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Thermochemistry: An Endothermic Reaction

A Carolina Essentials™ Demonstration

Overview

Students observe an extreme, spontaneous endothermic reaction between two solid compounds, measure changes in temperature and make observations during this thermochemistry engagement activity. The demonstration may be used with physical science or chemistry students as an introduction to thermochemistry or as an introductory example of enthalpy calculations for chemistry students. Prior to the demonstration students can review reaction writing and product prediction.

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Teacher Notes
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Grade & Discipline
6-12

Chemistry and Physical Science. Recommended for grades 6-12.

Time Requirements
Prep15 min
Activity10 min

Teacher Prep time: 15 min
Demonstration time: 10 min

Safety Requirements
Safety Gloves RequiredSafety Goggles Required

Overview

Students observe an extreme, spontaneous endothermic reaction between two solid compounds, measure changes in temperature and make observations during this thermochemistry engagement activity. The demonstration may be used with physical science or chemistry students as an introduction to thermochemistry or as an introductory example of enthalpy calculations for chemistry students. Prior to the demonstration students can review reaction writing and product prediction.

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Teacher Notes
Save & Print
Student Worksheet

Essential Question

How does energy flow during a chemical reaction?

Demonstration Objectives

  1. Observe an endothermic reaction.
  2. Describe the flow of heat energy during the reaction.
  3. Construct an energy diagram for the reaction.

Next Generation Science Standards* (NGSS)

Science and Engineering Practices

Practice: Constructing Explanations

  • Students will observe an endothermic reaction and explain the energy differences between reactants to products.

Disciplinary Core Ideas

DCI: Physical Sciences-Energy

  • Students will explain the flow of energy between the reaction process and the surroundings for an endothermic reaction.

Crosscutting Concepts

Concept: Energy and Matter: Flows, cycles, and conservation.

  • Students will explain the difference in energy between reactants and products and the flow of energy into and out of the surroundings.

Safety & Disposal

This reaction produces ammonia gas and should be performed in a fume hood. If no fume hood is available, perform in a large-well ventilated area and stir the contents of the beaker at a safe distance from your nose. An alternative venting method is to use a faucet aspirator connected to an inverted funnel over the reaction beaker.

Follow all federal, state, and local regulations as well as school district guidelines for the disposal of laboratory wastes. The product contains barium which may require special handling.

Procedures

Teacher Procedures

Teacher Preparation and Tips

Student:

Teacher:

  1. Weigh 32.0 g of Ba(OH)2 and place it in the beaker.

Allow students to examine both reactants before beginning.

  1. Weigh 11.0 g of ammonium chloride. Leave it in the weigh boat.

Review or introduce the terms-exothermic and endothermic.

  1. Place enough tap water on the wood block, (about 2 mL), to create a small puddle.

Introduce the term enthalpy.

  1. Place the wooden block in a fume hood or on a lab bench in a well ventilated room.

Use a beaker with a flat bottum.The bottum must make complete contact with the puddle.

  1. Turn on the fume hood.

For the block, a piece of 1" × 4" unfinished pine, cut 2 inches wider than the beaker, works well.

  1. Take the temperature of the beaker contents.

Record the temperature on the board.

  1. Put on temperature-resistant gloves

Winter gloves work well.

  1. Add 11.0 g of ammonium chloride to the barium hydroxide octahydrate.

Ammonia gas will be generated. Ask if any student can identify the smell. If not, inform them that ammonia is a product.

  1. Set the beaker onto the center of the water puddle on the block.

Make sure the puddle covers the entire area of the beaker bottom.

  1. Hold the beaker there with one hand while you stir the two solids with a stirring rod.

Record the temperatures on the board.

  1. Take the temperature of the beaker contents 3-4 times during the reaction process, about 3-5 minutes.

Look for the formation of ice on the board and a temperature in the beaker below -5.0 ℃

  1. Grasp the top of the beaker and lift it. The block will be lifted as well.

Have students record observations. You may want to walk around the classroom for students to observe the ice formation.

Data and Observations

Number Temperature (℃) Observations
1. 23
2. 17
3. 5
4. 0
5. -30

Analysis & Discussion

Physical Science

  1. Use the procedure diagram above and add arrows to show the flow of heat energy through the reaction process.

    Arrows should show heat energy flowing in from the environment into the reactants.

  2. Review: What type of chemical reaction is demonstrated?

    Double replacement

  3. Balance the reaction.

    Ba(OH)2 •8H2O(s) + 2NH4Cl(s) → BaCl2(aq) + 2NH3(g) + 10H2O(l)

  4. Was the reaction exothermic or endothermic? What data supports your answer?

    Endothermic because the temperature decreased.

  5. Did energy flow from the system, (beaker with chemicals) to the environment or from the environment to the beaker? Explain your thinking.

    Energy flow was from the environment to the system. I know this because the temperature went down.

  6. Did the products or the reactants have more energy? Explain.

    The products have more energy than the reactants. To obtain the additional energy, energy is transferred from the surroundings including the liquid water on the board to the products, resulting in the environment cooling.

  7. Fill in the blank in the title.

    Endothermic

  8. Chemistry

  9. Draw an energy diagram for the reaction progress.

  10. Calculate ΔH for the reaction. Show your work.

    Compound Δ Hf° / kJ mol-1
    Ba(OH)2•8H2O(s) -3345
    NH4Cl(s) -314
    NH3(g) -46
    H2O(l) -286
    BaCl2(s) -859
    BaCl2•2H2O(s) -1460

    ΔH = ∑Hproducts - ∑Hreactants
    ΔH = +164kJ/mole

*Next Generation Science Standards® is a registered trademark of Achieve. Neither Achieve nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of, and do not endorse, these products.