Activation Energy 

Kreidler, Kathy Thornridge High School

Objectives The student will define the terms endothermic, exothermic, and activation energy. The student will construct and interpret an energy diagram showing the progress of an exothermic reaction. Equipment and Materials For each pair of students: one vial containing 2 scoops of barium hydroxide, Ba(OH)2
one vial containing 1 scoop of ammonium thiocyanate, NH4SCN
one portable chemical reaction stick (strike-anywhere kitchen match)
One brick
One manicotti noodle

Recommended Strategies As the students enter the classroom, give each one a vial containing either barium hydroxide or ammonium thiocyanate. When class begins, ask for observations. Have students pair up so each pair has both reactants. Instruct the students to combine the two chemicals and shake to mix. They are to focus their attention on any energy changes observed. The vial becomes cold to the touch; this is an endothermic reaction. Next pass out the portable chemical reaction sticks and instruct the students to cause a reaction. Be sure that metal cans or ashtrays are plentiful to ensure safe disposal. Ask about the energy changes observed in this reaction. Heat and light are observed; this is an exothermic reaction. Ask about the amount of energy stored in the reactants and products in the exothermic reaction. Show the relative amounts of energy on a graph. Use different positions of the brick to illustrate the two energy states. When the brick is standing on one small end, it has the most potential energy (representing the unburned match). When it is lying flat, it has less potential energy (representing the burnt match). (Actually, there are three possible states for the brick. I use only two for simplicity.) Ask why the brick doesn't fall down spontaneously. The students will suggest that you need to tap the brick to make it fall. The energy released as the brick falls can be used to smash the noodle with a satisfying crunch. Tapping the brick adds energy. Tapping too lightly rocks the brick but doesn't knock it over. A certain minimal amount of energy is needed; this corresponds to the activation energy. This additional energy can be shown on the graph as the "hump" of activation energy. The endothermic reaction can also be illustrated with the brick. The brick can be moved from the flat position (lower potential energy) to the on-end position (higher potential energy) only by applying a continuous push or pull. Evaluation The students write a letter to "Sandy Sixth-Grader" explaining why Dad has to light the charcoal before he can barbecue the hamburgers. Credit for the brick analogy goes to Ken Spengler.
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