Chemical Equilibrium

Nancy Zipprich D.D. Eisenhower H.S.
12700 Sacramento
Blue Island, IL 60406


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


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

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!
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