temp change) b. Heat of system = - Heat of surroundings or q(sys) = - q(surr) q(sys) =-2,700J= -2.7kJ c. Enthalpy of reaction = Heat of system / moles of limiting reagent Δ H(rxn) = q(sys) / (mole limiting reagent)* *moles of limiting reagent = Molarity x Volume (L). Since both reagents have equal concentrations, equal volumes, and a 1:1 mole ratio in the balanced reaction, the reagents are stoichiometrically equal and either may be used as the limiting reagent. moles CHO2H = 1.00 M x 50.0 ml x (1 L/1000mL) =.0500 mol moles NaOH = 1.00 M x 50.0 ml x (1 L/1000mL) =.0500 mol ΔH(rxn) = -2.7kJ/0.0500mol = -55kJ/mol Chemicals: CAUTION: All of the reagents used in this experiment can cause chemical burns to the skin and damage to clothing. In the advent of a spill, report the incident to your instructor. Aqueous solutions of 2.00 M HCl, NaOH, NH4Cl Solid, Metal cylinder, Food Items Lab Equipment: Thermometers, Styrofoam cups with a Styrofoam lid, Tape for labels, Hot Plate 50.0 ml graduated cylinders, 10.0 ml graduated cylinders, 100 ml beakers, Pop Can, Aluminum Foil

3 Coffee Cup Calorimetor Procedure: Part 1: Determination of reaction enthalpy for NaOH-HCl HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l) Δ H =? You are to perform two trials for this reaction. All solutions can be washed down the drain with plenty of water. Rinse and dry the calorimeter and graduated cylinders before beginning a new reaction. 1. Obtain exactly 50.0mL of each reagent (NaOH & HCl) in separate, clean and dry graduated cylinders. Label with tape each graduated cylinder as NaOH or HCl. Record these values on your data sheet as volume of acid/base in cylinder before pouring into cup. 2. Measure the temperature of each solution using the same thermometer to 0.5 o C. Remember to rinse and dry the thermometer between measurements. If the two solutions are not at the same temperature ( 1 o C), calculate and record the average of the two solution temperatures as T initial. 3. This step must be done quickly! With the help of your lab partner, add the two reagent solutions to your calorimeter, place the lid on the cup (with the thermometer inserted into the hole in the lid) and begin monitoring the temperature of the reaction solution while slowly swirling the cup. When the maximum temperature has been observed, record it as T final. 4. Using a 10.0mL graduated cylinder, measure the volume of each reagent solution that remains in the larger graduated cylinders. Record these values as volume of acid/base in cylinder after pouring. Calculate and record the volume of acid/base reacted. 5. All solutions can be washed down the drain with plenty of water. 6. Repeat steps 1-4 for the second trial of this reaction. 7. Calculate q(water), q(surr), q(sys), H(rxn) and average H(rxn) Part 2: Determination of reaction enthalpy for dissolving NH4Cl NH4Cl(s) NH4 + (aq) + Cl - (aq) ΔH=? 1. Weigh out 10.0 grams of solid NH4Cl. Record the mass to the nearest g. 2. In a clean, dry graduated cylinder, obtain exactly 50.0 ml of R.O. water. Record this value on your data sheet as volume of H2O in cylinder before pouring into cup. 3. Measure the temperature of R.O. H2O using the same thermometer to 0.5 o C. Remember to rinse/dry the thermometer between measurements. Record this temperature as T initial. 4. Add the 50.0mL of H2O to the calorimeter. 5. This step must be done quickly! With the help of your lab partner, add the solid NH4Cl to your calorimeter, place the lid on the cup (with the thermometer inserted into the hole in the lid) and begin monitoring the temperature of the reaction solution while slowly swirling the cup. When the minimum temperature has been observed, record it as T final. 6. Using a 10.0mL graduated cylinder, measure the volume of H2O remaining in the graduated cylinder. Record this value as volume of H2O in cylinder after pouring. Calculate and record the volume of water added to the cup. 7. Repeat steps1-6 for the second trial of this reaction. 8. Calculate q(water), q(surr), q(sys), H(rxn) and average H(rxn) 9. Wash all glassware before returning it to your drawer. Rinse the Styrofoam cup and lid. Do NOT throw them away. Return them to the bin. Part 3: Determination of the specific heat capacity of a metal cylinder CAUTION: The hot plate, beaker, and steam are very hot! Use caution and tongs. 1. Record the mass of a metal cylinder to grams. 2. Obtain 75.00mL of HOT water from the sink. Place the hot water in a 100mL beaker on a hot plate. Set the heat setting to Setting # 6 or 250 o C. Cover with a watch glass.

4 3. After the water boils, use tongs to remove the watch glass and carefully submerge the metal cylinder in the boiling H2O. Caution: steam is very hot! 4. Heat for at least 15 minutes in the boiling H2O. Measure the temperature of the boiling water bath to 0.5 o C. Record this as the INITIAL temperature of the heated metal cylinder. 5. While the metal cylinder is heating, use a clean graduated cylinder to measure 50.0mL of room temperature H2O. Put this water in another 100 ml beaker. Measure and record the temperature to 0.5 o C. This is the INITIAL temperature of the room temperature H2O. 6. Assume 1 ml H2O = 1 gram H2O. Calculate and record the mass of the room temperature water. Remember: Density of H2O = 1 gram per 1 ml. 7. With tongs, remove the metal cylinder from the boiling water and submerge it in the room temperature H2O. Gently stir the water with the thermometer to transfer the heat from the hot metal cylinder to the water. Record the HIGHEST temperature that this H2O reaches as the FINAL temperature of the metal cylinder and the FINAL temp. of the H2O. When taking the temperature reading, the thermometer should not be touching the bottom of the beaker nor the metal cylinder. 8. Repeat this experiment again. Record your data as Trial Dry the metal cylinder and return it. Leave the hot plate in the hood to cool. 10. Calculate Delta T, q(water), q(metal) Part 4 Food Energy Procedure 1. Suspend a soda can using a ring stand in the fume hood, see Figure Fill the pop can with exactly 50.0 ml of water. Suspend the can above the solid support stand or burner. 3. Construct a holder for the food using a paperclip and an aluminum foil tray. Weigh the paperclip, foil, and food. 4. Carefully light the food on fire and center it under the pop can. Measure the temperature change in the water that results from the heat given off. Stir continuously with the thermometer during each burn to obtain a uniform water temperature. 5. Allow the water to be heated until the food sample stops burning. Record the maximum (final) temperature of the water in the can (usually reached about 30 seconds after extinguishing the flame). 6. The ash left after burning the food is very sooty, so try not to get it on your clothes. Use an Aluminum Foil tent to put the flame out if needed. Do not remove it from the hood if it is still on fire. Figure 1. Pop calorimeter can set-up 7. Analyze two or three food items. You may want to do the same item more than once. Record your results in the table.

5 Notes: It may take about 10 seconds to get the food ignited. Be sure that when the food sample burns, it is close to but not touching the soda can. If it is too close to the bottom of the can, it may extinguish too early due to a lack of oxygen. Black carbon soot will deposit on the bottom of the can when the food burns. For best results, this soot should be wiped off with a little water and a paper towel between trials. Figure 2. Cheeto example while burning. 8. Calculate the energy released per unit mass, in kilojoules per gram and Calories per gram, for each food. Calculations: 1. Determine the change in temperature of the water by subtracting the initial water temperature from the final water temperature. T = Tfinal Tinital 2. Calculate the heat gained by the water using Q = mc T. The specific heat of water (C) is 1.0 cal/g C. These values will give you the heat gained in calories. 3. Convert the heat gained from calories to food Calories (kilocalories, CAL) by dividing the answer above by Determine how much of the food burned by subtracting the final mass of the food container from the initial mass. 5. Calculate the energy content per gram of the food sample. This is done by dividing the heat gain of the water (in Calories), by the change in mass of the food sample, giving CAL/g Theoretical Results: Cheetos Puffs Peanuts in the Shell Cashews Almonds Macadamia Nuts Dietary calories ~ 6.1 CAL per gram ~ 7.2 CAL per gram