Lab: Heat and Calorimetry

Transcription

1 Objectives Lab: Heat and Calorimetry Gain applicable knowledge about calories Compare the calorie content of food samples Introduction Most people are aware that foods contain calories, but what is a calorie? A calorie is the amount of energy necessary to raise the temperature of 1 gram of water 1 degree Celsius. Your body burns the energy contained in the foods you eat. On average, most people burn approximately 2000 calories per day. Have you noticed that your body feels warmer the more you exercise? That is because heat is one way that energy is transferred. Figure 1: Federally required nutrition labels provide people with information about the amount of energy contained in each serving. The labels also provide other nutritional information. One of the physical properties of any substance is specific heat. Specific heat is the energy required to raise the temperature of 1.00 gram of the substance by 1 degree Celsius. In Table 1, you can view the specific heat of many common substances. In this laboratory experiment, you will burn food to measure the amount of heat energy it produces from that you will be able to determine the amount of calories (energy) that food contains. The equation below shows the relationship between the amount of energy, in the form of heat, which is added to a substance to change the temperature of that substance: E = mct E = Energy measure in calories m = mass of the substance in grams c = specific heat in cal/g - C T = the change in temperature ( C or K)

2 In order to measure the temperature of a food sample, you will need to burn it and measure the amount of heat it gives off. You will need to construct a calorimeter to measure the heat produced from the burning material this is referred to as an indirect measurement. The heat from the burning food sample will heat a container filled with water. A thermometer will measure the change in temperature of the water. Since energy is always conserved, the heat absorbed by the water from the burning food sample will provide us with the information necessary to calculate the amount of energy contained in the food sample. The amount of energy in food is measured in Calories. One Calorie (with a capital C ) is equivalent to 1 kilocalorie, or 1000 calories (with a lowercase c ). Substance Cal/g K Gold Lead Copper Iron Glass Aluminum Wood Alcohol Water Table 1: Specific heat of common substances. Example Calculation: Here s an example on the steps necessary to calculate the calories in a piece of food. This is accomplished by measuring the amount of energy transferred by heating milliliters of water from 24 to 36.5 degrees Celsius by burning a marshmallow which masses at grams. 1. Determine the mass of the water heated: Mass = Density x Volume m = (1.00 g/ml) x (100.0 ml) = 100 g 2. Apply the constant for the specific heat of water: c = 1.00 cal/g C 3. Calculate the change in the temperature of the water: T = (36.5 C - 24 C) = 12.5 C

3 4. Calculate the energy lost by the marshmallow this equals the energy gained by the water: E = mct E = (100.0 g) x (1.00 cal/g C) x (12.5 C) = 1250 cal 5. Calculate the calories per gram of marshmallow: calories/g marshmallow = 1250 cal g = cal/g 6. Convert from calories to Calories (kilocalories) per gram of marshmallow: calories/g marshmallow = cal 1 Cal x = Calories/g g 1000 cal Pre-lab Questions 1. How does a food calorie differ from a food Calorie? What does a calorie measure in terms of food? 2. A gummie bear was tested through a flame-calorimeter test. The bear had a mass of grams and the temperature of milliliters of water increased by 15.0 degrees Celsius. How many Calories were in the gummie bear? Show all of your calculations.

4 Experiment: Heat and Calorimetry CAUTION: In this experiment, you will be working with fire, hot water, and heated metal. The experiment will produce smoke. Be sure to work in a well ventilated area, such as a stove hood, or near an open window and with the supervision of an adult. DO NOT burn foods that you know you are allergic to such as peanuts. You will need to construct a calorimeter with the equipment listed. Be sure that the thermometer does not touch the sides or bottom of the soda can. Materials Eye googles or safety glasses and gloves Large (unfolded) paper clips Butane lighter Thermometer Cork from bottle (2) 12 ounce aluminum soda can Tripod (can be constructed from tin can or wire coat hanger and canning lid) Snack foods from container with nutrition label Water Glass jars Measuring cups Kitchen measuring scale

5 Procedure 1. Measure 1 cup (118 milliliters) of distilled water. 2. Carefully pour the measured water into an empty aluminum soda can. 3. Set the aluminum can on the tripod as shown in Figure Insert a thermometer into a split cork to use as a stopper. Place the thermometer into the can as shown in Figure 1, with the stopper resting on the top of the can. You can adjust the height of the thermometer by sliding the stopper up or down. The thermometer should touch the water but not the bottom or sides of the can. 5. Insert the end of the unfolded paper clip that is still folded into a cork. 6. Tear a junk food sample into a piece that is about 1 centimeter squared (cm 2 ). Determine the mass of the piece of junk food and record it in the data table. NO eating in the lab!

6 7. 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. 8. Fill a large glass jar or bowl approximately half full of water near your calorimeter. Use this to extinguish smoke after the sample has finished burning. 9. In Table 1, record the initial temperature of the water inside the aluminum can. 10. Light the butane lighter. CAUTION: Burns can occur with the use of flames. 11. Hold the cork 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. 12. Immediately and carefully bring the burning food approximately 1 centimeter 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, relight the sample immediately. 13. 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. 14. Immediately after the sample has completely burned, dip it into the beaker of water and wait for it to cool. 15. Place the remains of the sample in the trash. Wash end of the paper clip, and then dry it with a paper towel. 16. Repeat steps 7-16 using other foods from your pantry. (Hint: Use a sample size that has a similar mass to the previous sample.)

7 Data Junk Food Volume of Water (ml) Mass of Water (g) Mass of Food Sample (g) Initial Water Temp. ( C) Max. Water Temp. ( C) Water T ( C) Sample A (Specify) 118 ml Sample B (Specify) 118 ml Sample C (Specify) 118 ml Sample D (Specify) 118 ml Calculations 1. If all of the heat from the food sample was transferred to the water in the can, calculate the Calories per gram that each sample contains. Remember, Calories are measured in kilocalories. a. Food sample A b. Food sample B

8 c. Food sample C d. Food sample D 2. Use the information from the nutrition facts label, including the serving size, to calculate the amount of calories each food contains. a. Food sample A b. Food sample B

9 c. Food sample C d. Food sample D

10 Post-lab Questions 1. Which food sample had the highest number of calories per gram? Were you surprised by these findings? 2. Did your measurements match those reported by the food manufacturer? 3. Where did error most likely occur in this experiment? 4. Is it important for the food to completely burn? Why or why not? 5. How could the experiment be modified to reduce the amount of error?