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Heats of Combustion
Objectives:
-to use a burning nut to heat a sample of water
-to use the heat capacity and temperature of a sample of water to
determine the caloric content of different types of nuts
-to compare experimentally derived caloric content with label information
Equipment/Materials
two or more types of nuts1
small aluminum pie tin
modeling clay or Playdoh®
water
empty soda can
measuring cup
fireplace matches or lighter
scale (your trusty balance)
pin or needle (2-3 inches long)
tongs
thermometer (that can measure temperatures between 0 – 100°C, 32 − 212°F)
Introduction
Nutritional labels on the foods we eat provide a great deal of information. One
piece of information included is the caloric content of foods. We all know that fattening
foods contain greater amounts of calories than healthier alternatives, such as fruits and
vegetables. You also probably know that if you eat more calories than you expend in
metabolic processes and exercise, you will gain weight. If you consume fewer calories
than you expend, you will lose weight. However, if you look at the scientific definition
of a calorie, you will see it defined as the amount of heat required to raise the temperature
of 1.000 g of water from 14.5 °C to 15.5 °C. What does this have to do with food?
To understand this we must look at heat and energy from the perspective of a
chemical reaction. When chemical bonds break and re-form in a chemical reaction,
energy is often absorbed or released. A good example is photosynthesis. Green plants
use energy from the sun and process this energy through a molecule called chlorophyll to
convert water and carbon dioxide into glucose (sugar) and oxygen. The overall reaction
can be written as:
→ C6H12O6 + 6O2
6CO2 + 6H2O + energy from sunlight 
carbon dioxide
water
glucose
oxygen
The plant then metabolizes glucose and recoups that energy, using it for various
metabolic processes like growth, transpiration, reproduction, etc. The reaction for the
metabolism of glucose is ultimately just the reverse of photosynthesis:
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We have had good luck with peanuts, walnuts, hazelnuts, cashews, almonds, and pecans.
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C6H12O6 + 6O2 
→ 6CO2 + 6H2O + energy for metabolism (heat)
Interestingly, the overall energy associated with a chemical reaction is the same,
whether the reaction occurs stepwise at low temperature inside a living cell or rapidly at
high temperature inside a furnace. In this case, the oxidation of glucose that occurs
inside the cell through with the regulation of various enzymes is nothing more than the
combustion of glucose. So the energy released gradually inside the cell is overall the
same energy that would be released if you simply burned glucose. In the cell it is just
released in small manageable increments.
So what does all this have to do with the calories? Well let’s think about how we
measure and use heat. Let’s look at what occurs when you turn on the burner underneath
a pot of water on your stove. Assuming that you have a gas range, you are burning
methane (CH4) to produce carbon dioxide and water. Just like the metabolism of
glucose, this reaction gives off heat:
CH4 + 2O2 → CO2 + 2H2O + heat
The heat released by this reaction is absorbed by the pot and the water it contains. As the
pot/water combination absorbs this heat, the temperature rises. The greater the amount of
water, the more heat is required to raise it to a certain temperature. So a simple way to
measure the heat used and released by different chemical reactions is to connect it to the
change in temperature of a particular substance, like water. This is how a calorie is
defined. As mentioned above, it is the amount of heat that is required to raise the
temperature of water 1 °C.
We talk about consuming calories, but how can we eat heat? As we digest and
metabolize the food that we eat, bonds are broken and re-formed. These processes
release heat that your body can use to fuel other processes that require heat. The energy
derived from food is described by its caloric content. However, it is important to note the
difference between the calorie and the Calorie with an uppercase C. The calorie is
defined above (the amount of heat required to raise the temperature of water 1 °C). The
Calorie is equal to 1000 calories with little c’s. Another way to state this is that a Calorie
is in reality a kilocalorie or kcal. So when you heat a bagel that contains 280 Calories
(280 kcal), in reality it provides 280,000 calories of energy, enough to raise the
temperature of 280 kg of water 1°C!
In this experiment, you will be relating the caloric content of foods, specifically
nuts, to the energy required to heat water. The experiment requires a flame, so you
should exercise caution. You will use the heat capacity of water in calories and its
temperature change to determine the caloric content. The equation you’ll use is:
heat (cal) = Cwater x ∆T
2
where
Cwater is the heat capacity of water in calories:
Cwater = (1.0 cal/g) x (mass of water in grams)
∆T is the change in temperature = Tfinal − Tintial in °C
We will assume the density of water is ~1 g/mL, thus 100 mL of water ≈ 100 g of water.
You will also need to determine the caloric content given on the label for comparison to
your experimentally derived number. Remember the calories given are actually
kilocalories. Also serving sizes may be given in ounces so you may need the following
conversion factor:
28.35 g = 1 ounce
If your thermometer is Fahrenheit-based rather than °C, you’ll also need this conversion
factor:
5
°C = (°F − 32°)
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Procedure
Use a can opener to remove the top of the soda can. Measure 100 mL of water and place
it into the soda can.
Determine and record the mass of the nut. If the mass is greater than 2 – 3 g, you will
need to choose a smaller nut or break off a smaller piece to use in the experiment.
A nut or piece of nut 1 g or less will work just fine.
Place the nut firmly onto the end of the pin or needle. Wrap the other end of the pin or
needle with a clay cone (see figure) and secure it to the bottom of the pie tin.
Measure and record the initial temperature of the water.
Ignite the nut with one of the fireplace matches or the lighter. It may take repeated
efforts to get it lit, so you will want to avoid using smaller ordinary matches.
Once lit, use tongs to hold the soda can (filled with water) over the flame. With
your other hand, place the thermometer into the water and note the temperature
rise inside the can.
Continue heating the water with the burning nut and paying attention to the rising
temperature.
When the temperature stops rising and/or the nut extinguishes, record the final highest
temperature.
Repeat this process with another of the same kind or a different kind of nut. You should
do two trials per type of nut and examine at least two different kinds of nuts. Start each
trial with a fresh sample of cold water. You are also welcome to explore other nut-like
foods, e.g. “Wheat Nuts”.
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thermometer
tongs
soda can with
lid removed
100 mL water
nut
pin or needle
modeling clay or Playdoh®
Experimental set up
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Name_______________________________
Data sheet
1. First convert all your temperature readings from Fahrenheit to Celsius if applicable.
2. Enter your data into the table below.
Type of nut
Mass of nut (g)
Initial
temperature (°C)
Final
temperature (°C)
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∆T
∆T= Tfinal − Tinitial
Experimental
caloric content
(see below)
Label-derived
caloric content
(see below)
To determine the experimental caloric content, multiply the mass of the water by the
change in temperature,∆T:
heat (cal) = (1.0 cal/g) x (mass of water in grams) x ∆T
To determine the label-derived caloric content, you must determine the number of
calories in the small sample of nut you actually used in each trial. To do this, use the
label to determine the kcal/oz and convert that to kcal/g. Then multiply by the number of
grams in each sample in your various trials. For example from the label below from a jar
of almonds, you can see that in a 1.0 oz serving, there are 170 Calories, or 170 kcal.
Using our ounce to gram conversion,
Nutrition Facts
28.35 g = 1 ounce,
Serving Size 1 ounce (30 g)
we can set up the following ratio:
Servings Per Container 1
Amount Per Serving
Calories 170 Calories from Fat 140
% Daily Value *
23 %
5%
Total Fat 15 g
Saturated Fat 1 g
Polyunsaturated Fat 4 g
Monounsaturated Fat 10 g
Cholesterol 0 mg
Sodium
Total Carbohydrate 5 g
Dietary Fiber 4 g
Sugars 1 g
Protein 6 g
Vitamin A 0 %
Calcium 8 %
0%
0%
2%
8%
Vitamin C 0 %
Iron 6 %
Vitamin E 35 %
Folate 4 %
Magnesium 22 %
Phosphorous 14 %
*Percent daily values are based on a 2,000
calorie diet.
170 kcal ? kcal
=
28.35 g
1.0 g
to determine that there are about 6.0
kcal/g. If in our trial we used a piece of
almond with a mass of 1.2 g, the label
derived caloric content would be:
(6.0 kcal/g) x 1.2 g = 7.2 kcal.
Don’t forget about the Calorie/calorie
discrepancy. Labels provide information
about Calories, which are in reality kcal,
where 1000 cal = 1 kcal. The
measurements and calculations you’ll
make in this experiment will provide you
numbers of calories with a little c.
Answer the following questions:
3. How do the caloric content values that you determined experimentally compare
with those reported on the labels of the different kinds of nuts?
4. The heat transfer from the combustion of the nut to the increase in temperature of
the water was not perfect. List all the places where heat could be lost in this
process. How could you design an experimental apparatus that would minimize
the loss of heat to the surroundings.
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