Dietary Calories

nutrition label

The dietary calories formative assessment (FA) asks students to do an energy survey to probe how students think about energy in their diets. The FA task reveals students’ thinking about structure-property relationships and chemical mechanism by getting students to look at food labels to try to figure out where in the ingredients the calories come from. Students are asked to share and explain their thinking about the energy content in food and justify their explanations based on models of structural formulas of some ingredients. Students reveal even more thinking about structure-property relationships when asked to explain the relationships between the energy content in food and the energy changes witnessed in combustion reactions.

This FA task was tested with high school chemistry students. In the class in which it was tested, students were learning about the use of calorimetry to measure amounts of heat transfer, and they were focusing on clear ways of defining “system” and “surroundings”. They were also working on identifying different reactions as endothermic and exothermic.

The following question can be used to open the activity: Why do we care about calories? Then, students are given thePDF icon  survey and asked to work on it in groups as the teacher circulates and works with different groups.

 

Teaching reflections

Improvements: Formatting…I would provide room for students to explain their thinking as a separate space on the survey document. This would allow students to elaborate more on previous conclusions, thus revealing more student thinking.

Follow-up Option 1: Post Survey Compare and Contrast

  • Upon reviewing bond energy, enthalpy, or other internal energy topics (heat of formation or combustion) one could return to this topic by reminding students about their survey and share selected student responses (anonymously). Discusses ways that they would answer questions similarly or differently (think, pair, share). Alternatively, students could be given the blank FA and asked to take the survey again to collect pre- and post data.

Follow-up Option 2: Lecture/Discussion

Upon reviewing bond energy, enthalpy, or other internal energy topics (heat of formation or combustion) one could return to this topic, likely in the form of a lecture with a demonstration, about the abstract nature of measuring internal energy. Remind students that we cannot describe energy in absolute terms because we can only measure energy through transformation. Essentially, explain to students that internal energy (bond energy) is only measurable in relative terms based on common products generated in a chemical reaction.

  • To provide an example, we would use glucose. It is incomplete to say that glucose contains a certain quantity of energy per molecule, but rather its energy can be measured as a function of the amount of heat lost when glucose is combusted. This example, combined with other representative chemical reactions helps scientists to determine the average energy of representative bonds in molecules. It is just an approximation that gradually improves with additional testing/experimentation.

Follow-up Option 3: Open Discussion

  • Students demonstrate the ability to calculate the energy transferred based on bond energy data (published). Question students on how they think this energy was experimentally determined? Open the floor to student ideas or solutions.

 

Examples of student work (names are pseudonyms)

Part 1: The human body uses food to make energy. Peanut butter (nutrition facts label shown above) is a common snack food. A typical serving of peanut butter contains 250 Calories. Here are some common ingredients in peanut butter: 1) peanuts, 2) oil, 3) sugar, 4) salt.

Student Which peanut butter ingredient do you think gives your body the most energy? Explain your thinking. How could we find out which ingredient has the most energy? Explain your thinking. Where do you think the energy in our food comes from?
Tori sugar or peanuts We can light them on fire and see what burns the longest. Energy comes from carbs and protein.
Elephant Peanuts, because they are the major ingredient? Or sugar, which gives you brief energy burst. You could test something with what equilibrium temp. it reaches in water, but that might only be heat energy. The sun, ultimately (photosynthesis transfers it to food).
Clara Peanuts: they contain the highest caloric value out of those ingredients, plus they contain protein. Find out which ingredient has the highest caloric value. The calories. Calories are a measurement of energy. (calories and joules measure energy)
I’mWithHer Peanuts give the most energy because it is the leading ingredient in the peanut butter. We can figure out the caloric value of each ingredient and compare them. The “energy” in food comes from the sugars and nutrients in it as it is processed in our body.
Dan Peanuts Which sustains you longer wins. You ever heard something called the water cycle or like the cycle in general kind of shows how plants get energy, photosynthesis.
Alberta I think that sugar gives the most energy. It breaks down more easily in your body. The time it needs to break down into a simple sugar could perhaps be measured. I think that…
Mr. Worldwide Sugar
Glucose → energy
Look at calories Glucose/ carbohydrates
Agrippa Sugar – easiest to break down and convert to energy We could light each on fire and whichever burns the longest has the most energy. It comes from the –ose in each food
Sizzle Sugar b/c glucose is used in cell respiration to produce ATP. Look at which ingredient has the most calories. The broken bonds b/c energy is released.
Oswald Peanuts b/c they’re high in protein. Oil-fat used to make energy. Looking at their molecules/bonds. The molecules of H, O, N and C.
Frenchie Sugar because it breaks up easily. Break down each ingredient and see the time it takes. Glucose, carbs, nutrients

 

Part 2: The table below shows the molecular structure of some of these ingredients found in peanut butter.
part 2

 

Student What happens to these ingredients when we digest food? Where do you think the energy in our food comes from?
Tori They break up It comes from the chemical makeup breaking down.
Elephant They are broken down to extract nutrients. Released during the reaction to break them down.
Clara Enzymes attack them and break them down so that our body can actually utilize the energy they contain. All things contain energy, whether it be a blade of grass, a rock, a cow, etc. Plants contain the most energy because they are primary consumers (sun’s energy), and apex predators contain the least because they are secondary, third, fourth, etc., consumers. 
I’mWithHer They are broken down in our stomach and nutrients are absorbed. The energy comes from the sugars and nutrients of food as it is absorbed in our stomach.
Dan They are broken down and absorbed by human. Minerals or the sun or wherever the food got its nutrients from.
Alberta They break into simpler structures. I think that it comes from breaking down the sugar molecules.
Mr. Worldwide They break apart — chemical reaction. breaking + forming bonds  releases energy (ATP)
Agrippa They break apart The process of breaking them apart creates usable energy.
Sizzle They are broken down The broken bonds that release energy
Oswald Our bodies break them down into molecules and uses them for energy The molecules of H, O, N and C
Frenchie They break apart into smaller structures Carbon and hydrogen and oxygen

 

Part 3: The first law of thermodynamics tells us that energy cannot be created or destroyed during a chemical reaction. One interesting and common type of chemical reaction is combustion. Burning a candle and lighting a Bunsen burner are both examples of combustion react. Combustion reactions combine carbon-containing molecules with oxygen (O2) to make carbon dioxide and water. This process is exothermic (ΔEsystem < 0) and produces energy in the form of heat and light. Below is a balanced equation for the combustion of methane (CH4).

part 3

 

Student Where do you think the energy produced by a combustion reaction comes from?
Tori The energy comes from the change of chemical bonds.
Elephant The movement of the oxygen/methane molecules? Breaking/reforming of bonds during the reaction?
Clara If it is an exothermic reaction, the energy comes from the two reactants (I believe), and if a reaction is endothermic, the energy comes from the surroundings (I believe, yet again…).
I’mWithHer The energy comes from the heat produced in this reaction as it disperses from the system.
Dan If all combustion requires fire, then the heat and fire is what provides the initial energy.
Alberta Perhaps the friction between the  molecules provides energy for the reaction.
Mr. Worldwide Breaking bonds releases energy.
Agrippa The energy comes from the change in chemical bonds.
Sizzle When the bonds are broken in a combustion reaction, energy is released.
Oswald Bonds switching
Frenchie The friction between molecules.