Activation Energy 

Kreidler, Kathy                        Thornridge High School 
                                       1-708-841-5180
                           

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.