revised by Bruce E. Wilcox, Fall 2007
Purpose: This experiment will look at the consequence of the reversibility of chemical reactions and the equilibrium conditions in a qualitative fashion. Also of major importance are the results when external stresses are applied to the system at equilibrium - applications of Le Châtelier's
Principle.
No lab report will be written on this experiment. All questions should be answered in your notebook before leaving the laboratory.
Procedure
 Prepare a hot water bath by placing 100 mL of hot tap water and several boiling chips in a beaker on a hotplate. Turn on the hotplate and allow this water to warm up for later use.
I. Cobalt(II) Ion in Solution
Old and new chemistry you need to know:
(1) Transition metal ions do not really exist as simple monoatomic ions in solution, but rather as
complex ions. (Recall the spectrometry experiment from Chem 1 with the dark blue-purple species tetraamminecopper(II) ion, [Cu(NH
3
)
4
] 2+ .) In their simplest cases, when a simple transition metal ion salt is dissolved in water, typically between four and six water molecules (usually six) actually bond to the metal ion forming the complex ion. For example, the copper(II) actually exists in aqueous solution as [Cu(H
2
O)
6
] 2+ . Species bonded to transition metal ions in this way are called
ligands and can be anions or molecules. We write the formulas of these complex ions with the transition metal and the ligands within brackets and the charge of the overall species outside of the brackets. The charge of the complex is the sum of the charges of the metal ion/atom and all of the ligands.
(2) Ligands on transition metal ions can be replaced with other ligands that may be in solution creating new complex ions. Again, recall the spectrometry experiment from Chem 1, Cu 2+ solution actually existed as [Cu(H
2
O)
6
] 2+ (light blue), but when ammonia, NH
3
ion in
, was in solution with it, the dark blue-purple [Cu(NH
3
)
4
] 2+ ion formed, and the six water molecules were replaced with four ammonia molecules. The equilibrium reaction for this can be written…
[Cu(H
2
O)
6
] 2+
light blue
(aq) + 4 NH
3
(aq) colorless
[Cu(NH
3
)
4
] 2+ (aq) + 6 H
2
O (l) dark blue-purple
Keep this information in mind as you conduct the following exercise.
Caution: Cobalt(II) chloride solutions are toxic irritants. Hydrochloric acid is toxic and corrosive. Be careful not to ingest nor spill HCl on skin or clothing. Immediately notify the instructor if solution is spilled.
 Obtain 3 mL of 0.1 M CoCl
2
solution into each of two medium test tubes using a 10 mL graduated cylinder. Into one of then test tubes place 5 mL of concentrated (12 M) hydrochloric acid. Observe the colors of the solutions.
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o What species exist in the HCl solution?
 Which of these species can be a ligand?
 Assume four of these ligands can replace all of the water molecules on the cobalt(II).
Determine the formulas of the two colored species you have observed.
 Add concentrated HCl solution to the test tube with just the Co 2+ solution drop by drop
(counting drops!) while mixing, until a change is observed in the mixture. Keep adding HCl dropwise with stirring until no further change occurs (patience) and record your observations. o Write the net ionic equilibrium equation for the chemical system. o What evidence exists that the equilibrium was affected by the addition of hydrochloric acid? o Describe this in terms of Le Châtelier's Principle, and reference it to the equilibrium equation you have written. o Describe using a Q expression. NOTE: Solids and solvents do not appear in Q or K expressions because their values are constant.
 Add distilled water to the test tube drop by drop while mixing until a change is evident.
Continue adding water dropwise with stirring until no further change occurs and record your observations. o What evidence exists that the equilibrium was affected by the addition of water? o Describe this in terms of Le Châtelier's Principle and reference it to the equilibrium equation you have written. o Describe using a Q expression.
II.
 Place all the waste into the container labeled 'Spent Cobalt Solutions".
Dissolution of NH
4
Cl in Water
Caution: Ammonium chloride is an irritant.
 o Write the net ionic equation for the dissolution of solid ammonium chloride into water.
Obtain 3 mL of saturated NH
4
Cl solution in a clean, DRY 10-mL graduated cylinder and pour
 into a clean dry test tube. This will involve planning ahead to make sure the cylinder is both clean and dry when you need it! Record your observations.
Record your observations as you add concentrated HCl solution to the test tube drop by drop while stirring with a glass stir rod until a change is observed in the mixture. Add a few more drops. o What evidence exists that the equilibrium was affected by the addition of hydrochloric acid? o Describe this in terms of Le Châtelier's Principle and reference it to the equilibrium equation you have written. o Describe using a Q expression.
 Using a test tube holder, place the test tube in the hot water bath. Carefully stir the contents as you heat for 3-4 minutes. Record your observations. o What evidence exists that the equilibrium is affected by temperature? Add 'heat' as either a product or reactant to your equilibrium equation. o Is this evidence and reaction consistent with Le Châtelier's Principle? o Explain utilizing your Q expression.
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 Add enough crystals of solid NH
4
Cl to cover the bottom of a second dry test tube. Add 5 mL of water from a graduated cylinder to the solid ammonium chloride and stir. Measure the temperature of the solution in the test tube to determine if the dissolution of NH
4
Cl produces a temperature change. Record your observations. o Show a balanced reaction including 'heat' as either a product or reactant. o Is this evidence consistent the previous evidence and with Le Châtelier's Principle? o Explain utilizing your Q expression.
 Waste ammonium chloride can be disposed of down the drain with lots of water as you clean all your glassware.
III. The Chromate/Dichromate Equilibrium
Caution: Potassium chromate (K
2
CrO
4
) is a suspected carcinogen and mutagen. Thoroughly wash your hands after using this chemical. You are encouraged to wear gloves.
Solutions of both sulfuric acid (H instructor immediately.
2
SO
4
) and sodium hydroxide (NaOH) are toxic, corrosive and cause severe burns. If any of these solutions is spilled, notify the
 Observe the jars of solid potassium chloride (KCl) potassium chromate (K
2 potassium dichromate (K
2
Cr
2
O
7
). o What species exist in each of these compounds (names and formulas)?
o What is the color of each of these species?
CrO
4
) and
 Place 2 mL of 1 M K
2
CrO
4
solution into a test tube using a 10 mL graduated cylinder.
Observe the color of the solution.
 Add 3 M H
2
SO
4
solution to the test tube drop by drop while mixing until a color change is observed in the mixture. Record your observations. Add several more drops and record your observations. o What species exist in the sulfuric acid solution? o One of these species affects the chromate ion. Which is most likely? (Hint: We can use several other acids to accomplish the same thing.) o What species was being formed as you added the acid? o Write an equilibrium net ionic equation for the reaction you have observed. o Is the continued addition of acid giving results that are consistent with Le Châtelier's
Principle? o Explain utilizing your Q expression.
 Add 1 M NaOH solution to the test tube drop by drop while stirring with the stir rod until a change is evident. Record your observations. Add several more drops and record your observations. o What species exist in the sodium hydroxide solution?
 Which of these species will readily react with one of the species in the chromate/dichromate equilibrium? Write the equation for this reaction. o Explain how the addition of sodium hydroxide affects the chromate/dichromate equilibrium in terms of Le Châtelier's Principle? o Explain utilizing your Q expression.
 Place all the waste into the container labeled for Chromium waste.
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IV. Precipitation of Barium Chromate
 Put 1 mL of 0.1 M barium chloride (BaCl
2
) solution and one drop of 1 M K
2
CrO
4
into each of three test tubes. Record your observations. o Write the full and net ionic equilibrium equations for this reaction. o Based on your equation, the results of your earlier experiments, predict what might occur if you add some hydrochloric acid (HCl) to this solution. Record this prediction .
 Add a few drops of 6 M HCl to the solution in one of the test tubes (mix well). Record your observations. o Are your observations consistent with your prediction? If yes, explain what is going on in terms of Le Châtelier's Principle and a Q expression. If no, can you revise your thinking and explain what is going on? o Predict what will occur if you add sulfuric acid (H
2 the solution you made in the first step.
SO
4
) instead of hydrochloric acid (HCl) to
 Add a few drops of 3 M sulfuric acid (H
2
SO
4
) to a second test tube of solution. Let this test tube sit undisturbed for a few minutes. Look closely and record your observations. o Are your observations consistent with your prediction? If yes or no, explain what is going on in terms of Le Châtelier's Principle and a Q expression. o Predict what will occur if you add nitric acid (HNO
3 the solution you made in the first step.
) instead of hydrochloric acid (HCl) to
 Add a few drops of 6M nitric acid (HNO
3
) to the third tube. Record your observations. o Are your observations consistent with your prediction? If yes or no, explain what is going on in terms of Le Châtelier's Principle and a Q expression.
 Place all this waste into the barium chromate waste container.
Caution: Wash hands thoroughly with soap or detergent before leaving the laboratory.
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