Lesson 16:

Concepts conveyed | Materials | Procedure | Benefits | Resource

Concepts conveyed:

The purpose of this classroom demonstration is to allow students to experience endothermic and exothermic reactions.

Materials:

• 1 first aid instant-cold pack
• 1 first aid instant-heat pack
• 1 tall form beaker (~ 1000 mL)
• powdered sugar (1 lb. bag)
• 30 mL of concentrated sulfuric acid (use caution when handling this acid)

Procedure:

This demonstration should be preceded with the definitions of an exothermic and endothermic reaction.

Exothermic reaction: a reaction that releases heat.
Endothermic reaction: a reaction that absorbs heat.

Take the first aid instant-cold pack, and activate it by breaking the center disk. Pass it around, and explain that this is an endothermic reaction.

Take the instant-heat pack, and activate it by breaking the center disk. Pass it around and explain that this is an exothermic reaction.

Now, perform a demonstration of another exothermic reaction using powdered sugar and sulfuric acid. Use appropriate protective clothing, goggles, and gloves. Add 3/4 pound of powdered sugar to a tall form beaker. Pour 30 mL of concentrated sulfuric acid into the sugar and wait a couple of minutes. It will slowly start to grow into a tall black column. Sugar is a carbohydrate. This means that it contains carbon and water. The sulfuric acid slowly burns the sugar converting it to carbon which is black and releasing the water in a gaseous form (steam). Does the beaker get hot? Steam is just H₂O converted from a more condensed form using the energy released from the reaction. The energy given off in the reaction is absorbed by the H₂O which can then achieve a gaseous phase. The steam (water vapor) causes the black carbon to percolate upwards. Therefore, since energy is being released, this reaction is exothermic.

Benefits:

• The instructor can use this activity as a means to check attendance. He or she can pass around a sign-up sheet along with the instant hot and cold packs during the class period.
• Students can see some practical applications of exothermic and endothermic chemical reactions.

Resource:

• Chemistry for Museums: A Manual of Chemistry Demonstrations; Center of Science and Industry: Columbus, OH, 1976; Show #8, Demonstration #5; Project Funded by a National Science Foundation Grant #7423549.