Endothermic Reaction Demonstration

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Objective

Students will be able to observe an endothermic reaction which mixes barium hydroxide and ammonium thiocyanate to freeze a flask to a wooden plank.

Big Idea

Endothermic reactions are cold and more difficult to demonstrate. This is safe, cheap, easy, and exciting.

NGSS Background

This lesson is based on California's Middle School Integrated Model of NGSS.

PE: MS-PS1-2 - Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.

DCI: PS1.B – Chemical Reactions: Substances react chemically in characteristic ways. In a chemical process, the atoms that make up the original substance are regrouped into different molecules, and these new substances have different properties from those of the reactants. The total number of each type of atom is conserved, and thus the mass does not change. Some chemical reactions release energy, others store energy.

S&E Practice 4: Analyzing and Interpreting Data

CCC: Cause and Effect

This lesson demonstrates an endothermic reaction, which causes a decrease in temperature and is therefore a chemical reaction (MS-PS1-2). The students will need to determine if a chemical reaction has occurred based on the available evidence (PS1.B, SP4). Mixing two dry chemicals (cause) results in these chemicals turning into a slush (molecular water is released as a liquid) and absorbs energy, leading to a decrease in temperature (effect) (CCC).

Set-up

60 minutes

To perform this reaction the following materials are needed.

  1. Barium Hydroxide
  2. Ammonium Thiocyanate
  3. Small flask with a screw top cap
  4. Wood plank

Directions

  1. Weigh 50g of barium hydroxide and pour in the flask.
  2. Weigh 25g ammonium thiocyanate and add it to the flask.
  3. Immediately place the cap on the flask and give it a good shake (the two dry chemicals will turn to a slush and become cold).
  4. Pour a small amount of tap water on the wooden plank to form a puddle.
  5. Place the cold flask on the wooden plank making sure to place it in the puddle
  6. Wait approximately 10 minutes for the flask to freeze the puddle.
  7. Demonstrate the frozen flask to the class

CAUTION: This reaction produces ammonia gas. Perform with care. It's best to perform this reaction under a hood. If a hood is not available, open the cap outside and allow the ammonia gas to escape before bringing it back into the classroom. Ammonia stinks! 

Demonstration

15 minutes

I place the following chemical reaction on the board and explain that this reaction will absorb energy and therefore be classified as an endothermic reaction. The opposite would be a chemical reaction that releases heat (exothermic) - see Decomposing Hydrogen Peroxide Demonstration (Elephant Toothpaste). It is also defined as a replacement reaction, as the barium [Ba] reacts with the ammonium thiocyanate [NH4SCN] to replace the ammonia [NH4], then bonds with the thiocyanate [SCN] to form barium thiocyanate [Ba(SCN)2].

I mix the two chemicals according to the directions in the "Set-up", being very careful about the ammonia gas that is produced.

I explain that the ammonia gas smells awful and that if the flask were to break we would need to evacuate the classroom. To prove that the flask is frozen, I ask for a volunteer and dramatically tip the flask inside down revealing that it is indeed frozen to the wooden plank.

 

Student Activity

20 minutes

Students create a Venn Diagram, comparing and contrasting exothermic and endothermic reactions. I'm a huge fan of Dinah Zike Foldables for Science and had my students design their Venn Diagrams on one such foldable. Students place this foldable in their Interactive Science Notebook. This video provides directions on creating this foldable.

You may need to review on the board the concept of a Venn Diagram and how it works. They are required to compare/contrast exothermic and endothermic reactions, use three different colors, include a title, and provide labels. For comparison, Decomposing Hydrogen Peroxide (Elephants Toothpaste) or Exploding Hydrogen Balloon Demonstration are excellent examples of exothermic reactions.

 

TIP: One of the hardest parts of this activity was to convince my students that the circles didn't have to be perfect and they wouldn't need a compass.

Extensions

50 minutes

1) To better help teach this lesson I have created a mnemonic sentence to help remember the different types of chemical reactions along with a Powerpoint presentation.

Types of Chemical Reactions

Squids don't carry roses.

  1. synthesis
  2. decomposition
  3. combustion
  4. replacement

2) To teach the difference between exothermic (releases energy) and endothermic (absorbs energy) reactions I explain that the word exothermic has an X in it, the same as the word explosion. Since explosions (combustion reaction) are always hot (no such thing as a cold explosion) exothermic reactions are also hot. I have also included a powerpoint to help with this.

Exothermic & Endothermic Reactions

3) Since this reaction shows a decrease in temperature there is ample evidence that a chemical reaction has occurred. To teach the evidence of a chemical reaction I have included a powerpoint lesson.

Evidence of a Chemical Reaction