Calorimetry: Measuring energy in foods.

Many chemistry teachers lead a lab in which students burn food and capture much of the energy in water in an aluminum can.

Since so much of the energy is not absorbed by the water in the can (so much just passes around the can and rises into the classroom), this lab is more about practicing lab skills, including:

• Collecting before and after temperatures to calculate Δt.
• Determining the mass of water in the can by taring the mass of the can.
• Calculating heat using the formula Q=mcΔt.
• Some teachers may include calculating the percent of energy absorbed compared to the experimental results to the values from the nutritional label on the foods.

I’m not going to provide instructions for the lab here (there are so many out there that s quick web search will reveal), but I wanted to share a few techniques that help me.

Food

Students usually want to eat the chips (yes, I know we all tell our students no eating in the lab, but I also suspect many teachers know that students will still try to eat). In order to avoid this, this year I dumped all the chips into 2 L beakers, then put the chip bag into the beaker. I point out to the students that we really don’t know how clean the beaker is, and so they probably don’t want to eat the chips.

This method allows students to be able to read the nutrient labels on the chips, but not be able to feel like the chips are there for eating.

Basic setup

Here’s a photo showing my basic setup. We have electronic thermometers we use for some labs, but for this lab I prefer a simple thermometer so there’s no need for worry about dead batteries, tangled wires, etc. I hang the thermometers from a test-tube clamp so they don’t tip over the can.

Getting cans

I don’t like to offer ‘extra credit,’ but I do sometimes offer ‘bonus assignments.’ If students bring in a can, they get full credit on a ten-point ‘bonus assignment.’ Those who don’t bring in a can are excused from this assignment.

Spreadsheet

I’ve been working with my students on using spreadsheets for their calculations. Here’s a template of a Google Sheet that I provide to my students. The spreadsheet has sample data that they use to create all their equations and when the get to the bottom there is a value provided that they can check to look for typos in their equations (if their last value doesn’t match the check value, they know they need to go back and check their equations). Once they have their equations correct, they delete the sample data and add their actual data.

Comments or questions?

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