Equilibrium Introduction Le Chatelier’s principle states that an equilibrium will shift to alleviate a stress acted upon it. This lab will have an equilibrium established then shifted to demonstrate this principle. The equilibrium reaction being shifted in this lab is CuCl42-(green) + 4 H2O <==> Cu(H2O)42+(blue) + 4 ClExperimental observation will be used to determine if this reaction is endothermic or exothermic as written. Materials ● ● ● ● ● Beakers or cups Graduated cylinder, 50mL Scale (paper or waxed paper for weighing solid onto) Calcium Chloride (solid) Copper Sulfate (either 30mL of 1M Solution OR solid CuSO4) ● Cone Coffee filter ● Table Salt ● Hot water and Ice Background Equilibrium is a state of a reaction system where the forward and reverse reactions occur at the same rate and the concentrations of the reactants and products are not changing. At first, reactants could be introduced where they would produce products. Then the products would react to produce reactants. When this system "evens out" it is at equilibrium. An equilibrium can be established by introducing a number of different combinations of reactants and products. Equilibrium is a dynamic state - both the forward and reverse reactions continue to occur but there is no net change in the concentrations over time. Chemically this is shown with an arrow going both ways between the reactants and products. *Since the products, as written in the forward direction, are also the reactants of the reverse, it is accepted that as the reaction is written, the chemicals on the left are the reactants. The chemicals on the right are the products. aA + bB ⇆ cC + dD An equilibrium has no net reaction: reactants are converting to products and products are being converted to reactants, but there is no change. There is no change that occurs unless an outside force acts upon the equilibrium. An equilibrium will shift to alleviate any stress applied to the equilibrium. The following examples will use this equation as an example. N2(g) + 3H2(g) ⇆ 2NH3(g) ● An equilibrium will shift away from an addition to reestablish the equilibrium. Adding N2 will shift the equilibrium to the right; away from the addition. ● An equilibrium will shift towards a removal to replace what was removed. Removing N2 will shift the equilibrium to the left; towards the item removed. ● A gaseous equilibrium will shift in response to a change in pressure. ○ Compression (increase in P) will shift the equilibrium towards the production of fewer moles of gas. ○ Expansion (decrease in P) will shift the equilibrium towards the production of the greater number of moles of gas. For example, compression of N2(g) + 3H2(g) ⇆ 2NH3(g) will shift the equilibrium towards the right. The left side has 4 moles of gas total and the right side has 2 moles of gas. Compression shifts towards the side with fewer moles of gas. ● Temperature will also shift an equilibrium. ○ An endothermic reaction has heat on the reactant side. It requires heat to proceed. ■ Heat + reactants ⇆ products ○ An exothermic reaction has heat on the product side. It produces heat in the forward direction. ■ Reactants ⇆ products + heat Written this way, the addition (increase in T) or removal (decrease in T) of heat can be treated just like any other reactant or product. This will be observed in the lab with a change in colors. This is a difficult concept for many students so this point will be explained by the following example. Which way will the following reactions shift? Recall that: ● When ΔH is negative, it is an exothermic reaction. An exothermic reaction puts HEAT on the product side. It gives off heat. ● When ΔH is positive, it is an endothermic reaction. An endothermic reaction puts HEAT on the reactant side. It requires heat to GO. When you add HEAT to either the product or reactant side, you can view it as a product or a reactant and add or take away like a chemical. When temperature is increased, it is like increasing the concentration of one of the chemicals. Shift AWAY from an increase. If heat is added to each of these.... a) S + H2 ⇆ H2S + HEAT ΔH =(-20 kJ/mole) SHIFT LEFT (away from addition of heat) b) HEAT +C + H2O ⇆ CO +H2 ΔH =(131 kJ/mole) SHIFT RIGHT (away from addition of heat) c) HEAT + H2 + CO2 ⇆ H2O + CO ΔH =(41 kJ/mole) SHIFT RIGHT (away from addition of heat) d) MgO + CO2 ⇆ MgCO3 + HEAT ΔH =(-117 kJ/mole) SHIFT LEFT (away from addition of heat) e) 2CO + O2 ⇆ 2CO2 + HEAT ΔH =(-566 kJ/mole) SHIFT LEFT (away from addition of heat) The equilibrium being investigated in this lab is CuCl42-(green) + 4 H2O <==> Cu(H2O)42+(blue) + 4 ClThe resultant color of the solution you should have is a light blue green. Procedure Prepare the Copper Sulfate from Solid CuSO4 1. Verify your solid CuSO4 is Copper Sulfate Pentahydrate. This is the most common form. If it is not, contact your instructor. (molar mass is 249.68 g/mol) 2. Warm approximately 100mL of clean water. If local water is considered “hard”, bottled water would be appropriate for this lab. 3. Tare a beaker on the scale. 4. Mass 15.0g of CuSO4 into the beaker. 5. Measure about 75mL of warmed water with the 50mL beaker. (You will need to measure a volume twice.) You do NOT need to record this volume. 6. Pour the warm water into the beaker and swirl the solids. a. This solution will need to sit for 5-15 minutes to dissolve. 7. Swirl periodically. Prepare the Calcium Chloride solution from Solid CaCl2 1. Calculate the stoichiometric amount of CaCl2 needed to convert all of the CuSO4 into CuCl2. Show the calculation and the actual amount massed in the results section of your lab report. 2. Tare a beaker on the scale. NEVER mass a chemical directly on the scale. 3. Mass the CaCl2 into the beaker. (Mass should have been calculated above.) 4. Measure about 25mL of warmed water with the 50mL beaker. You do NOT need to record this volume. 5. Pour the warm water into the beaker and swirl the solids. a. This solution will dissolve fairly quickly. Create CuCl2 1. If your CuSO4 was purchased as a solution, measure ~60mL of 1M CuSO4. Pour into a beaker and set aside. 2. Pour the CaCl2 solution into the CuSO4 solution. Record observations. 3. Stir or swirl. DO NOT USE METAL UTENSILS or anything you would eat with. Use a popsicle stick or plastic spoon if you are going to stir. 4. The solution will remain blue and there will be a white solid precipitate. This reaction is very fast. Separate the Liquid and Solid 1. Pour the solution and solids through a cone coffee filter. NOTE - a basket type filter will not work. 2. Rinse the solids with approximately 50mL of clean, warm water. a. The solids are being discarded for this lab. b. The blue-green liquid will be used to create an equilibrium c. I found that I could use a binder clip to hold the filter to the edge of a glass while the liquid drained. Alternatively, I suspended the filter over my beaker by using a binder clip to hold the filter on a chopstick that was balanced on two glasses. Be creative. Show me what you did. In the video, I just held the filter. 3. When the filter has stopped dripping or approximately 50mL of liquid has been collected, set the filter paper with solids aside in a different beaker and move on to “Shift an equilibrium” with the beaker of blue CuCl2 solution. Shift an equilibrium 1. Separate the CuCl2 solution evenly into each of 4 small cups or beakers. (Your volume will vary and your beaker size will vary so I cannot give you a volume to aim for here. You have to have enough liquid to see the shift. A very small amount of liquid is only valid in a very small container.) 2. Beaker 1 will be the standard; it will be the reference color. 3. Add a large pinch or about ¼ teaspoon NaCl to Beaker 2. Record observations. 4. Place beaker 3 in an ice water bath for 3-5 minutes. 5. Place beaker 4 in a hot water bath for 3-5 minutes. 6. Take a picture comparing the colors of beakers 3 & 4 together. Include your ID in the picture. 7. Compare beakers 2-4 with the reference color of beaker 1. Take a picture of beakers 1, 2, and 3 together. 8. Consider the equilibrium equation. Consider what other means you have available to shift this equilibrium. What else may shift this equilibrium? Can you test it? Write a procedure and thorough discussion on what else could be tested to shift this equilibrium. Shift the equilibrium using other items you have (Vinegar, NaOH, etc. Experiment. Not experimenting here is -15 for the lab) Citations Chemistry. Provided by: OpenStax College. Located at: http://openstaxcollege.org . License: CC BY: Attribution. License Terms: Download for free at https://openstax.org/details/books/chemistryatoms-first-2e "Clipart of Camera" by Unknown is in the Public Domain, CC0 Text is an original work by Dr. Karey Marshall.
