LAB: Equilibrium and Le Chatelier’s Principle
Introduction: This demonstration begins with a chemical system at equilibrium. The
equilibrium is described by the following equation:
CoCl42-(aq)
+
BLUE (reactant)
6H2O
<===>
[Co(H2O)6]2+(aq)
+
4Cl-(aq)
+
heat
PINK (product)
Equilibrium means that the rate of the forward reaction is equal to the rate of the reverse
reaction, so no macroscopic changes are evident. The color of the solution is constant.
Part 1: Effect of Concentration on Equilibrium
1. Are there more reactants or products present in this initial equilibrium system? How
can you tell?
2. Le Chatelier’s principle states that if a system at equilibrium is subjected to a stress,
reactions will occur to relieve the stress and establish a new equilibrium. Let’s “stress”
this system by adding some water. Describe the color change that occurs.
3. The stress was increased water concentration, therefore the system reacted so as to
“use up” some of the added water. The color change indicates that there is (more/less)
reactant, and (more/less) product.
4. Has the new equilibrium been established? How can you tell?
5. Now let’s “stress” this new equilibrium by adding some Cl-. We will use an HCl solution,
which contains both H+ ions and the Cl- ions. The H+ ions, however are NOT a source of
“stress” for this equilibrium system. Why not?
6. Describe the color change you observe as excess Cl - ions are added.
7. The stress was increased Cl- concentration, therefore the system reacted so as to “use
up” some of the added Cl- ions. The color change indicates that there is now (more/less)
reactant, and (more/less) product.
8. We could remove the Cl- ions we just added by precipitating them as AgCl. To do this,
we add some AgNO3. Since NO3- is not in the balanced equation for this equilibrium, it
is just a spectator ion. The Ag+ ions combine with Cl- ions, effectively removing Cl- from
the equilibrium system. The system relieves this “stress” by reacting to replace some
of the lost Cl- ions. THIS stress is said to “favor” the forward reaction. What is the
color of the liquid above the AgCl precipitate? Has a new equilibrium been established?
How can you tell?
Part 2: Effect of Temperature on Equilibrium
1. What color would this equilibrium system be if equal amounts of product and reactants
were present?
2. We will set up two test tubes that have equal amounts of product and reactant. One will
be placed in hot water and one in cold. Record you observations.
a. Color of tube absorbing heat:
b. Color of tube releasing heat:
3. In the equation for this equilibrium (as written at the top of this page) endothermic or
exothermic? Explain your reasoning.
4. Adding heat stresses this equilibrium system. How does the system relieve this stress?
5. Is a new equilibrium established? How can you tell?
6. Is the temperature effect reversible? How do you know?
CoCl42- (aq)
+
6H2O
<===>
BLUE (reactant)
[Co(H2O)6]2+(aq)
+
4Cl-(aq)
+
heat
PINK (product)
So now that you understand Le Chatelier’s Principle, try this: Use and  to indicate the
change in concentration of each species as a result of the indicated “stress.” Refer to the
equation to decide each one.
STRESS
CoCl4
2-
Effect on Concentration of…
[Co(H2O)6]2+
Cl-
More H2O is added
More Cl- is added
Some Cl- is removed
Heat is added
Heat is removed
Analysis Question: Paper, soaked in an anhydrous alcohol solution of cobalt chloride, is used
to detect the presence of water vapor in the air. What color would this paper be in a dry
environment? In a moist environment?