Exp 14A: Le Chatelier’s Principle
http://www.youtube.com/watch?v=4-fEvpVNTlE
• When a system in chemical equilibrium is disturbed by a
change in temperature, pressure or concentration (“stress”),
the system shifts in equilibrium composition to relieve the
effect of the change
• Types of stress
–
–
–
–
Change in concentration
Change in temperature
Change in pressure
Effect of a catalyst
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Exp 14A: Le Chatelier’s Principle
N2 (g) + 3 H2 (g)  2 NH3 (g)
1)
Add H2  N2 reacts with the excess H2 until equilibrium  shift to the
right
2)
Remove H2  NH3 will decompose to make more H2 until equilibrium
 shift to the left, relative increase in [H2] and [N2]
3)
Add NH3  Excess NH3 will be decomposed to N2 and H2 until
equilibrium  shift to the left
4)
Remove NH3  N2 and H2 will react to form more NH3 to re-establish
equilibrium  shift to right
Add product: equilibrium shifts towards reactant (to the left)
Add reactant: equilibrium shifts towards product (to the right)
Remove product: equilibrium shifts towards product
Remove reactant: equilibrium shifts towards reactant
2
Exp 14A: Le Chatelier’s Principle
Reactions
 Fe3+ + SCN-  Fe(SCN)2+
(yellow) thiocyanate
(colorless)

iron (III) thiocyanate
(red)
Ni2+ + 6 NH3  Ni(NH3)62+
color change

Acid-base equilibrium with indicator
color change determined by acidity

Solubility of Ca(OH)2 in H2O as a function of acidity

Co2+ + 4 Cl-  CoCl42color change as function of temperature
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Exp 14A: Le Chatelier’s Principle
Reaction #1

Fe3+ + SCN-  Fe(SCN)2+
[Fe(H2O)6]3+(aq) + SCN-(aq) [Fe(H2O)5(SCN)]2+(aq) + H2O(l)
(yellow)
thiocyanate
(colorless)
iron(III) thiocyanate
(red)
1)
2)
3)
4)
5)
6)
7)
8)
Mark 3 test tubes
Add 20 mL diH2O to a 100-mL beaker
Add 20 drops of 0.1 M Fe(NO3)3 + 20 drops 0.1 M KSCN. Mix/Stir
Add 3 mL of the solution to each test tube
Add 20 drops 0.1 M Fe(NO3)3 to tube 1. Mix gently
Add 20 drops 0.1 M KSCN to tube 2. Mix gently
Add 20 drops dH2O to tube 3 (reference tube). Mix gently
Compare the colors in all 3 tubes. View down the tubes to a piece of
white filter paper
9) Record observations on the Results Sheet (p. 269)
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Exp 14A: Le Chatelier’s Principle
Reaction #2
 Ni2+ + 6 NH3  [Ni(NH3)6]2+
[Ni(H2O)6]2+(aq) + 6 NH3(aq)

[Ni(NH3)6]2+(aq) + 6 H2O(l)
color change
1) Add 10 drops of 0.1 M Ni(NO3)2 to a clean test tube
o What is the color?
2) Add drops of 6 M NH3 until the color changes and intensifies
o What is the color?
3) Add drops of 6 M HCl until the color changes again
o What is the color?
 The acid has reacted with NH3 to form NH4+
4) Record observation on p. 269
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Exp 14A: Le Chatelier’s Principle
Reaction #3
 Acid-base equilibrium with indicator
H3O+(aq) + OH-(aq)  H2O(l)
1) Mark 2 small beakers with A (acid) and B (base)
2) Add 10 mL of dH2O and 4 drops of 6 M HCl to beaker A. Swirl &
mix
3) Add 10 mL of dH2O and 4 drops of 6 M NaOH to beaker B. Swirl &
mix
4) Add 1 mL of dH2O to a clean test tube
5) Add 2 drops of indicator solution and 2 drops of dilute acid from
beaker A. Mix gently and record color
6) Add drops of dilute base solution from beaker B until color changes.
Shake gently and record color on page 269
7) Add drops of dilute acid until color changes again. Shake gently and
6
record color on page 269
Exp 14A: Le Chatelier’s Principle
Reaction #3
 Acid-base equilibrium with indicator
Methyl orange
1) Methyl orange is one of the indicators commonly used in
titrations. Its structure is simple enough to be able to see what
is happening as it loses and gains hydrogen ions.
2) You have the same sort of equilibrium as in the litmus case but the colors are different.
3) You should be able to work out for yourself why the color
changes when you add an acid or an alkali. The explanation is
identical to the litmus case - all that differs are the colors.
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Acidic
Basic
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Exp 14A: Le Chatelier’s Principle
Reaction #4: Solubility of Ca(OH)2 in H2O
 Ca2+(aq) + 2 OH-(aq)  Ca(OH)2(s)
Ca(OH)2 solubility changes with acidity
1)
2)
3)
4)
5)
6)
7)
Put 5 mL of 6 M NaOH in small beaker using graduated cylinder
Rinse cylinder thoroughly with dH2O
Use cylinder to add 5 mL of Ca(NO3)2 to beaker
Stir thoroughly with stirring rod. White precipitate forms
Add 5 mL 6 M HCl to beaker. Record the results on page 269
Add 5 mL NaOH to the beaker. Record the results on page 269
Which substance is formed in the reaction(s)?
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Exp 14A: Le Chatelier’s Principle
Reaction #5
 Co2+ + 4 Cl-  CoCl42[Co(H2O)6]2+ + 4Clpink
1)
2)
3)
4)
5)
6)
7)
8)
9)
 [CoCl4]2- + 6H2O
blue
Set up ring stand, ring, metal gauze, and bunsen burner
Place 50 mL beaker with dH2O and bring to gentle boil
Add 5 drops CoCl2 to a clean test tube. Record color (p. 270)
Add 5-10 drops of concentrated HCl (12 M) until the solution
changes color. Shake test tube and record color on p. 270. This is
the color of CoCl42Add 5 drops of dH2O until the solution changes color.
Add 5-10 drops of water to the tube. Shake gently and record color
(p. 270)
Place tube in boiling water and wait until color changes. Record
color
Cool tube in ice water until color changes. Record color
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Repeat steps 6 & 7 as often as you want
Next Monday, Oct 22
• Postlab for Expt. 14A
• Prelab assignment for Expt. 14B
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