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Le Chatalier Virtual Lab
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Introduction
In this experiment you will be introduced to chemical equilibrium. You will then be presented with a number of systems at equilibrium and will be asked to "stress" these systems by changing the concentration of one of the reactants or products or by changing the temperature of the system.
The experiment is composed of four parts (background, prelab, experiment, postlab) that should be completed in the order listed below.
Background:
Unless something is done to disturb a system at equilibrium it will remain at equilibrium. Any changes that disturb a system at equilibrium are calledstresses. We are often able to predict the outcome of applying a stress to an equilibrium system using LeChatelier's Principle. It states that when an equilibrium system is disturbed, the system will shift in such a way as to relieve the applied stress. The three most common ways to stress a system at equilibrium are changing the concentration of one of the reactants or products, changing the temperature of the system, or changing the pressure on the system. As the last stress applies primarily to gaseous systems, we will concern ourselves with only the first two stresses.
Prelab Questions (5 marks)1. / (2) Which describes a solution that contains a system at equilibrium? Explain your answer.
One in which the color of the solution is changing slowly, or one in which the color is not changing.
2. / The following equilibrium is established when copper ions and bromide ions are placed in solution.
heat + Cu(H2O)6+2+ 4 Br-6 H2O + CuBr4-2
The tube on the left contains only copper sulfate dissolved in solution. The tube on the right is the result of adding some potassium bromide solution. Given that the Cu(H2O)6+2ion is blue and that the CuBr4-2ion is green, answer the questions below.
a. (1) What happened to the concentration of each of the ions when the KBr was added?
b. (1) Explain why the solution changed color.
c. (1) Would the tube feel hot or cold when the KBr was added?
The Experiment (34 marks)
In this experiment you will be shown a series of equilibrium systems and asked to apply certain "stresses" to the systems. The results of applying the stress will then be shown to you. For each reaction, record:
(a) The name of the equilibrium system (You only need to do this for this first experiment within a system)
Include the reaction as well.
(b) Your observations (1 mark each)
This means a color change, a precipitate formed or some like statement. For example, "the red solution turned green". Do not use this part to state what chemical change occurred. For example, you should not say "more cobalt ion was formed".
(c) What you think happened chemically (1 mark each)
State what chemical reaction occurred or species was formed that resulted in your observation. For example, "by adding chloride ion to the solution it shifted the equilibrium to the left which caused more CoCl4-2ion to be formed. Since this ion is blue, the solution became more blue."
I. Cobalt system(10 marks)
CoCl4-2{blue} + 6 H2OCo(H2O)6+2{red} + 4 Cl-+ heat
NOTES:
- Silver ions react with chloride ions to form the insoluble compound silver chloride.
a)Add heat
b)Remove heat
c)Add water
d)Add KCl
e)Add AgNo3
II. Ammonium system (6 marks)
heat + NH4++ OH-NH3+ H2O
NOTES:
- The purplish color is due to phenolphthalein. The darker the purple color, the larger the hydroxide ion concentration in solution.
a)Add NH4Cl
b)Add HCl
c)Add NaOH
III. Iron thiocyanate system (8 marks)
Fe+3{pale yellow} + SCN-FeSCN+2{red} + heat
NOTES:
- the HPO4-2ion forms a complex with the Fe+3ion.
a)Add KSCN
b)Add Fe(NO3)3
c)Add heat
d)Add Na2HPO4
IV. Chromate system (4 marks)
2 CrO4-2{yellow} + 2 H+Cr2O7-2{red} + H2O
NOTES:
- the tube above contains a mixture of CrO4-2and Cr2O7-2ions.
a)Add HCl
b)Add NaOH
V. Nitrogen dioxide system (4 marks)
2 NO2{brown}N2O4{colorless} + heat
NOTES:
- the tube above contains a mixture of gaseous NO2and N2O4in a water bath at room temperature.
a)Remove heat
b)Add heat
VI. Copper sulfate system (2 marks)
CuSO4.5 H2O + heatCuSO4+ 5 H2O
NOTES:No notes for this experiment
a)Add heat
b)Postlab Questions (6 marks)1. / (1) Predict what would happen if the ammonium system described in the experiment was heated.
2. / (2) Consider the following equilibrium.
Cu+2+ 4 NH3Cu(NH3)4+2
The copper ion in solution is light blue. The Cu(NH3)4+2ion is a deep blue. The tube on the left contains only copper ions. Identify what chemical was added to produce the results shown in the center tube. Identify what chemical may have been added to the center tube to produce the results shown in the right tube.
3. / (1) The barium ion is toxic to humans. However, barium sulfate is commonly used as an image enhancer for gastrointestinal x-rays. What does this imply about the position of the equilibrium shown below.
BaSO4Ba+2+ SO4-2
4. / (2) Hemoglobin (Hb) and oxygen gas form a complex (HbO2) that carries oxygen throughout the human body. Unfortunately, carbon monoxide also binds to hemoglobin so that an equilibrium is established. Carbon monoxide poisoning occurs when the concentration of HbO2in the blood is reduced.
HbO2+ COHbCO + O2
The first aid for a person suffering from carbon monoxide poisoning is to (1) remove them to an area of fresh air, and (2) administer oxygen. Using the principles of equilibrium, explain how each of these helps to restore the HbO2concentration.
This completes the experiment.