- Plants have evolved processes that convert light energy into the chemical bonds of complex molecules. The chemical bonds in carbohydrates, fats, and proteins store energy until needed by the plant. The plant can then release the energy by breaking the appropriate chemical bonds.
- Every animal maintains its life processes by consuming complex molecules that store energy. The processed plants and animals we eat as foods contain varying amounts of proteins, carbohydrates, and fats. Because each of these types of foods contains varying amounts of energy, these foods will release varying amounts of energy when they are used by cells. Within our bodies, the energy is released slowly by a series of chemical reactions.
PRE-LAB PREPARATION:
- By burning pieces of food, the chemical energy stored in molecular bonds is released as heat and light. The heat can be measured in units called calories. A calorie is the amount of heat (energy) required to increase the temperature of one gram of water by one degree C. This process is the basis of the technique of calorimetry.
- The more calories a food contains, the more heat is given off when burned. Foods high in calories will release large amounts of energy. One gram of a protein will release far fewer calories than one gram of fat. You will study foods with different proportions of protein, fats, and carbohydrates to see how much energy (calories) they release.
MATERIALS: Test Foods Test tube (18 x 150 mm) Balance 25-mL Graduated cylinder Utility clamp Large cork with pin Ring stand Matches Thermometer Distilled water PROCEDURE:
1. Assemble the ring stand and clamp so that a test tube placed in the clamp will be one cm above the food sample (see drawing).
2. Place 15.0 mL of water in the test tube and put the test tube in the clamp. Place the thermometer in the test tube.
3. Obtain a 1 to 3 g sample of test food number 1. Find the mass of the test food sample to the nearest 0.01 g (two decimal places), and record its name and mass in the DATA TABLE.
4. Measure the temperature of the water in the test tube to the nearest 0.5 degrees C and record in the DATA TABLE as initial water temperature.
5. Use the pin to affix the sample to the cork. Place the cork on the table away from the test tube. Then strike a match and set the food on fire. Immediately move the sample under the test tube. Gently stir the water with the thermometer, using an up and down motion.
6. After the food sample is completely burned, measure the temperature of the water again to the nearest 0.5 degrees C, and record in the DATA TABLE as final water temperature. Be sure to watch the thermometer carefully, to catch the highest temperature reached.
7. Find the mass of the sample remaining to the nearest 0.01 g and record in the DATA TABLE as mass of sample after burning (ash weight).
DATA TABLE:
Test food # ____
Food name: __________________________
Mass of sample
before burning. _______ g
Initial water
temperature. _______ oC
Mass of sample after
burning (ash weight). _______ g
Final water
temperature. _______ oCCALCULATIONS:
8. Subtract the mass of the sample after burning (ash weight) from the mass of the sample before burning. This is the change in mass.
Change in mass = ________ g
9. Calculate the change in temperature for the water by subtracting the initial water temperature from the final water temperature.
Change in water temperature = ________ oC
10. To estimate the calories in the food sample you will need the mass of the water you heated. By definition the density of water is 1g/mL, so 1 mL of water has a mass of 1 g. The 15.0 mL of water you used would be 15.0 g.
Mass of water = 15.0 g
The following formula will calculate Kilocalories (Kcal). One kilocalorie = 1000 calories.
The specific heat of water is 1 kilocalorie/Kg deg.C. So the formula would look like this.
You will see that all units of measurement except kilocalorie cancil each other out of the equation. Everything is already in the equation except your change in temperature for the water. Put in your change in temperature and work the calculation. You now have the total kilocalories of energy given off by the food sample.
Energy given off by sample = ________ Kcal
11. Calculate the kilocalories per gram of the food sample. This is the total kilocalories divided by the change in mass of the sample. The unit will be Kilocalories/gram.
Kilocalories per gram of sample = ________ Kcal / g
Now repeat the procedure with the next food sample. You may collect the data for all the samples, and then do the calculations. Use a clean test tube each time.
Compair the answer to step 11 for all the food samples.
FURTHER INVESTIGATION:
- The calorimeter you used is very crude, and some of the energy was lost to the air. It will still give you a good comparison of different food samples. You might like to design a better calorimeter, and repeat the experiment.
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- Every animal maintains its life processes by consuming complex molecules that store energy. The processed plants and animals we eat as foods contain varying amounts of proteins, carbohydrates, and fats. Because each of these types of foods contains varying amounts of energy, these foods will release varying amounts of energy when they are used by cells. Within our bodies, the energy is released slowly by a series of chemical reactions.
CALORIMETRY
INTRODUCTION: