Peanut Calorimetry (Mara Desso)

Principle(s) Investigated: Energy, units, states of matter, latent heat of fusion

Standards :

California Physics Standards

Heat and Thermodynamics

3. Energy cannot be created or destroyed, although in many processes energy is trans­ferred to the environment as heat. As a basis for understanding this concept:

a. Students know heat flow and work are two forms of energy transfer between

systems.

California Chemistry Standards

Chemical Thermodynamics

7. Energy is exchanged or transformed in all chemical reactions and physical changes of matter. As a basis for understanding this concept:

d. Students know how to solve problems involving heat flow and temperature changes, using known values of specific heat and latent heat of phase change.

Materials:

Various nuts

Matches/ Lighters

Graduated cylinder

Water

Empty can with lid taken off

Empty can with sides cut out

Large paperclips/ metal wire (or cork and pin)

Balances

Procedure:

CAUTION: This lab contains peanut products. If you are allergic to peanuts please notify the instructor and make arrangements to leave the lab during this period. We will be working with open flames so please TIE LONG HAIR BACK, REMOVE LOOSE PAPER FROM LAB SPACE, WEAR EYE PROTECTION.

1. Choose a nut and measure the mass (g). Record in data sheet.

2. Place nut in the metal holder in the can with sides cut off.

3. Fill topless metal can with ~10ml water (record exact mass of can + water used). Place this can on top of the one with sides cut off. Put thermometer in this can and record initial temperature. Be sure to observe temperature changes as the experiment progresses.

4. Use a match or lighter to light the peanut (be careful not to knock it over!). It may take a few seconds for the nut to catch fire so be patient and hold the flame to the nut until it does catch.

5. Let nut burn completely and reignite if flame dies prior to the entire peanut burning out.

6. CAUTION: Can will be hot! Hold can by the top, not the bottom. Once the nut has burned out, reweigh the mass of the water and can. Record this value in the data sheet.

7. Weigh the remaining charcoal from the burnt nut or snack food. Record this value in the data sheet.

8. Look up the calories per gram of the snack food you have been given. You may need to convert between ounces and grams: 1 ounce = 28.3495231 grams.

Student prior knowledge:

Students will have learned about chemical reactions, states of matter, and have had an introduction basic chemical thermodynamics. They will have been studying thermal equations, latent heat of vaporization, and specific heats. They will have an understanding of the concept of energy and the units used to represent it. Additionally, students should have already taken high school biology so should have been taught about cellular respiration previously. This is an analogous concept to the combustion of foods and will promote student engagement as they will want to understand food Calories more than chemistry calories.

Explanation:

Peanuts are a high calorie food. They contain large amounts of both saturated and unsaturated fats. Burning peanuts is a reaction with oxygen (combustion) that releases the energy contained in the bonds of these fats in addition to the energy released from burning proteins and carbohydrates. Some of this energy is used to heat up water from room temperature (around 20C) to potentially near boiling. Unfortunately, the majority of the energy is lost to the environment due to flaws in the experimental design. Usually this process is performed in a bomb calorimeter that is capable of maintaining and measuring much more of the energy released from food than we can do in the laboratory.

Figure 1: Bomb Calorimeter

Once the peanut has burned out the volume of water remaining and remaining mass of the charcoal are measured. The energy released is used to first heat the liquid water to it's boiling point. The temperature will increase steadily during this time period (see figure 2). If the nut has enough energy to bring the water to it's boiling point, the energy will then be used to vaporize the water molecules rather than raise the temperature. IDENTIFY MISCONCEPTIONS: Students may think that temperature and heat are identical. Make sure to note that heat is a form of energy used to increase the temperature but that it can also be used to break intermolecular forces and release water molecules from each other, resulting in a phase change from liquid to gaseous. Two different calculations can be used to determine the amount of energy released from the peanut:

1. heating the water to it’s boiling point
   • Using specific heat of water:
     • q=mcdT
     • heat energy= mass of water x specific heat of water (1cal/gC) x change in temperature (C)
2. evaporating the volume of water missing after burning
   • Using latent heat of vaporization for water:
     • q=mL
     • heat energy

Figure 2: Changes in states of matter.

Questions & Answers:

1. After looking at the nutrition facts for peanuts, which organic macromolecule do you think is responsible for storing the most energy in the peanut? Why do you think this is so?

Peanuts contain large amounts of both saturated and unsaturated fats. The energy released through the burning of the peanut mainly comes from combustion of these fats. Fats are great sources of energy because of the many high energy bonds that they contain.

2. How is burning a peanut similar to cellular respiration?

Burning a peanut is a combustion reaction. The organic material in the peanut (fats, carbohydrates, proteins) will react with oxygen in the air once enough heat is provided (lighter). This is analogous to the process of cellular respiration in that cells convert energy from glucose (carbohydrate monomers) and other organic compounds into a different form using oxygen. The chemical formula for this process is:

C₆H₁₂O_6(s) + 6O_2(g) → 6CO_2(g) + 6H₂O_(l) + Energy (as ATP)

3. How would you improve this experiment so that there is less error in the system?

A lot of energy is lost to the environment during this experiment. In order to improve the system one could decrease the distance between the flame and the water by bringing the peanut up higher towards the top of the can. One could also decrease the size of the window in the bottom can to increase the amount of insulation from the metal there is. Additionally, the top of the bottom can could be removed so that it did not absorb as much heat that could otherwise go towards heating up the water. Of course, this is a homemade calorimeter and in order to achieve an accurate measurement of the calories in a certain food it would be necessary to use a bomb calorimeter.

Applications to Everyday Life:

1. Understanding food Calories: Many students have heard of Calories and are used to seeing them on the nutrition facts of foods that they eat regularly. Students usually do not understand that Calories are a unit of energy. Many health conscious people count calories to ensure that the energy that their body uses throughout the day is roughly matched or perhaps less than (in the case of those who wish to lose weight) the amount of Calories in the food they eat.

2. Why is it important to eat before playing a sport? Students who play sports know that they need a lot of energy to succeed. They get this energy from their food. Energy is never created or destroyed but is transformed into different usable forms. When a hockey player eats a good breakfast before her big game, she is providing her body with chemical energy that can be transformed into different usable forms (ATP, NADH) by the cells of her body. The mitochondria in her cells convert NADH and FADH2, made from the break down of glucose in the cytoplasm, into ATP using oxygen and creating CO2 and water.

Photographs: Please see attached for photos of this lab.

Videos:

This youtube video uses a can and ring stand to determine the calories in common snack foods.

This video shows an alternative way that this experiment can be performed with a test tube and ring stand.