Colorado College ?Determining Equilibrium Constant of a Chemical Reaction Lab Report answering question LABORATORY 6 Determining the Equilibrium Constant o

Colorado College ?Determining Equilibrium Constant of a Chemical Reaction Lab Report answering question LABORATORY
6
Determining the Equilibrium Constant
of a Chemical Reaction
Objectives
The objectives of this laboratory are to:
• Determine the equilibrium constant, Kc, by measuring the equilibrium concentrations of reacting species.
Introduction
Cobalt(II) does not exist in aqueous solution as a free ion, but forms a complex ion where
6 water molecules, acting as a Lewis bases (electron pair donors), donate electrons into
the vacant orbitals of the cobalt (II). This results in the pink complex ion Co(H2O)62+. In
the presence of chloride ions, a different complex forms, the blue CoCl42- complex ion.
We use their different colors to indicate the equilibrium concentrations for the following
reaction.
Eq. 1
Co(H2O)62+ (aq)+ 4 Cl- (aq) ? CoCl42- (aq)+ 6 H2O (l)
In this simulation you not only observe the equilibrium concentrations through their
colors, but also directly read their concentrations. From the concentration of the ions
under different conditions, you will be able to determine the equilibrium constant (Keq).
For this experiment, we will use simulation software developed by the ChemCollective.
You will need to access the simulation on either Chrome or Firefox (and perhaps Safari)
by visiting http://chemcollective.org/vlab/85.
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Chemistry 143 Experimental Chemistry
Part I: Equilibrium Shift & Observations
Data Collection
1. Add 4-250 mL Erlenmeyer Flasks to the workbench by selecting it from the available “Glassware”
in the “Stockroom” (left panel) five times. Rename each flask to “Flask 1” through “Flask 4” as shown in
the table below.
Sample
Volume (mL) of
1.0 M CoCl2
Flask 1
25.0
0.0
0.0
Flask 2
25.0
4.0
0.0
Flask 3
25.0
10.0
0.0
Flask 4
25.0
10.0
20.0
Volume (mL) of
12 M HCl
Volume of
6 M AgNO3
Color of Solution
2. Add the 1 M CoCl2, 12 M HCl, and 6 M AgNO3 to the workbench by selecting them from the
available “Solutions” in the “Stockroom”. Then add the volumes of each solution indicated in the above
table to each flask. Record the observed color of the solution in the above table.
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Determining the Equilibrium Constant of a Chemical Reaction Laboratory 6
Interpretation
1. What happens to the initial equilibrium (Eq. 1) when HCl is added to the CoCl2 solution? Support
your answer using your observations.
2. What happens upon addition of AgNO3? Which direction does the initial equilibrium (Eq. 1) shift?
How can you justify this with your observations?
Part II. Determination of Equilibrium Constant
Data Collection
1. Clear your workbench by right-clicking and selecting “Clear Workbench”.
2.
Prepare a set of samples indicated by the table below.
Sample
Volume (mL) of
1 M CoCl2
Flask 1
25.0
0.0
10.0
Flask 2
25.0
5.0
5.0
Flask 3
10.0
10.0
15.0
Flask 4
5.0
25.0
5.0
Volume (mL) of
12 M HCl
Volume (mL) of
H2O
3. Calculate the initial concentrations of Co2+ and Cl- in each sample. Remember to account for dilution
in your calculations. Record your concentrations in the table below.
Sample
Co2+
Cl-
1
2
3
4
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Chemistry 143 Experimental Chemistry
4. The equilibrium concentrations can be determined by spectroscopy. This was done for you, and the
resulting concentration of Co(H2O)62+ and CoCl42- can be found in the
“Information” for each sample (click on the sample to view the
information in the left-hand panel). The Cl- concentration can then be
determined from the initial concentration of Cl- and the concentration
CoCl42- at equilibrium using stoichiometry. Show how to calculate the Clconcentration in the space provided below.
Fill out the table below with the equilibrium concentrations of all the species and calculate the
equilibrium constant for each sample. Show one example of each calculation below.
Sample
[Co(H2O)62+]equil
[Cl-]equil
[CoCl42–]equil
1
2
3
4
What is the average of your Keq value?______________________.
4
Kequil
Determining the Equilibrium Constant of a Chemical Reaction Laboratory 6
Conclusions
1. The solution concentrations used to find Keq varied considerably. Do you expect
the Keq values for these solutions to vary as well? Explain.
2. If AgNO3 was added to Flask 4, would you expect the Keq to change? Explain. (You can
check your answer by adding 20 mL AgNO3 and calculating the Keq).
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