Chemical Equilibrium

Nancy Zipprich                 D.D. Eisenhower H.S.
                               12700 Sacramento
                               Blue Island, IL 60406
                               708-597-6300

Objectives:

     The student will:
       1. distinguish between reactions that go to completion and those that 
          are reversible.
       2. explain the concept of chemical equilibrium.
       3. understand how Le Chatelier's Principle works on a chemical reaction
          at equilibrium.

Materials needed:

     (for teacher demonstration with student participation)
     matches, "Match Box" car that changes colors

water scooping demo: 2 equal sized battery jars (or 2 2000 mL beakers), 2 medium 
plastic cups, one smaller plastic cup 

cup demo: 4 or 5 sets of: 2 plastic cups (may or may not be same size), water, 
2 eyedroppers (or 2 straws of different sizes), plain water and colored water 

NO2-N2O4 demo: 3 sealed tubes containing these gases at equilibrium, beaker full 
of ice, beaker full of boiling water, empty beaker, hotplate 

cobalt (II) chloride hexahydrate equilibrium: this chemical solution is made by 
putting .6 g into 20 mL ethanol, safety goggles, apron, test tube, rack and 
holder, dropper bottles full of: (1) distilled water, (2) .1 M silver nitrate, 
(3) conc. HCl, beaker full of ice water, beaker full of boiling water 

Strategy:

1. Demonstrate how a toy (such as hot wheels that change color) will change 
color in response to different temperatures.  Hopefully this will instill 
curiosity about reversible reactions. 

2. Burn a match and get the class to realize this is a reaction that has gone to 
completion.  Explain other reactions that go to completion (ex. complete 
combustion, rusting, decomposition) and put a sample equation of this type on 
the board, discussing its one way arrow and arrangement. 
      match + oxygen  gives  ash + carbon dioxide + water + HEAT

3. Review the placement of energy on the right side as meaning an exothermic or 
energy releasing reaction and energy on the left as endothermic or an energy 
absorbing reaction.
 
4. Perform the water scooping demonstration using two battery jars and either 
the same size or different size cups to scoop with.  The water level starts out 
filled on one side and empty on the other.  Ask the students to make predictions 
about what will happen.  Then after their predictions have been verified or 
disproven, draw a comparison between this demo and a reversible reaction that 
has reached chemical equilibrium.  No water spilled = a closed system.  Equal 
scooping technique in opposite directions = forward and reverse reactions 
proceed at the same rate.  Eventually the water levels do not change.  The 
levels are not necessarily equal.  Define chemical equilibrium (not only are 
forward and reverse reaction rates equal, but the concentrations of reactants 
and products eventually become constant at equilibrium).  Explain that under 
VERY specific conditions, most all chemical reaction are reversible (when in a 
closed system).  Chemical equilibria usually occurs in all gaseous or all 
aqueous systems.  (Irwin Talesnick, Idea Bank Collection, Vol I, Idea #1). 

5. Have five students come to the front of class and do a similar experiment, 
but this time using two beakers or transparent cups (do not have to be the same 
size).  One cup will have plain water in it and the other will have colored 
water.  Mark original water levels.  Water transfer will be accomplished by 
using eyedroppers (identical technique of transfer, as in last demo) and the 
students will tell when they have reached equilibrium and why.  (Irwin 
Talesnick, Idea #284) 

6. This will serve as a review of equilibrium learned thus far and perhaps it 
will come out in discussion that it does not matter how much reactant or product 
there is to start with, a certain equilibrium will be reached at that particular 
temperature.

7. Demonstrate a real chemical equilibrium using the NO2-N2O4 gas tubes in both 
cold and hot water.  At this point introduce the concept that a chemical 
equilibrium's position can be shifted by certain factors.  Temperature is one of 
them.  Define Le Chatelier's Principle in relation to this demo.  According to 
Le Chatelier, equilibrium systems can also be stressed so that they shift to 
relieve this stress.  Changes in pressure and changes in concentration of 
reactant or product are two ways to stress this closed system.  (This gas demo 
is in most H. S. Chemistry texts.) 

8. EMPHASIZE that whatever is done to stress a system at equilibrium, the system 
tries to relieve the stress by doing the exact opposite. 

9. Perform the demonstration using the cobalt II chloride dehydrated-hydrated 
complex.  A closed system's equilibrium system can be shifted by changing 
temperature, changing concentration of reactants and products and changing 
pressure (Le Chatelier's Principle).  In this demo, pressure was not 
demonstrated as a means to shift equilibrium.  The color can be shifted from 
blue to pink by either adding more pure water or by putting the tube in cold 
water.  The color can be shifted from pink to blue by either adding conc. HCl, 
putting in some .1 M AgNO3 or by heating the tube.  Tie in how the color can be 
shifted in this demo and ask the class to predict how to get the color to shift 
after thoroughly reviewing Le Chatelier's Principle.  Lee Summerlin, Chemical 
Demonstrations, American Chemical Society, Washington D.C., 1987. 

10. Review the objectives.

11. Ask the class for examples of phenomena or toys that could be examples of 
equilibria.  For homework, ask them to design a "controlled" paper wad fight 
that would simulate the idea of establishing chemical equilibrium.  The best 
idea will be carried out the next day.  This is a great learning experience, but 
pick a class that you really trust!