Thermochemistry: An Endothermic Reaction
Chemistry and Physical Science. Recommended for grades 6-12.
Teacher Prep time: 15 min
Demonstration time: 10 min
Safety Gloves Required
Safety Goggles Required
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Essential Question
How does energy flow during a chemical reaction?
Demonstration Objectives
- Observe an endothermic reaction.
- Describe the flow of heat energy during the reaction.
- Construct an energy diagram for the reaction.
Next Generation Science Standards* (NGSS)
PE HS-PS1-4. Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.
Science and Engineering Practices
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 activity involves barium, which requires special handling.
Procedures
- Weigh 32.0 g of Ba(OH)2 and place it in the beaker.
- Allow students to examine both reactants (barium and ammonium chloride) before beginning.
- Weigh 11.0 g of ammonium chloride. Leave it in the weigh boat.
- Review or introduce the terms exothermic and endothermic.
- Pour enough tap water on the wood block, (about 2 mL), to create a small puddle.
- Introduce the term enthalpy.
- Place the wooden block in a fume hood or on a lab bench in a well ventilated room.
- Use a beaker with a flat bottom.The bottom must make complete contact with the puddle.
- Turn on the fume hood.
- For the block, a piece of 1" × 4" unfinished pine, cut 2 inches wider than the beaker, works well.
- Take the temperature of the beaker contents.
- Record the temperature on the board.
- Put on temperature-resistant gloves
- Winter gloves work well.
- 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.
- Set the beaker at the center of the water puddle on the block.
- Make sure the puddle covers the entire area of the beaker bottom.
- Hold the beaker with one hand while you stir the two solids with a stirring rod.
- Record the temperatures on the board.
- Take the temperature of the beaker contents 3-4 times during the reaction process, which takes 3-5 minutes.
- Look for the formation of ice on the board and a temperature in the beaker below -5.0 °C.
- 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
Analysis & Discussion
Physical Science
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Fill in the blank in the title.
Endothermic
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Was the reaction exothermic or endothermic? What data support your answer?
Endothermic because the temperature decreased
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Did energy flow from the system (beaker with chemicals) to the environment or from the environment to the beaker? Explain your thinking.
Energy flowed from the environment to the system. I know this because the temperature decreased.
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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.
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Balance the reaction.
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Draw an energy diagram for the reaction progress.
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Calculate ΔH for the reaction. Show your work.
ΔH = ∑Hproducts - ∑Hreactants
ΔH = +164kJ/mole
Chemistry
*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.
