CHEMISTRY 161 – EXPERIMENT 7
HEAT OF COMBUSTION: How Our Bodies Get Energy
INTRODUCTION:
Most of the energy we get from our food is supplied by carbohydrates and fats. These food molecules are
broken down in our cells in oxidation reactions. In the presence of excess oxygen, the food molecules react in a
complex series of reactions to produce CO2 and H2O. We breathe in oxygen to sustain this process and exhale
carbon dioxide and water vapor as reaction products. The energy released in these reactions fuels our basal
metabolism (the chemical reactions constantly occurring to maintain basic body functions) and our physical activity.
Because the products of these oxidation reactions are the same when food is burned in a calorimeter as when it is
metabolized in the body, we can approximate the energy value of carbohydrates and fats in our bodies by doing a
simple calorimetry experiment.
Food chemists use a bomb calorimeter to measure the energy evolved from the combustion of food. A
bomb calorimeter is a sealed steel vessel or “bomb” designed to withstand high pressures and surrounded by a
known volume of water – a sealed container at constant volume. The quantity of energy (measured in joules or
calories) produced by the combustion of a known amount of food inside the bomb can be calculated from the
temperature increase of the water. The heat absorbed can be calculated as follows:
q water = mass * C * ΔT q water = q food burned
where m is the mass of the solution in grams, C is the heat capacity in calories per gram per degree (for water this is
1cal/gºC), and ΔT is the temperature change (in ºC). This will give the heat absorbed in units of calories;
nutritional information labels report the heat in Calories (1 Calorie = 1 kcal and 1 kcal = 1000 calories).
It should be noted that this approximation does not work as well for proteins because:
* Their nitrogen is released as N2 in a bomb calorimeter whereas the body produces mainly urea, (NH2)2CO.
*Laboratory measurements of the energy content of foods tend to be higher than the energy obtained by the
body because a small percentage of the food is not digested or absorbed as it moves through the digestive tract.
This percentage varies depending on the efficiency of a person’s digestive system.
*A portion of the food’s energy must be used for the process of digestion and metabolism.
PROCEDURE:
1. Using a paper clip and a cork (or rubber stopper), construct a “nut support stand” to hold your burnt
offering. Place nut on the stand.
2. Fill a soda can with exactly 175g of water.
3. Suspend the soda can above the nut support stand.
4. Insert a thermometer in the can and measure the temperature of the water (after about 1 minute)
5. Carefully light the food on fire and measure the temperature change in the water that results from the
heat given off. (record the highest temperature)
 Stir the water thoroughly to be sure it has a uniform temperature.
 DO NOT leave the thermometer in the can while the nut is burning, unless you can suspend
the thermometer so it does not touch the bottom of the can.
6. Repeat the type of nut and a cheese puff.
 The ash left after burning the food is very sooty, so try not to get it on your clothes.
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HEAT OF COMBUSTION DATA & RESULTS
Nut
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
l.
m.
n.
Calories per serving (from nutritional info label)
Grams per serving (from nutritional info label)
Calories per gram (from nutritional info label)
Mass of water, g
Mass of food + holder (before burning)
Mass of residue + holder (after burning)
Mass of food burned (line e-line f)
Initial temperature of water, oC
Final temperature of water, oC
Change in temperature, ΔT,(line i-line h)
Heat generated, calories
Heat generated, Calories
Energy value of food, Calories/ gram of food
Percent Error
Do all calculations below:
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Cheese curl
POST-LAB QUESTIONS:
1. Did your food burn completely? How do you know?
2. Is the energy liberated in the combustion of your food the entire amount of energy contained in the
sample?
3. How would your energy amount compare to the energy liberated from the same sample if metabolized
in your body?
4. Assuming that you were able to use a sealed bomb calorimeter to completely burn the food, what
would you expect to find inside the bomb when the combustion was complete?
5. The fat in peanuts is primarily a triester of oleic acid and has an empirical formula of C57H104O6.
a. Write a balanced chemical equation for the metabolism of peanuts in your body.
b. What type of reaction is this?
Adapted from Sandra Laursen and Heather Mernitz, The Fuss About Fats in Our Diet, John Wiley & Sons, Inc., New
York, 2000, pp. 75-77.
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