Experiment purpose: To determine the effect of a change on a system at equilibrium and to correlate the observed responses with Le Chatelier’s principle. Experiment Chemical list:
Student Provided
1 Tap water
1 Toothpicks
1 Distilled water
1 Crushed ice
2 Coffee spoons
1 Rubber bands
2 Beaker, 50 mL, plastic
1 Magnifier, dual
1 Pencil, marking
1 Thermometer-in-cardboard-tube
1 Well-Plate-24
In the Experiment Bag: LeChatelier’s Principle
1 E.D.T.A., .15 M (Na2EDTA) – 2 mL in, Pipet
1 Hydrochloric Acid, 6 M – 2 mL in Pipet
1 Magnesium Chloride, 2 M – 3 mL in Pipet
1 Phenolphthalein Solution, 1% – 0.5 mL in Pipet
2 Pipet, Empty Short Stem
1 Sodium Bisulfate Crystals – 0.5 g in Vial
1 Sodium Bisulfate, 0.1 M – 4 mL in Pipet
1 Sodium Hydroxide, 1 M – 2 mL in Pipet
1 Sodium Sulfate, 1 M – 2 mL in Pipet
1 Thymol Blue Indicator, 0.04% – 0.5 mL in Pipet
Experiment procedure:
Part I:
1. Prepare an ice water bath by partially filling one of the 50-mL beakers from your LabPaq with tap water and crushed ice. Set aside.
2. Prepare a hot water bath by partially filling the second 50-mL beakers from your LabPaq with very hot tap water. Set aside.
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3. Using a 24-well plate add 5 drops of 2.0 M MgCl2, magnesium chloride
solution to wells C1 and C2. Add 10 drops to wells C3 and C4.
4. Add 1 drop of phenolphthalein to wells C1, C2, C3 and C4. Stir with a clean toothpick. Precipitation noted. 5. Add 5 drops of 1.0 M NaOH, sodium hydroxide solution to wells C1, C2, C3, and C4. Stir : solution thickens, turns dark pink.residue also formed in the solution. 6. Add 1 drop of 6.0 M HCl, hydrochloric acid solution to well C1. Stir and record your observations. Continue to add HCl drop-by-drop to well C1 until you see a color change. Pink solution turns to milky solution again
7. Add 1 drop of 0.15 M Na2EDTA to well C2. Stir and record your observations. Continue to add Na2EDTA drop-by-drop to well C2 until you see a color change. Color changes to clear.
8. Use the marker pencil to label an empty pipet “C3.” Then, squeeze the pipet’s bulb, place it in well C3’s solution, and slowly release the bulb to carefully suck up all the mixture from the well into the bulb.
9. You need to place the pipet bulb from Step 8 into the hot water bath, but it will tend to float. To avoid this, securely attach the pipet stem to a spoon handle with a rubber band. Then place the pipet bulb in the water bath with the spoon handle outside of the beaker so as to not obstruct your observations. Cloudy pink solution observed. 10.Repeat Steps 8 and 9 with a second pipet filled with the mixture from well C4. Place © Hands-On Labs, Inc. LabPaq CK-2 68
this pipet into the cold water bath. Still fuschia pink color 11.As the temperature of the mixtures in the pipets changes, C3- cloudy light pink, C4- fuchsia pink 12.After about 10 minutes, exchange the pipets in the baths. (Put the pipet from the cold water bath into the hot water bath and vice versa).
C3-fuschia pink in the hot water bath and C4 – turns to cloudy pink. 13. Dispose of your solutions by pouring them down the drain with running water. Rinse your 24-well plate and dry it. Remove the marker pencil marks from the pipets and rinse them well by sucking up distilled water and squeezing it out. They will be used in the next part, so dry the pipets as well as possible by squeezing their bulbs repeatedly and tapping the tips on a hard surface covered with a paper towel. Questions:
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In Part I the reaction you observed was Mg(OH)2 (s) Mg2+ (aq) + 2 OH- (aq).
Compare the colors you observed in the experiment and answer these questions: A. Which way should the equilibrium shift when HCl is added? How do your results support your answer? The purple solution changed to milky shifting to the right because of the chloride ion. B. Which way should the equilibrium shift when Na2EDTA is added? How do your results support your answer? Color changes rom pink to clear. The equilibrium composition of a reaction was altered because there was a change in concentration. It should shift to the right because adding the Na2EDTA lowers the concentration of the chloride ion. C. Is this reaction endothermic or exothermic? How do you know? The reaction from question (B) is a shift to the right, more products are formed making it endo thermic. It is consuming energy. Heat is required and heat is a reactant.
Part II:
Again prepare hot and cold water baths as instructed in Part I, Steps 1 and 2. Note: the reactions in Part II are much more subtle than those in Part I. You may have to wait 30 seconds or longer and look very carefully to observe the reaction and changes taking place! Use your magnifier to better see the changes.
1. Using a 24-well plate, add 5 drops of NaHSO4 sodium bisulfate solution to wells A1, A2, and A3. Add 10 drops to wells A4 and A5.
2. Add one drop of thymol blue indicator to each well. Stir with a clean toothpick and Solution turns wine .
3. Add one drop of Na2SO4 sodium sulfate solution to well A1 and stir with a clean toothpick. Compare the color of well A1 with that of well A2. Well A1 has an orange color compared to well A2 that is wine red. 4. Repeat Step 3, drop-by-drop, until you see a color change. 5. Add a few crystals of solid NaHSO4 to well A3. Stir with a clean toothpick, wait a while, and record your observations. Compare the color of well A3 with that of well A2. Well A3 is purplish compared to A2 that is wine red
6. Repeat Step 5 until you see a color change. Precipitation occurs, the purplish solution becomes cloudy. © Hands-On Labs, Inc. LabPaq CK-2 69
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7. Use the marker pencil to label an empty pipet “A4.” Then squeeze the pipet bulb, place it in well A4’s solution, and slowly release the bulb to carefully suck up all the mixture into the bulb. 8. You need to place the pipet bulb from Step 7 into the hot water bath, but it will tend to float. To avoid this securely attach the pipet stem to a spoon handle with a rubber band. Then place the pipet bulb in the water bath, with the spoon handle outside of the beaker so as to not obstruct your observations.
9. Make sure the water bath is as hot as possible. If the water bath is not very hot the expected color change may be difficult to see. Place the pipet bulb from step 8 into the hot water bath. Record the beginning temperature of the hot water bath. Temperature: 97 degrees Celsius 10.Repeat Steps 7 and 8 with a second pipet for the mixture from well A5. Place this pipet into the cold water bath.
11.Record the beginning temperature of the cold water bath. Beginning temperature; 100 degrees Celsius. 12.When you observe changes in the mixtures in the pipets record your observations and the water bath temperatures. Solution turns light pink
13.Compare the final solutions to that in well A2 Pipet A4 is light pink compared to Pipet A5 that is light wine red. 14.After about 10 minutes exchange the pipets in the baths. (Put the pipet from the cold water bath into the hot water bath and vice versa.) Switching pipet A4 to cold water bath turns solution darker fuchsia pink color and pipet A5 to hot water bath turns solution even more lighter wine red color. 15.Dispose of your solutions by pouring them down the drain with running water. Rinse and thoroughly dry your 24-well plate. Rinse your pipets thoroughly with distilled water. Dry the pipets as well as possible by squeezing their bulbs repeatedly and tapping the tips on a hard surface covered with a paper towel.
Questions Part II:
In Part II the reaction you observed was HSO4- (aq) + H2O (l) H3O+(aq)+SO42-. Compare the colors you observed in the experiment and answer these questions: A. Which way should the equilibrium shift when Na2SO4 is added? How do your results support your answer?
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Few drops of crystal turns purple and cloudy and the equilibrium shift to the left. B. Which way should the equilibrium shift when NaHSO4 is added? How do your results support your answer? The solution turns purple and the equilbrum shift to the left. It produces heat and energy is released. C. Is this reaction endothermic or exothermic? How do you know? This reaction is exothermic because of the precipitation, the solution undergoes a change in concentration, volume so equilibrium will shift to the counter the change and establish a new equilbrum.
