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CHEMICAL EQUILIBRIUM & Le CHATELIER’S PRINCIPLE VL134-1 Virtual Lab Step by Step Instructions

This virtual lab should help your understanding of Le Chatelier’s Principle by observing the effect of induced stress on the equilibrium distribution of two cobalt(II) complexes in aqueous solution. It is recommended that you review the chapter about equilibrium in your lecture text. Cobalt (II) ion exists in aqueous solution as the Co(H2O)62+ complex ion where 6 water molecules, acting as Lewis bases, make coordinate bonds to Co2+ by donating pairs of electrons into vacant orbitals of the Co2+. This results in a red solution. When excess Cl- ion is added to a solution of Co(H2O)62+, the blue complex CoCl4-2 forms and the

following equilibrium is established: Co(H2O)62+ (aq) + 4 Cl- (aq)

↔

CoCl4-2 (aq) + 6 H2O (l)

(Eq 1)

We can use the different colors to indicate the relative concentrations of the two complexes at equilibrium. This simulation permits you to observe the colors and gives the concentrations of all species as reagents are added. There are three parts to the simulation: Part 1: Concentrated HCl is added stepwise to a given volume of Co(H2O)62+ of known concentration. After each stepwise addition of HCl, the simulation provides the concentrations of the reactants and products from which you can determine the equilibrium constant, K for the reaction shown in (Eq 1). -

Part 2: Free Cl is removed from the final solution from Part 1 by adding controlled amounts of AgNO3 solution. (The Ag+ ion reacts with free Cl- to form solid AgCl and thus removes Cl- from the solution.) Part 3: The dependence of the Equilibrium constant on temperature is determined. From these data, it can be determined whether the reaction in Eq 1 is endothermic or exothermic. To access the exercise, open the virtual lab at: : http://chemcollective.org/vlab/85 Retrieving Chemicals and Equipment: Retrieve solutions from the Stockroom (Fig 1), located to the left of the Workbench, by clicking the solutions icon first and then clicking “Cobalt Chloride Expt Solution tab” to see the drop down menu of solutions. Click on 1M CoCl2, the solution will appear on the workbench in Erlenmeyer flask.

Similarly, retrieve 12M HCl, and 6M AgNO3 solutions and distilled Water by clicking stockroom tab first, and then following the same steps as you did to retrieve 1M CoCl2 solution. The relevant solutions will appear on the work bench also in Erlenmeyer flasks. Click on Glassware icon, in the dropdown menu of the Glassware option (Fig 2), click on Beakers, and then on the 250-mL Beaker icon. Similarly retrieve a 50-mL Graduated Cylinder from the sub-menu. 1

Fig 1 Fig 2 Part 1 : Determining K for the Reaction at 25oC

Fig 3

Summary: 12 M HCl is added in small increments to a known volume (2X.0 mL) of 1M CoCl2 and the equilibrium constant is computed at several stages from the given concentrations Drag the CoCl2 Flask icon on to the 50 mL graduated cylinder until it overlaps the graduated cylinder. When the flask is in the pouring position over the graduated cylinder, as shown in Fig 3, a transfer menu appears on the bottom as also shown in Fig 3. Type 2X.0 (where X is the last digit of your ID #) in the Transfer Volume cell. Click the Pour button once to transfer the indicated volume (2X) to Graduated Cylinder. Drag the CoCl2 Flask away from the Graduated Cylinder. Next drag the Graduated cylinder until it overlaps 250-mL Beaker and a transfer menu appears underneath. Type the indicated volume which is in the Graduated Cylinder (2X) in the Volume cell. Click the Pour button once to transfer the indicated volume into the Beaker. Drag the Graduated Cylinder away from the Beaker. Similarly, transfer 3X.0 mL (where X is the last digit of your ID #) of Water to the empty Graduated Cylinder. Transfer 3X mL of Water to the Beaker containing the CoCl2 solution. Similarly, transfer 25.0 mL of 12 M HCl into an empty Graduated Cylinder

Fig 4

2

Fig 5

Fig 6

Start the reaction by dragging the 50-mL Graduated Cylinder containing 12 M HCl to the Beaker containing CoCl2 solution (Fig 4). Enter 3.0 mL in Volume bar and click pour. Keep adding 3.0 mL increments of 12 M HCl until you detect a color change from red towards purple. The Information window (Fig 5) at the left side of the Workbench will show the concentration of each species at equilibrium shown in Eq 1. Record the equilibrium concentrations of Co(H2O)62+, Cl- and CoCl42- species on the datasheet and calculate the value of K, the equilibrium constant for the reaction in Eq 1. Now continue to Pour more HCl in 3.0 mL increments into the Beaker until a further color change is observed. Record this volume on data sheet. Again record equilibrium concentration of reactants and products from the Information window on data sheet and calculate K.

Part 2: effect of removing ClSummary: The equilibrium in Eq 1 is followed as the concentration of Cl- in the solution from part 1 is reduced by the incremental addition of AgNO3. Empty the 50 mL Graduated Cylinder and follow the procedure of Part 1 only this time instead of HCl add AgNO3 Solution to remove Cl- from the solution of part 1 in the form of solid AgCl. Add AgNO3 in 1.0 mL batches to see a distinct color change. Record equilibrium concentrations of species involved in reaction as well as total volume of AgNO3 added on data sheet and calculate K. Repeat this step one more time by adding more AgNO3 and determine K. Save this solution for use in part 3.

Part 3: Effect of Heating (endothermic or exothermic reaction) Summary: The value of the equilibrium constant is determined at several temperatures and from its changes with temperature, it is determined whether the reaction in Eq 1 is exothermic or endothermic. Right click on the Beaker containing the final solution at equilibrium from Part 2. From the drop-down menu select Thermal properties. Change the temperature to 5X oC (X = last digit of your ID#) check (Click) insulated from surrounding box tab and then click OK. The temperature is shown in the temperature box (Fig 6). Allow the system to reach thermal equilibrium (constant temperature). Record equilibrium concentrations from the information window on the data sheet and calculate K. Repeat the procedure at 1X oC (X=last digit of your ID #) and record the equilibrium concentrations on the data sheet to calculate K. Use a print screen program 1 to print the work bench of Part 3 and attach the printout to the data sheet. 1 Such a utility is available on the Stony Brook Softweb site (restricted): https://softweb.cc.stonybrook.edu/

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CHEMICAL EQUILIBRIUM & Le CHATELIER’S PRINCIPLE DATA Sheet Name: ___________________________________________

Section #: ________

Last digit of Stony Brook ID#: ________ Part 1 : Determining K for the Reaction at 25oC Volume of CoCl2 used:

________

Temperature of Solution: ________ oC

Volume of HCl added (mL)

[Co(H2O)62+]

[ Cl- ]

[CoCl4-2]

Color of solution

Calculated K

[ Cl- ]

[CoCl4-2]

Color of solution

Calculated K

[CoCl4-2]

Color of solution

Calculated K

Part 2: Effect of removing ClVolume of AgNO3 added (mL)

[Co(H2O)62+]

Part 3: Effect of Heating (endo or exothermic reaction ?) Temperature of Solution (oC)

[Co(H2O)62+]

[ Cl- ]

Use a print screen program2 to print the work bench and attach the printout of Part 3 to the data sheet. Name: ___________________________________________

Section #: ________

Observations & Discussion 2 Such a utility is available on the Stony Brook Softweb site (restricted): https://softweb.cc.stonybrook.edu/

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CHEMICAL EQUILIBRIUM & Le CHATELIER’S PRINCIPLE DATA Sheet a) Write down the equilibrium expression for the reaction as written in Eq 1.

b) Discuss the results obtained in Part 1 & 2 in terms of equilibrium and Le Chatelier’s principle.

c) In which direction would the equilibrium shift if pure water were added to the reaction mixture at the end of Part 1?

d) Discuss the results of Part 3 showing how the change in temperature affects the position of equilibrium. Rewrite Eq1 to show to include heat on the appropriate side of the reaction.

e) Is the reaction endothermic or exothermic? Is ΔH greater or less than 0?

f) Use a print screen program 3 to print the work bench and attach the printout to the data sheet *.

3*Such a utility is available on the Stony Brook Softweb site (restricted): https://softweb.cc.stonybrook.edu/

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