Download 13 Calories of nuts

Chemistry 151
Lab 13: Determining the Caloric Content of Nuts
Last Updated Dec. 2013
Introduction
Calorimetry is the study of heat as it’s transferred between systems or produced by a chemical reaction.
The principle equation that’s used for such studies is
q = mcT
Here, q is the amount heat transferred to or from the system being studied. The value of q will be negative
if the system is losing heat during the process (exothermic) or positive if heat is gained (endothermic).
The symbol m represents the mass of the system in grams and c is the specific heat capacity, the amount of
heat required to raise the temperature of 1 gram of a substance by 1 degree (this temperature can be in
Celsius or Kelvin, since both scales have the same increments; the Kelvin scale is just 273 units higher).
The specific heat capacity of water, for example, is 4.184 J/gC (or J/gK). In other words, it requires
4.184 Joules of heat to raise the temperature of 1 gram of water by 1 degree. Finally, T is the change in
temperature. The mathematical symbol for change, , is calculated by subtracting the initial value from the
final (e.g., Tfinal – Tinitial). Like specific heat capacity, this can be measured in either Celsius or Kelvin.
A common method for determining the amount of heat produced by a system is to have it heat a known
mass of water. By measuring the change in water’s temperature, we can determine the heat absorbed by
the water. Ideally, this is the same amount of heat lost by the system.
qwater = -qsys
A device used to perform such an experiment is known as a calorimeter.
When food is digested by the body for energy, it essentially undergoes a combustion process similar to
those found for nonbiological systems. For example, cellular respiration can be shown by the following
general equation
C6H12O6 + 6O2  6CO2 + 6H2O
The energy produced by such reactions is the same whether it occurs inside the body or burned from an
open flame. This allows one to use a calorimeter to experimentally determine the Calories in many types of
food.
In this lab you will use calorimetry to approximate the number of Calories in varies types of nuts. After
determining the amount of heat transferred to the calorimeter in units of Joules, you can convert this to
Calories and compare it to the value listed in the nutritional information. One note, however: there are
actually two units of calories. A “normal” calorie (cal – lower case c) is the amount of heat required to heat
a gram of water one degree Celsius.
1 cal = 4.184 J (exactly)
The information on food labels is given in “food calories” (Cal – capital C), which is equal to a kilocalorie.
1 Cal = 1 kcal
Procedure
1.
2.
3.
4.
5.
6.
7.
Assemble your calorimeter as follows, using the (admittedly crude) drawings in Figure 1 as a guide.
a. Make four small holes in a soda can, 90 degrees apart, about a quarter of the way down from
the top.
b. Straighten two paper clips and place them through the can so they crisscross each other.
c. Using a graduated cylinder, measure 200.0mL of deionized water then transfer the water to
the can.
d. Attach a ring clamp to a stand and set the can inside the ring.
e. Place a thermometer in the opening of the can and clamp it in place. Make sure it’s
submerged in the water, but not touching the bottom or sides of the can.
f. Make a stand for the nuts by partially unfolding a third paper clip and sticking in a cork
stopper.
g. Place the stand on the ring stand beneath the can. The bottom of the can should be
approximately 2 cm above the top of the stand.
Record the mass of a cocktail peanut then place it on the stand.
Ignite the peanut with a match and allow it to burn
Immediately after the flame has extinguished, unclamp the thermometer and stir the water inside the
can. Record the maximum temperature reached by the water.
Carefully record the mass of the peanut remains.
Repeat steps 2-5 twice more for three determinations. Allow the water to cool between trials.
Repeat steps 2-6 with another type of nut.
Figure 1
Waste Disposal
Peanut remains can be thrown in the trash. Return the cans to the reagent area so other sections can reuse
them.
Name: _____________________________
Section: ________
Data
Cocktail Peanut
Trial
1
2
3
Volume of water, mL
____________
____________
____________
Mass of water, mL
(density = 1.00g/mL)
____________
____________
____________
Initial mass of peanut, g
____________
____________
____________
Initial temperature of water, C
____________
____________
____________
Final temperature of water, C
____________
____________
____________
Final mass of peanut, g
____________
____________
____________
Mass combusted, g
____________
____________
____________
Energy absorbed by water, J
____________
____________
____________
Energy released by peanut, J
____________
____________
____________
Energy released by peanut, Cal
____________
____________
____________
Energy released per gram, Cal/g
____________
____________
____________
Average, Cal/g
Actual energy per gram, Cal/g
____________
____________
Percent different, %
____________
(exp. – actual)/actual
Show your work for each of the following calculations from Trial #1.
1) Mass combusted
2) Energy released by peanut (Joules)
3) Energy released by peanut (Calories)
4) Energy released per gram
Other brand of nut
Type of Nut
_______________________
Trial
1
2
3
Volume of water, mL
____________
____________
____________
Mass of water, mL
(density = 1.00g/mL)
____________
____________
____________
Initial mass of nut, g
____________
____________
____________
Initial temperature of water, C
____________
____________
____________
Final temperature of water, C
____________
____________
____________
Final mass of nut, g
____________
____________
____________
Mass combusted, g
____________
____________
____________
Energy absorbed by water, J
____________
____________
____________
Energy released by nut, J
____________
____________
____________
Energy released by nut, Cal
____________
____________
____________
Energy released per gram, Cal/g
____________
____________
____________
Average, Cal/g
Actual energy per gram, Cal/g
____________
____________
Percent different, %
____________
(exp. – actual)/actual
Name: _____________________________
Section: ________
Post-lab Questions
1. How would each of the following errors affect the experimentally determined value of energy released
per gram (incorrectly low, incorrectly high, or no effect). Explain your answers
a) Unbeknownst by the student, when transferring water to the can, some ran down the side of can and
leaked out of one of the holes that were made for the paper clip.
b) The initial temperature of the second trial was higher than the first.
c) Some of the charred remains fell off the nut before the final weighing.
2. Assuming most of what burned was peanut oil (C57H104O6), write a thermochemical equation for the
reaction that took place (use the value of Calories from the label to determine H).
3. A Peanut Butter Cup Fudge Ripple Cheesecake from The Cheesecake Factory clocks in at around 930
Calories per slice. An average slice weighs around 120 grams.
a) If you repeated this lab using a piece of cheesecake that was equal to mass of your peanut from Trial 1,
how high would the temperature have risen?
b) Using the actual value of Calories and your peanut mass from Trial 1, how many peanuts would you
have to eat in order to burn the same amount of energy from a slice of Peanut Butter Cup Fudge Ripple
Cheesecake?
c) A typical person burns around 45 Calories per mile from walking. How many miles would you have to
walk to burn that slice of cheesecake?
d) Is it still worth it?
Name: _____________________________
Section: ________
Pre-lab Questions
1. Define the following
a) Combustion
b) Calorimetry
c) Specific heat capacity
2. If the temperature of 150.0g of water increased from 23.3C to 44.8C, how much heat was absorbed
a) in units of Joules?
b) in units of Calories
3. When you determine the energy released per gram (Cal/g), which mass are you going to use?
4. Given the method used in this lab, is the experimentally measured value of calories in a peanut expected
to be higher than or lower than its actual value? Why?