Chemical Equilibrium
Le Chatelier’s Principle Teaching Notes
GENERAL COMMENTS
In science classes it is common to
distinguish between physical changes and
chemical changes by declaring that physical
changes are reversible (e.g. the freezing and
melting of water) while chemical changes
are not. It is useful for students to observe
examples of chemical changes that are
reversible.
An important concept of equilibrium is that
certain stresses cause equilibrium to shift.
These stresses are concentration,
pressure/volume (for gaseous equilibrium
systems), and temperature. This lab
exercise examines the effects of changing
ion concentration and temperature on
equilibrium systems.
SOLUTION PREPARATION
Solution
Mass solute needed; dilute
to 1 L with distilled water
0.1 M K2CrO4
19 g K2CrO4
0.1 M K2Cr2O7
29 g K2Cr2O7
0.1 M FeCl3
16 g FeCl3 dissolved in 50
mL 6M HCl. Dilute to 1 L
with distilled water
0.1M KSCN
10 g
0.1 M KCl
7g
0.2 M CoCl2 .
6 H2O
2.6 g CoCl2 . 6 H2O, dilute
to total solution volume of
100 mL (not 1 L). Add
enough concentrated HCl to
make this solution turn
blule.
PURPOSE
ƒ
To observe the effect of various stresses
(ion concentration; temperature) on
equilibrium systems.
SAFETY
1. Use extreme caution when handling the
acidified cobalt(II) chloride hexahydrate,
CoCl2. 6 H2O. HCl, used to make the
acidified solution, is highly corrosive.
Any spills should be reported immediately.
Spills on the skin should be flushed with
cold water.
2. Potassium chromate, K2CrO4 is a hazardous
substance. Use with care.
3. Use caution when using the hot water bath.
Le Chatelier’s Principle
1
SAMPLE RESULTS
Part 1. Chromate – Dichromate Equilibrium
2 CrO42-(aq) + 2 H3O+(aq)
yellow
K2Cr2O7
Fe3+(aq) + SCN-(aq)
Cr2O7-(aq) + 3H2O(l)
light brown
orange
Solution
K2CrO4
Part 2. Iron(III) – Thiocyanate Ion Complex
Fe(SCN)2+(aq)
red
Test
Tube
Stress
Applied
Initial
Color
Final
Color
initial colour
yellow
1
Control
light
brown
--
HCl added
orange
2
Fe3+ added
light
brown
darker
red
NaOH added
yellow
3
SCN- added
light
brown
darker red
4
Cl- added:
decreases
[Fe3+]
light
brown
very light
brown to
clear
initial colour
orange
NaOH added
yellow
Comments.
HCl added
orange
The addition of either reactant, Fe3+ or SCN-,
causes the equilibrium to shift to the right, or
Comments.
Adding an acid will increase the concentration
of H3O+ ions in the equilibrium system, forcing
the equilibrium to shift to the right, or product
side. This is demonstrated by the solution
becoming orange.
product side, causing a deepening of the colour.
Adding more Cl- ions causes Fe3+ to be
removed from the solution (due to the
formation of FeCl4-). The decrease in the
concentration of Fe3+ causes the equilibrium to
shift to the left, or reactant side, which results
in a lightening of the colour of the solution.
Adding a base reduces the concentration of
H3O+ ions in the solution, causing equilibrium
to shift to the left, or reactant side. This is
demonstrated by the solution becoming yellow.
Le Chatelier’s Principle
2
Part 3. Cobalt(II) Chloride Complex;
Effect of Temperature
[CoCl4]2- + 6 H2O
blue
Temperature
[Co(H2O)6]2+ + 4 Cl-
pink
Solution Colour
room temperature
blue
hot water bath
blue
cold water bath
pink
This allows us to predict, in simplified form, on
which side of the equation the energy term
should appear:
blue
pink + heat
or
[CoCl4]2- + 6 H2O
[Co(H2O)6]2+ + 4 Cl- + heat
Comments.
According to Le Châtelier’s Principle, adding
heat to an equilibrium system favors the
endothermic reaction. Since the equilibrium
favored the blue side in the hot water bath we
can conclude that the reverse reaction is the
endothermic direction.
Conversely, Le Châtelier’s Principle tells us
that the removal of heat favors the exothermic
direction. When placed in the cold water bath
the solution turned pink; thus the forward
direction is the exothermic direction.
Le Chatelier’s Principle
3
CONCLUSIONS AND QUESTIONS
Part 1. Chromate – Dichromate Equilibrium
1. Use Le Chatelier’s Principle to explain the
color changes observed in both test tubes
with the addition of both HCl and NaOH.
2 CrO42-(aq) + 2 H3O+(aq)
yellow
Cr2O7-(aq) + 3H2O(l)
orange
Adding HCl, an acid, increases the
concentration of H3O+ ions; adding NaOH,
a base, decreases the concentration of H3O+
ions.
Le Chatelier’s Principle
Part 2. Iron(III) – Thiocyanate Ion
Comlex
2. Use Le Chatelier’s Principle to explain the
color changes observed in Test Tubes 2 – 3
with the addition of both FeCl3, KSCN, and
KCl.
Fe3+(aq) + SCN-(aq)
light brown
Fe(SCN)2+(aq)
red
4