LEARN NC

photo of glassware in a chemistry lab

Why does chemistry matter in my life?

By Lisa Hibler

6 Units of heat: How much energy is that anyway?

This introductory lesson requires students to convert between various units of energy (Calories, calories and joules) as they determine the energy released when they eat a snack and the energy used when they are active.

Learning outcomes

Students will:

  • understand that energy is released by the food we eat and used by our bodies.
  • convert between units of calories, Calories and joules.
  • relate and apply chemical principles to their life.

Teacher planning

Time required

One 85-minute class period

Materials needed

  • “How Much Energy is That Anyway?” student sheet (1 for each student)
  • a variety of snack packs (1 per group) for the “How Much Energy Is That Anyway?” activity. Author’s note: Ensure that each snack pack has a nutrition label on it. I purchased different snack packs for each lab group to discourage sharing of calculations between lab groups.
  • materials for endothermic demonstration:
    • barium hydroxide
    • ammonium chloride
    • a small wooden board (6” X 12”)
  • hand warmers or feet warmers (brand names include “Hot Hands” and “Toasty Toes”) for the exothermic demonstration (1 per class) Author’s note: These can be purchased at a drug store or online. They are inexpensive, but not reusable so I use them as a demonstration. The directions are on the packet. The reaction is a simple oxidation of iron.

Pre-activities

  • Before class, prepare the endothermic demonstration:
    1. Put 32 g of Ba(OH)2 8H2O g in a 250 ml Erlenmeyer flask.
    2. Have 11 g of NH4Cl weighed on a piece of weigh paper.
    3. Have a piece of wood available to place the flask on.
  • Students should have an understanding of types of reactions and dimensional analysis.
  • Students should be able to distinguish between heat and temperature.

Activities

  1. Review the difference between heat and temperature with a focus on heat as a form of energy.
  2. Lead into a discussion about the human body as a machine that requires energy and sources of that energy.
  3. Hand out the “How Much Energy Is That Anyway?” activity sheet and have students work in groups of three or four to complete the activity.
  4. After students complete the activity, conduct a class discussion that leads students to understand that food is the source of energy for our bodies. We use a tremendous amount of energy each day and so we require a significant amount of food as the fuel. Have students compare the joules of energy needed to run their car to those needed to run their body. You could base this on the same ten-mile distance that is given in question 3a of the conclusion on the activity sheet.
  5. The reaction glucose + oxygen → carbon dioxide + water + energy, is just one example of how a reaction can produce energy. Ask the students if all reactions produce energy (exothermic). Some will recognize that reactions can also require energy (endothermic).
  6. Demonstrate an endothermic and an exothermic reaction to show that energy (heat) is involved in reactions other than digestion. Perform an endothermic demonstration (prepared prior to the start of class):
    1. Put water on the wooden board.
    2. Add the ammonium chloride to the flask containing barium hydroxide prepared prior to class and swirl to mix.
    3. Walk around the room and ask students to comment on what they observe. They should notice that the two powders mix and begin to form a slurry and eventually a liquid.
    4. Allow students to touch the outside of the flask and describe how it feels to the class. The flask will become very cold, and there may be frost visible.
    5. Place the flask on the wet wooden board and it will freeze to the wood within a minute. This is a very endothermic reaction: Heat is pulled from the water so that the temperature of the water drops below the freezing point.
    6. Follow this with a demonstration of hand or foot warmers, which is an exothermic reaction. Write the equations for each of these reactions on the board. Students can help with formulas and balancing.
      • Heat + Ba(OH)2 + NH4Cl → NH3 + H2O + BaCl2
      • 4 Fe + 3 O2 → 2 Fe2O3 + heat

Assessment

Assess by students’ completion of the “How Much Energy Is That Anyway?” activity.

Critical vocabulary

  • Calorie
  • calorie
  • joule
  • enzyme
  • energy
  • thermochemistry

North Carolina curriculum alignment

Science (2005)

Grade 9–12 — Chemistry

  • Goal 4: The learner will build an understanding of energy changes in chemistry.
    • Objective 4.02: Analyze the law of conservation of energy, energy transformation, and various forms of energy involved in chemical and physical processes.
      • Differentiate between heat and temperature.
      • Analyze heating and cooling curves.
      • Calorimetry, heat of fusion and heat of vaporization calculations.
      • Endothermic and exothermic processes including interpretation of potential energy.
      • Diagrams (energy vs reaction pathway), enthalpy and activation energy.

National Standards

In addition to meeting objectives of the North Carolina Standard Course of Study, this lesson plan addresses the following national standards.

National Science Content Standard A

All students should develop an understanding of:

  • Abilities necessary to do scientific inquiry
  • Understandings about scientific inquiry

National Science Content Standard B

All students should develop an understanding of:

  • Structure of atoms
  • Structure and properties of matter
  • Chemical reactions
  • Motions and forces
  • Conservation of energy and increase in disorder