Investigation #13: An Application of  Le Chatelier's Principle

Challenge:
How can the equilibrium of a reversible chemical reaction be shifted by applying different stresses according to Le Chatelier's Principle? Can the colors of the rainbow be made by applying Le Chatelier's principle to a variety of chemical reactions?

Context for the investigation:
Systems at chemical equilibrium involve what we call “reversible “ reactions. When these reactions reach a point where the rate of the forward reaction (reactants turning into products) equals the rate of the reverse reaction (products turning back into reactants) the reaction has reached equilibrium. Once equilibrium is reached, it can be stressed or altered by some outside interference. The stress applied to an equilibrium system can be a concentration, temperature, or volume (thus changing pressure) change. Le Chatelier’s Principle is a guideline that helps us predict whether the reaction will shift right (making more products) or shift left (making more reactants). In this experiment you will investigate the equilibrium shifts for two of the following six equilibrium systems. In each of the equilibrium systems you will be given a container with the necessary materials. Your job is to use these materials to cause equilibrium shifts. You will need to record how you attempted to shift the equilibrium, and what the results were for each attempt.

Pre-Lab Questions:
View and interact with the following three animations found on the linked webpage. Answer the following questions on your own sheet of paper. Most of these animations require shockwave.

 1. Cobalt Chloride Equilibrium (open in Internet Explorer to work properly)
    a. What immediate effect does adding Cl^- ions to the cobalt complex ion have on either the forward or reverse reaction rate?

    b. What immediate effect does adding water to the blue complex ion have on either the forward or reverse reaction rate?

    c. How do each of these two stresses shift the equilibrium (right or left)?

2. NO₂/N₂O₄ Equilibrium (open in Internet Explorer to work properly)
    a. Each time the animation stops, count the number of NO₂ and N₂O₄ molecules present. What do you observe?

    b. After watching the animation panels that include the yellow arrows, describe what is happening simultaneously to keep the number of each type of molecule constant.

    c. Explain how this animation illustrates the dynamic nature of chemical equilibrium.

3. Bromine Equilibrium (open in Internet Explorer to work properly)
    a. When the animation stops count the number of Br₂(l) and Br₂(g) molecules present. What do you observe?

    b. How would you design an experiment to determine if this animation is an accurate representation of what actually occurs?

MSDS: omit individual since so many in this lab but copy below statements

The solutions used in this lab require careful handling and adherence to all safety guidelines, as some of them can cause skin burns and eye damage. When working with a syringe of gases under pressure, direct the syringe away from people and toward the wall or floor. If solutions are spilled on skin, wash with copious amounts of water. Once used in the experiment, all the used solutions can be safely disposed following standard procedures as directed.

Procedures:

Equilibrium System #1: Bromothymol Blue (BTB, Acid / Base Indicator)

Equation: BTBH --> BTB^- + H⁺

Chemicals: BTB Solution; 0.10M HCl; 0.10M NaOH; 0.10M NaCl

Procedure: Add 1 mL of BTB to 25 mL of water. Investigate the shifts that can be observed by applying a stress to small amounts of this solution in test tubes.

Equilibrium System #2: Iron Thiocyanate

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

Chemicals: 0.10M KSCN; 0.20M Fe(NO₃)₃; Solid KSCN; Solid FeCl₃; Solid Na₂HPO₄, NaH₂PO₄, or Na₃PO₄

Procedure: Put about 20 mL of KSCN solution in a beaker. Add 20 mL of water and 5 drops of Fe(NO₃)₃ solution. Investigate the shifts that can be observed by applying a stress to small amounts of this solution in test tubes.

Equilibrium System #3: Copper Ammonia Complex

Equation: Cu₂(aq) + 4 NH₃(aq) --> Cu(NH₃)₄²⁺(aq)

Chemicals: 0.25M CuSO₄; Conc. NH₃; 1.0M HCl

Procedure: Put 25 mL of CuSO4 solution in a beaker. Add NH3 drop by drop to observe a precipitate forming. Continue adding the ammonia until the precipitate disappears. Investigate the shifts that can be observed by applying a stress to small amounts of this solution in test tubes.

Equilibrium System #4: Copper Chloride Complex

Equation: Cu(H₂O)₆²⁺(aq) + 4 Cl^- (aq) --> CuCl₄^2- (aq) + 6 H₂O(l)

Chemicals: Solid CuCl₂•2H₂O; Conc. (12M) HCl; Water

Procedure: Add about 2 grams of copper(II) chloride dihydrate to 25 mL of water. Investigate the shifts that can be observed by applying a stress to small amounts of this solution in test tubes.

Equilibrium System #5: Cobalt Chloride Complex 

Equation: Co(H₂O)₆²⁺(aq) + 4 Cl^- (aq) + heat --> CoCl₄^2- (aq) + 6 H₂O(l)

Chemicals: Solid CoCl₂•6H₂O; 95% Ethanol; Water; Acetone; Solid NaCl

Procedure: Add 2 grams of cobalt(II) chloride hexahydrate to 25 mL of 95% ethanol. There is a small amount of water dissolved in the ethanol. Investigate the shifts that can be observed by applying a stress to small amounts of this solution in test tubes. You also have an ice bath and a hot water bath.

Equilibrium System #6: Soda Water & Methyl Red (Acid / Base Indicator)

Equations: CO₂(g) --> CO₂(aq)    
CO₂(aq) + H₂O(l) --> H₂CO₃(aq)         
H₂CO₃(aq) + H₂O(l) -->  HCO₃^- (aq) + H₃O⁺(aq)
HCO₃^- (aq) + H₂O(l) --> CO₃^2- (aq) + H₃O⁺(aq)

Chemicals: Cold, fresh soda water; Methyl Red Indicator; 0.10M AgNO₃

Procedure: Put 20 mL of soda water in a beaker and add 1 mL of methyl red. Draw 10 mL into a large syringe. Invert the syringe and push all the air out. Close the valve. Investigate the shift produced by pulling back on the plunger and holding it. You may need to shake the syringe to see the color change. Investigate how adding silver nitrate will shift the equilibrium.

Data Tables: (you will need 6 of the following data table below for each system)

Equilibrium System #____:
Original Chemical Reaction Equation: ________________________________________

Post Lab Analysis Questions:

1. A student obtained a test tube with a suspension of white, slightly soluble calcium hydroxide in water. The system was at equilibrium as represented by the following equation: Ca(OH)₂(s) <--> Ca⁺² (aq) + OH^- (aq)
    a. write the equilibrium constant expression for this reaction.

    b. What would you expect to observe if hydrochloric acid, HCl (aq), was added? Explain using Le Chatelier's principle.

    c. What would you expect to observe if calcium nitrate was added? Explain using Le Chatelier's principle.

    d. What the solution was placed in an ice bathe and cooled it was observed that more solid calcium hydroxide was produced.

        i) Based on their observation would you expect the reaction to be exothermic or endothermic? Explain using Le Chatelier's principle.

        ii) If the solution was placed in a hot water bath and heated, what would you expect to observe? Explain using Le Chatelier's principle.

2. The following questions concern this equilibrium system:  Br₂ (l) + Cl₂ (g) <--> 2BrCl(g)  ΔH= +29.4kJ/mol
How will the following factors influence the equilibrium listed above? Indicate whether the system will shift left, right, or remain unchanged, and give a short explanation for your choice. In all cases, the listed change is the only change and all other variable remain constant.
    a. Increasing temperature

    b. Increasing the pressure in the flask by adding Ar

    c. Increasing the volume in the flask

    d. Adding Br₂ (l)

    e. Removing Cl₂ (g)

    f. Adding BrCl (g)

    g. Adding a catalyst

3. Indicating Drierite is a material used in the laboratory to remove water vapor from gasses and as a desiccating agent. What purchase it is blue in color and it changes to pick upon absorbing moisture.
    a. Based upon your observations in this lab, explain how Indicating Drierite works.

    b. Indicating Drierite can be regenerated and used over and over again. Propose a method for regenerating Drierite quickly, efficiently, and at low cost. Explain how your method will work, using Le Chatelier's principle.