CECsci.199


TITLE:    PV = nRT = BANG !

AUTHOR:   DAVID J FRANKLIN,  WEISER HIGH SCHOOL,
          WEISER, IDAHO

GRADE LEVEL/SUBJECT:   grades 11-12
                       Chemistry and Physics.

OVERVIEW:  The ideal gas law is often difficult for
students to deal with.  Manipulating the equation with
algebra can be confusing.

PURPOSE:  This experiment will excite and focus the
students on this important prediction tool :
 PV = nRT

OBJECTIVE(s):  Students will be able to:
1. understand the ideal gas law and how it is used
   as a prediction tool.
2. know the properties of 2 liter soda pop bottles.
3. explain the behavior of nitrogen (both liquid and
   gas).

RESOURCES/MATERIALS:
Liquid nitrogen ( we get ours from the local vet),
thermos, 2 liter PET bottle with lid, garbage can,
water safety equipment.

ACTIVITIES AND PROCEDURES:
1.   Place a large plastic garbage can (50 gal) at
     least 30 yards away from buildings, cars, PEOPLE,
     STUDENTS, etc.
2.   Fill the garbage can at least 1/2 full of water.
3.   Using a funnel, place about 100 ml of liquid N2 in
     a 2 liter PET Coke Bottle.  Teachers should do
     this; wear safety equipment.
4.   Cap the 2 liter bottle tightly and toss into water
     filled garbage can.  Run back toward students.
     (You have 15-20 seconds prior to bottle failure.)
5.   Enjoy the loud explosion of the bottle.
6.   Now, analyze the situation using the ideal gas
     law:

          P1V = nRT1
          P2V = nrT2

          P1  =  nR   =  P2
          --     --      --
          T1      V        T2

          100 ml N2   -->   ________ moles N2
          V = 2 liters

7.   Convert units: atm --> psi, etc.
     I have heard rumors that the PET bottles fail at
     200 psi. See if you agree.

TYING IT ALL TOGETHER:  This spectacular demonstration
opens the door for using the Ideal Gas Model: PV=nRT.
Similar cases can be considered quantitatively.  Graphs
(P vs.T) can be considered.  Other gases can be
considered.
Have fun !