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Monday, November 14, 2011

How Much Energy Do Different Foods Contain?

How much energy do these foods contain?

This lab is always a favorite of my students!

I have just finished teaching cellular respiration to my students.  As teachers, we spend so much time talking and teaching about the path of glucose through the reactions of respiration, that I worry that students will come away from a biology class thinking that only glucose can serve as a fuel for respiration.  It is important to help them realize that any organic compound (with modifications) can serve as a fuel for respiration.

This lab is just a basic calorimetry lab.  Students are given three different foods (peanuts, fritos, and marshmallows) and asked to determine the energy content in each.  Students will use this equation to do their calculations:

Energy gained by water = (mass of water) X (Δt of water) X (4.18 J/g°C)

This lab can easily be completed by students who have not yet completed a course in chemistry.

After finding the mass of a food sample, and the initial temperature of water in a calorimeter (coke can), the food is ignited and allowed to completely burn.  The heat lost by the burning food is gained by the water in the can.

Students can determine from their data the amount of energy in each food.  The energy is first calculated in kJ/mol and then converted to cal/mol.

I am fortunate to have the Vernier Probe System at my school.  This makes data collection easy and accurate.  You can still do this lab using a traditional thermometer instead of the probe system.

I am sure that the percent error in this lab is quite high.  However, students get a good idea about the energy content of foods.  Most students predict prior to the lab that the marshmallow will have the most energy.   They mistakingly believe that the higher the sugar content, the higher the energy content of the food.  In this lab, peanuts come through with flying colors with an energy content much, much higher than the other foods.

If interested, here is the lab that I do with my students:  Measuring the Energy in Foods

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