Le Chatelier's Principle

Introduction

Le Chatelier's principle states that an equilibrium will attempt to shift in a direction that will counteract a stress that is placed on it. Think about this. If you increase the concentration of a reactant or a product in a chemical equilibrium, what do you think will happen to the equilibrium? If you decrease the concentration of a reactant or a product, what do you think will happen to the equilibrium?

Bromcresol green in acid and basic solutions is involved in an equilibrium. We will represent bromcresol green with the formula, HIn. In water solution the reaction that takes place is:

HIn(aq) --> H⁺(aq) + In^-(aq)

We will also want to consider this equilibrium:

H⁺(aq) + OH^-(aq) --> H₂O

Iron(III) nitrate solution reacting with a potassium thiocyanate solution also is involved in an equilibrium.

Fe³⁺+(aq) + SCN^-(aq) --> FeSCN²⁺(aq)

Also important are:

Fe³⁺(aq) + 3OH^-(aq) --> Fe(OH)₃

and

H⁺(aq) + OH^-(aq) --> H₂O

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1. Place an acetate sheet on a white background with large, labeled squares. Add 3 drops of distilled water to each of the "squares" on the sheet.
2. Add 1 drop of bromcresol green solution to each square. View the colors against a white background. Note and record the color. Add 1 drop of 0.1 M HCl to the first spot. Mix. Note and record any color change.
3. Add 1 drop of 0.1 M NaOH to the third spot. Mix. Note and record any change.
4. Add 0.1 M NaOH, one drop at a time, to the first spot, with mixing after each addition, until you notice any change. Add 0.1 M HCl, one drop at a time, to the third spot, with mixing after each addition, until you notice any change. Note and record all changes.
5. Try to reverse the colors of the first and the third spots again. Record your procedure and results.
6. Rinse the acetate sheet at the sink.
7. Place an acetate sheet on a white paper marked with squares. Place 1 drop of 0.01 M KSCN into each of 5 "squares" on the acetate sheet. Add one drop of 0.01 M Fe(NO₃)₃ to each spot.
8. Make no additions to the first spot.
9. To the second spot, add one drop of 0.1 M Fe(NO₃)₃.
10. To the third spot, add one drop of 0.1 M KSCN.
11. To the fourth spot, add one drop of 0.1 M NaOH. Stir with a toothpick. Note any changes.
12. To the fifth spot, add one drop of 0.1 M NaOH. Stir with a toothpick. Then add 1 drop 0.1 M HCl. Stir. Note any changes.
13. Wait a few minutes. Reexamine the fourth and fifth spots. Note any changes. Add 1 drop of 0.1 M KSCN to each. Note any changes.
14. Rinse the acetate sheet at the sink.

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HIn(aq) --> H⁺(aq) + In^-(aq)

color:

1. What must have been true about the amounts of HIn and In- at equilibrium when the solution was:
   1. green ______________________________________

   2. yellow _____________________________________

   3. blue _______________________________________
2. How did you get the solution to change from blue to yellow?

   ____________________________________________

3. Why did the solution turn from blue to yellow?

   ____________________________________________

4. How did you get the solution to change from yellow to blue?

   ____________________________________________

5. Why did the solution turn from yellow to blue?

   ____________________________________________

6. Are these color changes reversible? Explain.

   ____________________________________________

   Fe³⁺(aq) + SCN^-(aq) --> FeSCN²⁺(aq)

   color:____________________

7. How do you know that the colored compound was not KNO₃?

   _____________________________________________

8. Assume that the above reaction did happen and that you originally added equal numbers of moles of each ion. What happened when you added more Fe³⁺ ions to the equilibrium?

   _____________________________________________

9. What could be an explanation for this?

   _____________________________________________

10. What happened when you added more SCN^- to the equilibrium?

    _____________________________________________

11. What could be an explanation for this?

    _____________________________________________

    Fe³⁺(aq) + 3OH^-(aq) --> Fe(OH)₃

    H⁺(aq) + OH^-(aq) --> H₂O

12. What happened to the color when you added NaOH?

    _____________________________________________

13. Why? (Hint: You have more than one equilibrium.)

    _____________________________________________

14. What happened to the color when you added HCl to the last drop?

    _____________________________________________

15. Why? (Hint: Think in terms of three equilibria!)

    _____________________________________________

16. When HCl was added the reaction was very faint. How do you know that in fact Fe^± ions were produced?

    _____________________________________________

17. How did you demonstrate Le Chatelier's Principle?

    _____________________________________________

    _____________________________________________

    _____________________________________________

    _____________________________________________

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Bromcresol Green Experiments

#2 original indicator _____________________________

#1 indicator + HCl _______________________________

#3 indicator + NaOH _____________________________

#1 indicator + HCl + NaOH _________________________

#3 indicator + NaOH + HCl _________________________

#1 indicator + HCl + NaOH + HCl ____________________

#3 indicator + NaOH + HCl + NaOH __________________

Fe³⁺ and SCN^-

#1 original solution (control) _______________________

#2 solution + Fe³⁺ ions ___________________________

#3 solution + SCN^- ions __________________________

#4 solution + NaOH _____________________________

#5 solution + NaOH + HCl _________________________

#5 solution + NaOH + HCl _________________________

(after a few minutes)

#4 solution + NaOH + SCN^- ______________________

#5 solution + NaOH + HCl + SCN^- _________________

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Safety

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HIn(aq) --> H⁺(aq) + In^-(aq)

color:yellowblue

1. 1. Both HIn and In^- are present in approximately equal amounts.
   2. The concentration of HIn is greater than the concentration of In-.
   3. The concentration of In^- is greater than the concentration of HIn.
2. By adding H± ions.
3. The stress was an increase in the concentration of H± ions. The equilibrium responded by using up H± ions and forming more HIn (yellow).
4. By adding OH^- ions.
5. OH^- ions reacted with H⁺+ ions. The stress was the decrease in the concentration of H⁺ ions. The equilibrium responded by forming more of these ions as well as In^- (blue).
6. Yes the reaction is reversible. The color of the first drop changed from yellow to blue when NaOH was added. It went back to yellow when HCl was added.

   Fe³⁺+(aq) + SCN^-(aq) --> FeSCN²⁺(aq)

   color colorless colorless orange

   (pale yellow)

7. A solution of KNO3 was colorless.
8. The color deepened.
9. The stress was an increase in the concentration of Fe³⁺. The equilibrium responded by forming more product.
10. The color deepened.
11. The stress was an increase in the concentration of SCN^-. The equilibrium responded by forming more product.
12. The intensity of the color decreased.
13. The Fe³⁺ ions were used up forming Fe(OH)₃. The iron thiocyanate equilibrium responded by releasing more Fe³⁺ ions thereby reducing the intensity of the orange color.
14. The color intensified.
15. Hydrogen ions used up hydroxide ions. Fe(OH)₃ dissociated forming more iron and hydroxide ions. The increase in Fe³⁺ caused the iron/thiocyanate equilibrium to respond by forming more FeSCN²⁺ and the color intensified.
16. The color intensified.
17. In each of these cases, a stress was placed on an equilibrium. The position of the equilibrium changed in response to the stress, as judged by a noticeable color change.

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Bromcresol green experiments

original indicator: green
indicator + HCl: yellow
indicator + NaOH: blue
indicator + HCl + NaOH: blue
indicator + NaOH + HCl: yellow
indicator + HCl + NaOH + HCl: yellow
indicator + NaOH + HCl + NaOH: blue

Fe³⁺ and SCN^-

#1 original solution (control): orange
#2 solution + Fe³⁺ ions: darker
#3 solution + SCN^- ions: darker
#4 solution + NaOH: almost colorless
#5 solution + NaOH + HCl: pale orange
(wait a few minutes)
#4 solution + NaOH + SCN^-: no change
#5 solution + NaOH + HCl + SCN^-: darker

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• 12 well strip
• toothpicks
• cotton swabs
• bromcresol green indicator (dissolve 0.1 g bromcresol green in a mixture of 20 mL 95% ethanol and 50 mL distilled water. Add enough distilled water to bring the final volume to 100 mL.)
• solution of KNO₃ (dissolve 5 g KNO₃ in 100 mL of distilled water)
• 0.1 M HCl (dissolve 4.2 mL concentrated HCl in enough distilled water to make a final volume of 500 mL)
• 0.1 M NaOH (dissolve 2.0 g NaOH in enough distilled water to make a final volume of 500 mL)
• 0.10 M KSCN (dissolve 0.97 g KSCN in enough distilled water to make 100 mL of solution)
• 0.01 M KSCN (dilute 10 mL of the 0.1 M KSCN solution above with 90 mL of distilled water)
• 0.10 M Fe(NO₃)₃ (dissolve 3 mL concentrated nitric acid in 25 mL distilled water. Add 4.0 g Fe(NO₃)₃•9H₂O, and add enough distilled water to bring to total volume to 100 mL.)
• 0.01 M Fe(NO₃)₃ (dilute 10 mL of the above solution with 90 mL of distilled water)

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The Woodrow Wilson microscale version of this lesson was developed by:

Barbara M. Hilli
Stratford High School
45 North Parade
Stratford, CT 06497

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