Cool Reaction

Introduction

Many reactions produce heat, in fact when people think of chemical reactions, heat production is often expected. However, endothermic reactions, reactions which consume heat, can be just as exciting. One of the most striking examples of this is when the solids barium hydroxide and ammonium thiocyanate are mixed together in a beaker.

Materials

Safety Precautions

Barium salts are toxic by ingestion. Ammonium thiocyanate is also toxic by ingestion. Use caution when handling the beaker or flask. Use tongs if available. The temperatures involved are cold enough to freeze skin. Ammonia vapor is very irritating to eyes and the respiratory tract. Do not allow students to inhale this gas. Review the Safety Data Sheets before beginning this activity. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Use Flinn Suggested Disposal Method #27h for the products of this reaction.

Procedure

1. Transfer 20 g of barium hydroxide and 10 g of ammonium thiocyanate to a flask and mix with a glass or plastic stirring rod.
2. In less than two minutes the solids become liquid. A thermometer placed in the mixture shows the temperature falling far below freezing. An ammonia odor is evident to all who are near the flask.
3. Place the flask in a small puddle of water and your students will clearly see just how cool this reaction is; the water will freeze the flask to the counter top. Alternatively, spray the outside of the flask with water from a wash bottle.
4. After a short time the flask with stopper may be passed around so that everyone can feel it. We encourage the use of a stopper because ammonia gas can be very irritating.

Student Worksheet PDF

13011_Student1.pdf

Teacher Tips

• A 50-mL beaker and Parafilm^® M may be used in place of the Erlenmeyer flask and stopper.

Answers to Questions

1. Describe what happened in this demonstration.

   Two solids were mixed in a flask. Both solids melted together to form a liquid. As this was happening, the temperature of the system was dropping dramatically. It also smelled like ammonia. When the flask was placed in a puddle of water, the water froze and the flask was stuck to the table. The outside of the flask was very cool to the touch.

2. What is the difference between an endothermic and an exothermic reaction?

   An endothermic reaction is a reaction that takes in heat, using heat as a reactant. An exothermic reaction, on the other hand, is a reaction that gives off heat as a product.

3. Was this reaction endothermic or exothermic? How do you know?

   This reaction was an endothermic reaction. Since an endothermic reaction consumes heat as it occurs, it causes the temperature of the system to drop. The temperature was very low as the reaction proceeded, and the flask it occurred in was able to freeze water when placed in a puddle.

4. Write a balanced chemical equation for the reaction between barium hydroxide octahydrate [Ba(OH)₂•8H₂O] and ammonium thiocyanate (NH₄SCN). Make sure to include heat on the correct side of the equation.

   Ba(OH)₂•8H₂O(s) + 2NH₄SCN(s) + Heat →Ba(SCN)₂(aq) + 2NH₃(aq) + 10H₂O

Discussion

This demonstration would be a great way to introduce the concept of heat as a reactant or product. In this reaction, heat is a reactant. It is absorbed from the surroundings as the reaction proceeds. The surroundings lose so much heat that water freezes!

The reaction between the solids is:

Ba(OH)₂∙8H₂O(s) + 2NH₄SCN(s) + heat → Ba(SCN)₂(aq) + 2NH₃(aq) + 10H₂O(l)

This demonstration can be expanded into a lab on thermodynamics by using other ammonium salts. These include ammonium chloride (7 g) and ammonium nitrate (10 g). Have your energetic students (no pun intended) write the equations for these reactions. Thermodynamic data is available for the reaction between barium hydroxide and ammonium chloride or ammonium nitrate. These data can then be tested using calorimetry.

References

Atkins, P. W. General Chemistry; W. H. Freeman: New York, 1989; p 188.

Shakhashiri, B. Z., Chemical Demonstrations: A Handbook for Teachers of Chemistry; University of Wisconsin: Madison, WI. 1983; pp 10–12.

Summerlin, L. R., Ealy, J. L., Chemical Demonstrations: A Sourcebook for Teachers; American Chemical Society: Washington, DC, 1988; Vol. 1, p 66.