Write A Lab Report About Environment

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Exercise 2 THE CALORIE

 

INTRODUCTION All animals, including humans, need to consume energy in order to survive. We call this energy food. The unit which is used to describe the energy content of food is the Calorie (food calorie, note the capital ‘C’). One calorie (note the lower case ‘c’) is the energy required to raise one gram of water by one degree Celsius. The food Calorie is defined as 1000 calories or one kilocalorie (kcal). Most scientists not dealing with food now use other units such as joules or British Thermal Units (BTUs) to measure energy, but the Calorie is still used to describe food energy. The Calories in food are measured using a CALORIMETER, which is a special combustion device designed to burn food samples completely and capture the energy in a water jacket. For a 100% efficient calorimeter, the energy captured is equal to the energy content of the food as shown below:

Energy captured = (mass of water) × c × (Temp. change) = Energy Content (1)

The constant c is the specific heat of water and is equal to 1 Cg

cal °

(one calorie per gram

per degree C). In our case, however, the calorimeter we use will capture only a fraction of the total energy, so we must account for this reduced efficiency. THE CALORIMETER The device that scientists use to capture the heat energy of a combustible mixture is a bomb calorimeter. It is a metal chamber which is supplied with a measured amount of oxygen during the combustion of the test material. The heat from the burning material is transferred to a water reservoir that surrounds the metal chamber containing the test material. The change in temperature is used to determine the number of calories that the material originally held. A bomb calorimeter is very nearly 100% efficient. It is also very dangerous (thus the name ‘bomb’ calorimeter) and so we will use a simpler (but less efficient) type of calorimeter. A SIMPLE CALORIMETER The simple but effective calorimeter we will use consists of a calorimeter can (a metal cylinder with a notch out of the bottom), a calorimeter lid to hold a 125 ml flask, a flask stopper and thermometer, a sample holder and some tools. Each group of students will have their own calorimeter to use. Because these simple calorimeters are not 100% efficient, the water will not capture all of the energy content of the food. Therefore, the equation for this calorimeter must be modified to:

Energy captured = (mass of water) × c × (Temp. change) = Energy Content × ε (2)

where ε is the efficiency of energy capture by the water. With this equation and a food item of known caloric content, we can determine the efficiency of our calorimeter and explore why the

 

 

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efficiency might vary from one group to another. Once a consistent efficiency is achieved, we can use that efficiency to determine the caloric content of any combustible item. OBJECTIVES: 1) To measure the energy content of a food sample. 2) To determine the efficiency of a calorimeter. LABORATORY PROCEDURE

We will take a sample of a food with a known caloric energy, weigh it, and then burn it in the calorimeter to determine the amount of energy captured by the calorimeter. We will then use the true energy content of the food and the amount of energy captured by the calorimeter to determine the efficiency of the calorimeter and its operators (that would be you and your classmates).

1. Choose a single almond and carefully push it onto the end of a partially straightened paperclip.

2. Zero the scale and then place a weighing dish (a small aluminum foil dish) and the almond/paper clip on the scale to determine the total mass (weight). Record your data.

3. Using the graduated cylinder, measure out 100 ml of distilled water from the water bottle and pour it into the flask. Place the flask stopper with thermometer inserted into the neck of the flask. The place the neck of the flask through the hole in the calorimeter lid and rotate a quarter turn.

4. Measure the initial temperature of the water. Record your data. Make sure to leave the thermometer in the water for a while before reading the temperature.

5. Position the weighing dish in front of the calorimeter. 6. Put on your safety glasses and light the almond. It may take a while to catch on fire. As

soon as the almond catches fire, carefully position the paperclip with the burning almond on the weighing dish. The set up should be such that any ash or broken pieces of almond will fall onto the weighing dish.

7. Carefully slide the weighing dish with the burning almond into the calorimeter so that the flame is directly under or touching the bottom of the flask.

8. Allow the almond to burn until it goes out. If possible, try to keep an eye on it and if it goes out quickly (less than a minute), relight the almond.

9. Once the almond has finished burning, let it cool for a minute while continuing to monitor the water temperature. The temperature may continue to rise for a short period (perhaps 5 to 15 seconds) after the almond stops burning or is removed from the calorimeter. Once the temperature reaches a maximum value and then begins to fall, record the highest temperature reading.

10. After the almond has cooled, (and while you are waiting for the temperature to reach its maximum value) carefully move the weighing dish, almond and paperclip to the scale and determine the final mass. Record your data.

11. Calculate the amount of calories in the food item using the equation provided. 12. Provide the TA with your estimate of the calorimeter efficiency to share with the class.

 

 

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DISCUSSION QUESTIONS: Be sure to address the following as part of the QUESTIONS section of your lab report. The questions should be repeated with detailed answers, using complete sentences.

1. Share the efficiency that your group achieved with the rest of the class and list the efficiencies of each group in tabular form.

2. What is the range of values achieved? 3. What is the average value? Provide at least two reasons why your group’s efficiency

differs from that of the other groups. 4. What would cause the calorimeter efficiency to be less than 100%? Provide at least two

possible causes. 5. What human errors might explain the variation in the observed efficiencies? 6. What improvements could be made to this calorimeter or experimental procedure in order

to reduce the variation in observed efficiencies and achieve a higher efficiency? Discus at least four improvements.

 

 

 

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Experiment (Almonds)

Data Collection

1. Mass of Water = 100 ml (volume) × 1g/ml(density) =__________gm

2. Initial Water Temperature = ______________°C

3. Initial weight of almond/paperclip and aluminum weighing dish =________________gm

4. Residual weight of almond/paperclip and aluminum weighing dish (after burning) =

__________gm

5. Weight of combustion = initial weight (step 3) – residual weight (step 4) =_________gm

6. Final Water Temperature =____________°C

7. Temperature Change = final water temp (step 6) – initial water temp (step 2) =______°C

Calculations

8. Unit Energy in Almonds (from Nutrition Facts on the bag) converted from kcal/ounce to

cal/gm:

_________ kcal/oz × ozgm kcalcal

/28 /1000

=_______________cal/gm

 

9. Energy Burned = weight of combustion (step 5) × unit energy in Almonds (step 8)

=_________ cal

10. Energy Captured = mass of water (step1) × 1 cal/gm/°C (specific heat of water) × Temp.

Change (step7) = ______________cal

11. Efficiency of Calorimeter, ε = ( ) ( )9

10 stepburntenergy

stepcapturedenergy

× 100 = __________

12. The “accepted” efficiency of the calorimeter you used is 65%