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14
Chemical Equilibrium: Finding a Constant, Kc
Objectives
• To determine an equilibrium constant.
•
To write equilibrium expressions.
•
To prepare a standard solution.
•
To understand dilution calculations.
•
To use colorimetric data to find concentrations of solutions.
In the Lab
• Students will work in pairs.
Waste
• All solutions should be disposed of in the aqueous waste container.
Safety
• Iron nitrate solutions are prepared in nitric acid—use with caution.
The purpose of this lab is to experimentally determine the equilibrium constant, Kc, for
the following chemical reaction:
Fe3(aq)  SCN(aq)  FeSCN2(aq)
iron(III)thiocyanate thiocyanoiron(III)
When Fe3 and SCN are combined, equilibrium is established between these two ions
and the FeSCN2 ion. In order to calculate Kc for the reaction, it is necessary to know
the concentrations of all ions at equilibrium: [FeSCN2]eq, [SCN]eq, and [Fe3]eq. You will
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1 4 • Chemical Equilibrium: Finding a Constant, Kc
Fe3+(aq)
+SCN– (aq)
I
[Fe3+]i
[SCN–]i
0
C
–x
–x
+x
E
[Fe3+]i − x = [Fe3+]eq
[SCN–]i − x = [SCN–]eq
x = [FeSCN2+]eq
prepare four equilibrium systems containing different
concentrations of these three ions. The equilibrium
concentrations of the three ions will then be experimentally determined. These values will be substituted
into the equilibrium constant expression to see if Kc is
indeed constant.
Using the equation shown above we can set up an ICE
(initial, change, equilibrium) table to set up the calculations. Pay careful attention to the subscripts.
Expt.
14
In order to determine [FeSCN2]eq, you will use a colori­
meter (see Figure C4.9). The FeSCN2 ion produces solutions with a red color. Because the red solutions absorb
blue light very well, the blue LED (470 nm) setting on
the colorimeter is used. The colorimeter measures the
amount of blue light absorbed by the colored solutions
(absorbance, A). By comparing the absorbance of each
equilibrium system, Aeq, to the absorbance of a standard
solution, Astd, you can determine [FeSCN2]eq. The standard solution has a known FeSCN2 concentration.
To prepare the standard solution, a very large concentration of Fe3 will be added to a small initial concentration
of SCN (hereafter referred to as [SCN]i). The [Fe3] in
the standard solution is 100 times larger than [Fe3] in
the equilibrium mixtures. According to Le Châtelier’s
principle, this high concentration forces the reaction
far to the right, using up nearly 100% of the SCN ions.
According to the balanced equation, for every one mole
of SCN reacted, one mole of FeSCN2 is produced. Thus
[FeSCN2]std is assumed to be equal to [SCN]i.
We can calculate [FeSCN2]std and measure its absorbance
and the absorbance of [FeSCN2]eq. Assuming [FeSCN2]
and absorbance are related directly (Beer’s law), the concentration of FeSCN2 for any of the equilibrium systems
can be found by using the ratio:
 70
FeSCN2+(aq)
[FeSCN2] eq
A eq

Astd
[FeSCN2] std
Knowing the [FeSCN2]eq allows you to determine the
concentrations of the other two ions at equilibrium. For
each mole of FeSCN2 ions produced, one less mole of
Fe3 ions will be found in the solution (see the 1:1 ratio
of coefficients in the equation on the previous page).
The [Fe3] can be determined by the following because
the amount of [Fe3] consumed equals the amount of
FeSCN2 formed.
[Fe3]eq  [Fe3]i  [FeSCN2]eq
Because one mole of SCN is used up for each mole of
FeSCN2 ions produced, [SCN]eq can be determined by:
[SCN]eq  [SCN]i  [FeSCN2]eq
Knowing the values of [Fe3]eq, [SCN]eq, and [FeSCN2] eq,
you can now calculate the value of Kc, the equilibrium
constant.
Tips for Procedure
• Record exact volumes used in each step to one
digit beyond the markings on the glassware.
Volumes given in the table are approximate values.
•
Always use distilled water.
•
Check the solution concentration carefully; there
are two iron nitrate solutions.
•
Mix each solution thoroughly with a stirring rod.
Clean and dry the stirring rod after each use.
•
A blank or reference cuvette must be prepared
by filling a cuvette approximately 3/4 full with
distilled water.
E x p e r i m e n t
•
Always use a cap on cuvettes.
•
The outside of all cuvettes should be wiped clean
and dry on the outside with a tissue.
•
Rinse the cuvettes and caps with distilled water and
return them to your TA.
•
Check the sample compartment of the colorimeter
for any moisture and dry, if needed. Leave the colorimeter lid open when you return it.
Suggested Materials
MeasureNet
1 4 • Chemical Equilibrium: Finding a Constant, Kc 
Procedure
You must write your procedure and prepare your lab
notebook to record data before coming to lab to do the
experiment. Make sure that you will collect the data
necessary to complete the data analysis questions. It’s
better to have too much information and not need it,
than to need something and not have it.
Instructions on using graduated pipets can be found
in Chapter 3. Steps for using the colorimeter with the
MeasureNet workstation are in Chapter 4.
Data Analysis
1. Write the general Kc expression for the reaction.
colorimeter
two cuvettes with caps
2. Using exact volumes used, calculate the initial concentration of Fe3, based on the dilution that results
from adding KSCN solution and water to the original
0.00200 M Fe(NO3)3 solution.
five six-inch test tubes
thermometer
three small beakers
0.00200 M KSCN
3. Using the exact volumes used, calculate the initial
concentration of SCN for each trial, based on its
dilution by Fe(NO3)3 and water.
0.00200 M Fe(NO3)3
0.200 M Fe(NO3)3
Kimwipes
4. Determine the [FeSCN2]std based on the amount
of SCN2 used and the total volume of the solution
prepared (Tube #5).
funnel
pipets
pipet bulb
Expt.
14
Table 14.1. Approximate volumes for each solution.
Sample #
Volume of 0.00200 Volume of 0.00200 Volume of 0.200 M
M Fe(NO3)3 (mL)
M KSCN (mL)
Fe(NO3)3 (mL)
Volume of Water
(mL)
1
5
2
0
3
2
5
3
0
2
3
5
4
0
1
4
5
5
0
0
Standard
0
2
18
0
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1 4 • Chemical Equilibrium: Finding a Constant, Kc
5.[FeSCN2]eq is calculated using the ratio of concentations and absorbances.
[FeSCN2] eq
A eq

2
Astd
[FeSCN ] std
where Aeq and Astd are the absorbance values for the
equilibrium and standard test tubes, respectively.
Calculate [FeSCN2]eq for each of the four trials.
6.[Fe3]eq: Calculate the concentration of Fe3 at equilibrium for Trials 1–4 using the equation:
[Fe3]eq  [Fe3]i  [FeSCN2]eq
7.[SCN]eq: Calculate the concentration of SCN at
equilibrium for Trials 1–4 using the equation:
[SCN]eq  [SCN]i  [FeSCN2]eq
Expt.
14
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8. Using equilibrium concentrations, calculate Kc for
Trials 1–4. Be sure to show the Kc expression and the
values substituted in for each of these calculations.
9. Using your four calculated Kc values, determine an
average value for Kc. What was the standard deviation in your Kc values?