Lab 7: Heat and Calorimetry

Transcription

1 Concepts to explore: Gain applicable knowledge about calories Compare the calorie content of food samples Introduction Have you ever roasted marshmallows over a campfire? You have probably heard that foods have calories, but do you know what a calorie is? A calorie is defined as the amount of energy it takes to raise the temperature of one gram of water by one degree Celsius. Heat is a transfer of energy, and measuring the amount of heat given off by burning food is one way to determine how much energy (calories) that food contains. Specific heat is a physical property of a substance. It is the amount of energy needed to raise the temperature of 1.00 g of a substance by 1 o C. The specific heats of many common substances are known; for example, glass has a specific heat of 0.22 cal/g oc and copper has a value of cal/g oc. From the definition of the calorie, you already know the specific heat of water, which is defined as 1.00 cal/g oc. As you can see in Table 1, water has a very high specific heat compared to most common substances. This means that it takes more energy to increase the temperature of water than an equal mass of copper or glass. The high specific heat of water makes it useful as a coolant in everything from engine radiators to simple cold compresses. In this lab you will determine the caloric content of different junk foods by measuring the amount of heat a sample gives off when burned. The following equation relates the amount of heat energy added to a substance to the change in temperature of that substance: E = mc T Figure 1: You have probably looked over the nutrition facts label on many of your favorite foods but what does the number of Calories actually mean, and how does it relate to chemistry? In this equation E is energy measured in calories, m is the mass of the heated substance in grams, ΔT is its change in temperature (⁰C or K), and c is its specific heat in cal/g oc. Since it is difficult to measure the change in temperature of a food directly, we can use a calorimeter to obtain the heat indirectly. A simple form of calorimeter is a metal container filled with water and suspended above a combusting material. The energy given off by the burning material transfers as heat to the water sample, increasing its temperature by ΔT. Knowing the mass of the water sample and it s specific heat (1.00 cal/g oc), we can calculate the energy contained by the original food sample using the above equation. The assumption is that energy is always conserved, so that all the heat given off by the food sample is gained by the water. In other words: Q lost, A = Q gained, B where Q is the quantity of heat energy lost by substance A and gained by substance B. 77

2 An important thing to remember is that food calories are often expressed as Calories with a capital C. One Calorie is actually equivalent to one kilocalorie, or 1000 calories with a lowercase c. This means that the number calories found using the formula above must be divided by 1000 to convert to the food Calories listed in the nutrition facts of your favorite food. The conversion can also be written out: 1 Cal = 1 kcal = 1000 cal You now have all the information needed to determine the caloric content of a substance. For example, if 0.500g of a mini marshmallow is completely burned underneath a can containing ml of water and the water temperature changes from 24 o C to 28.5 o C, the number of Calories per gram contained in the marshmallow can be calculated by the following steps: Example 1. Use the density of water to determine the mass of water heated: Mass = Density x Volume m = (1.00 g/ml)*(100.0 ml) = 100 g 2. Use the constant for the specific heat of water: c = 1.00 cal/g C 3. Determine the change in temperature of the water: ΔT = (28.5 C 24.0 C) = 4.5 C 4. Calculate the energy gained by the water, which equals the energy contained (lost) by the marshmallow: E = mc T E = (100.0 g)*(1.00 cal/g C)*(4.5 C) = 450 cal 5. Calculate the number of calories per gram of marshmallow: 450 cal calories/g marshmallow = = 900 cal/g g Table 1: Specific heats of some common substances Substance In cal/ g K Aluminum Copper Gold Lead Ethyl Alcohol Water Iron/Steel Glass Wood Convert to Calories (kilocalories) per gram of marshmallow: 900 cal 1 Cal calories/g marshmallow = x = 0.90 Calories/g g 1000 cal 78

3 Pre lab Questions 1. In terms of food, what is a calorie? 2. A piece of chocolate chunk cookie was tested the same way as is used in this procedure. The cookie sample had a mass of g and the temperature of a ml of water increased by 7.87 ⁰C. How many Calories per gram did the cookie contain? Show all calculations. 79

4 Experiment: The Calorimetry of Junk Food In this experiment, you will measure the caloric content of junk food using a calorimeter made from an aluminum can. The experimental number of calories can then be compared to the actual number of calories reported by the manufacturer on the package. Make sure to treat the samples the same in order to get a more accurate comparison. CAUTION: This experiment can produce excessive smoke. If possible, perform the following procedure under a stove s ventilation hood or near a window that can be opened. Do not use foods that contain ingredients to which you have a known allergy, such as peanut products. Materials Safety Equipment: Safety goggles, gloves Split rubber stopper 2 Large metal paper clips Tripod 50 ml beaker Scale 100 ml graduated cylinder Distilled water* Butane lighter Thermometer Clay Cheetos or similar snack food* Mini marshmallow* Aluminum can with tab (12oz)* *You must provide Procedure 1. Measure between 75 ml of distilled water into a 100 ml graduated cylinder. Record the exact amount of water in Table Carefully pour the measured water into an empty aluminum can that still has a tab attached. 3. Set the aluminum can on the tripod as shown in Figure Insert a thermometer into a split rubber stopper. Place the thermometer into the can as shown in Figure 2, with the stopper resting on the top of the can. This allows you to adjust the height of the thermometer by sliding the stopper up or down. The thermometer should touch the water but not the bottom of the can. 5. Gently straighten the outside fold of another large paper clip. This will be used as a holder for the food sample. 6. Insert the end of the paper clip that is still folded into a hunk of clay. The hunk of clay should be approximately the size of a ping pong ball. 7. Tear a junk food sample into a piece that is about 1 cm 2. Determine the mass of the piece of junk food and record it in the data table. Remember NO eating in the lab! 80

5 8. Insert the straightened end of the food holder into the sample. HINT: If this does not hold the sample you can make a loop at the end of the paper clip to rest the sample in. 9. Place a 50 ml beaker approximately half full of water near the apparatus to extinguish smoke after the sample has finished burning. 10. Record the initial temperature of the water inside the aluminum can in Table Light the butane lighter. CAUTION: Burns can occur with the use of flames. 12. Hold the clay end of the sample holder and carefully bring the sample into the flame until it ignites. HINT: The sample should be held in the flame for a few seconds to assure the sample will burn strongly. 13. Immediately and carefully bring the burning food approximately 1 cm below the bottom of the aluminum can in order to minimize the amount of heat lost. CAUTION: Excessive smoke can result from the ignited sample and can be a respiratory irritant. If there is excessive smoke the sample should be relit immediately. 14. Watch the thermometer as the food sample completely burns to ash. If the food sample goes out before it is completely burned or is producing only a little flame and excessive smoke, quickly relight it in the lighter flame and place it back under the aluminum can. Record the maximum temperature that is reached. 15. Immediately after the sample has completely burned, dip it into the beaker of water and wait for it to cool. 16. Place the remains of the sample in the trash. Wash end of the paper clip, and then dry it with a paper towel. 17. Repeat the procedure from steps 7 16 using other foods from your pantry. (Hint: Use a sample size that has a similar mass to the previous sample.) Thermometer Aluminum can with tab Burning food sample Tripod Food holder (clay and paperclip) Figure 1: Apparatus for calorimetry experiment 81

6 Data Table 2: Data for Calorimetry Experiment Junk Food Volume of Water (ml) Mass of water (g) Mass of Junk Food (g) Initial water Temperature ( o C) Maximum Temperature ( o C) Water ΔT ( o C) Sample A: Cheetos Sample B: Marshmallow Sample C: Sample D: Calculations (Make sure you use the mass of the water for the mass in the first part of the calculation). 1. Assuming that all the heat from each food went into heating the water in the aluminum can, calculate how many Calories per gram each sample contains. HINT: Food calories are measured in kilocalories! A. Food Sample A 82

7 B. Food Sample B C. Food Sample C D. Food Sample D 83

8 2. Using the nutrition facts and serving size reported by the manufacturer, calculate how many Calories per gram each food contains as reported by the company. A. Food Sample A B. Food Sample B C. Food Sample C 84

9 D. Food Sample D Post lab Questions 1. Which food sample had the most calories per gram? Was this what you expected? Why or why not? 2. Were your measured Calories/g values the same or close to the ones you calculated from the information provided on the manufacturers labels? 85

10 3. What are the primary sources of error in this experiment that could explain any differences between your data and the label information? (HINT: How do these sources of error relate to our assumption that Q lost = Q gained?) 4. Why is it important for the food to burn completely? 5. How might you alter the experiment to allow for a more accurate transfer of heat between the burning sample and the calorimeter water? 86