Equilibrium
Introduction
Many reactions reach a steady state condition in which there
are appreciable amounts of both reactants and products in
the reaction vessel. The quantities of reactants and products
do not change with time (at constant temperature) once this
steady state is reached. These reactions have reached the
state of chemical equilibrium. In the generic reaction:
In this experiment, you will check out the validity of the
Law of Chemical Equilibrium using the reaction:
This reaction is particularly well suited to investigation
because equilibrium is reached very quickly. The FeSCN2+
is intensely colored and its concentration is easily measured
using a Spec 20. You will prepare 5 mixtures of Fe3+ and
SCN-. Each mixture will contain the same concentration of
Fe3+, and varying concentrations of SCN-. You will then
measure the amount of FeSCN2+ formed in each mix. You
will use this data to calculate how much of the Fe3+ and
SCN- are still left in each mix. Finally, you will arrange
the concentrations of each substance for each mix in the
equilibrium expression for the reaction:
FeSCN 2 +
Kc =
Fe3+ ⋅ SCN −
and see if the Kc is truly constant for the 5 mixtures.
You will use pipets to measure the volumes of the reactants.
Refer to the pipet use discussion on page 104. Good results
will depend on good measurements. Note that when solutions are mixed together, the solvent in the one solution will
dilute the other solution. If, for example, you mix 5 ml of a
0.2M solution of substance A with 5 ml of a 0.1M solution
of substance B, immediately after mixing, substance A will
become 0.1M and substance B will become 0.05M. The
equation for calculating concentrations after diluting is:
Vol before
Conc after = Conc before x
Vol after (1)
The c on Kc refers to the concentrations of the species in the
reaction. For gases, the equilibrium constant is often given
as Kp , where p refers to the pressure of the species.
If, in a reaction, there are stoichiometric relationships other
than 1 to 1, the Law of Chemical Equilibrium observes
that the factors on each species in the balanced reaction
appear as exponents on the concentration (or pressure)
of that species in the K expression. For example, for the
generic reaction:
[
2A + B
3C + 2D
the equilibrium constant expression has the form:
[ C] 3 ⋅ [ D] 2
Kc =
[ A] 2 ⋅ [ B]
The general case, for the reaction:
aA + bB
Kc =
FeSCN2+(aq)
(Fe3+ is the iron(III) ion, present here in a solution of
Fe(NO3)3 acidified with HNO3. SCN- is the thiocyanate
ion, present here in a solution of KSCN. Fe(SCN)2+ is the
mono­thio­cyan­atoiron(III)­ion, one of the so-called complex
ions.)
A+B
C+D
the concentrations of A, B, C, and D will obey the Law of
Chemical Equilibrium. This law notes that reactions obey
certain conditions for the relative values of the concentrations
of the products and reactants at equilibrium. These values
depend on the particular reaction and on the temperature.
The values are usually given in the form of an equilibrium
constant, K, which takes the form:
[ C ] ⋅ [ D]
Kc =
[ A] ⋅ [ B]
is:
Fe3+(aq) + SCN–(aq)
cC +dD
[ C] c ⋅ [ D] d
[ A ] a ⋅ [ B]b
[
][
]
]
You have used this relationship before, and will use it often,
so understand it and practice with it.
1
Work in Pairs
Supplies:
• 2 cuvets
• One 5 ml fixed volume pipet (Volumetric Pipet), two 5 ml
variable volume pipets (Mohr-type Measuring Pipet)
• 1 pipetting aid
• A calibration curve
• 5-18x150 test tubes (large) from drawer
• Use clean, dry 50 ml beakers to bring about 30 ml of
2.00 x 10-3M Fe(NO3)3 solution and about 20 ml of
2.00 x 10-3M KSCN solution to your work area. Put about
20 ml of deionized water into another 50 ml beaker.
• 400 ml beaker for waste
Mixing instructions: The pipets should be clean and dry. If
not, refer to the pipet use discussion for the proper method
of rinsing.
Label the 5 test tubes 1 to 5 and set them in a test tube rack.
Use the volumetric pipet to add 5.00 ml of the Fe3+ solution into each of the 5 test tubes. Use one of the Mohr-type
measuring pipets to add 1.00 ml of the SCN- solution into
test tube 1, 2.00 ml into test tube 2, etc., until 5.00 ml goes
into test tube 5. Now use the other Mohr-type pipet to add
4.00 ml deionized water into test tube 1, 3.00 ml into test
tube 2, 2.00 into test tube 3, and 1.00 ml into test tube 4.
Each test tube now has 10.00 ml total volume. The table
below summarizes the mixing quantities.
You will use a Spec 20 to determine the concentration of
the FeSCN2+ formed.
Use a stirring rod to mix each solution. Mix with vigorous
up and down elliptical agitation, being careful not to poke
out the bottom of the test tube. Rinse and dry the stirrer
between test tubes.
Spec 20 setup:
Analog Spec 20: Refer to page 5 for Spec 20 information.
Turn the Spec 20 on with the left front knob as you enter the
lab to allow for warm-up. Make sure the sample holder is
empty and its cap is closed. After you prepare the solutions
described below, with the machine warmed up, use the left
front knob to set the pointer to the left zero on the dial. Set
the wavelength on the Spec 20 at 45 nm, the wavelength of
maximum absorption for this complex ion. The upper right
dial is the wavelength set dial. Insert a cuvet with about 3
cm height of deionized water into the sample holder. Use
the right front knob to set the needle to the right zero on
the dial, the zero on the absorption scale. This zeroes out the
solvent. The machine is now ready to measure the absorption of each of the 5 mixtures you will use.
Digital Spec 20: Refer to page 5 for Spec 20 information.
Turn the Spec 20 on with the left front knob as you enter the
lab to allow for warm-up. Make sure the sample holder is
empty and its cap is closed, and the lever on the front left
of the machine is turned to the left. After you prepare the
solutions described below, with the machine warmed up, set
the wavelength on the Spec 20 at 45 nm, the wavelength of
maximum absorption for this complex ion. The upper right
dial is the wavelength set dial. Turn the left front knob until
0 is displayed. Set the Mode to A. Insert a cuvet with about
3 cm height of deionized water into the sample holder. Turn
the right front knob until 0A shows on the display. This may
take several full turns. The machine is now ready to measure
the absorption of each of the 5 mixtures you will use.
One cuvet contains deionized water, which you used to zero
the spectrophotometer. Pour about 3 cm height of sample 1
into the other cuvet, place it into the zeroed Spec 20, record
the absorption on the data sheet. Dispose of the liquid from
the cuvet into the 400 ml beaker.
Add about one ml from sample 2 to rinse the cuvet. Dump
the rinse liquid into the 400 ml beaker. Add another ml
from sample 2 to further rinse the cuvet. Dump into the
400 ml beaker. Finally add about 3 cm height of liquid
from sample 2 into the cuvet and place it into the Spec 20.
Record the absorption.
Repeat this procedure for each of the remaining 3 samples.
There should be a smooth trend of values for the absorptions
in samples 1 to 5, close to about 0.1 apart. If the trend is
much less than 0.1 apart, check with the instructor. If any
one value does not follow the trend, repeat the measurement
for that sample, or even re-mix and then repeat the measurement. You are now ready to do the calculations.
To dispose of the solutions: Combine them all into the
400 ml beaker, add about 3 g of sodium hydrogen carbonate (6 cm on the scoopula) from the plastic bottle on the
lab table. When the bubbling ceases, dump the liquid into
the sink.
S U MM A R Y O F SAM PLE P R EP A R A T ION
Test Tube Nu mber
1
Volume Fe
3+
solution
2
3
4
5
5.00
5.00
5.00
5.00
5.00
Volume SCN – solution
1.00
2.00
3.00
4.00
5.00
Volume H 2O (deionized)
4.00
3.00
2.00
1.00
0
2
D AT A T A B L E
Test Tube Num ber
1
Absorbance of F eS C N
2
3
4
5
2+
C A L C U L AT I O N S
1.
C alculate the concentrations of the starting m aterials after m ixing. Use equation 1 on page 43. The stock
concentrations of the F e3+ and the S C N – are 2.00 x 10 -3 M . The concentration after m ixing and before reacting is
called the initial concentration.
1
2
3
4
5
[F e3+]i ni t i a l
M
M
M
M
M
[S C N ]i ni t i a l
M
M
M
M
M
–
2.
Use the calibration curve to find the [F eS C N 2+]e q for each sam ple. Write the values in the table below. F ind the
equilibrium concentrations of the F e3+ and S C N - using the relationships:
[F e3+]i ni t i a l – [F eS C N 2+]e q
[F e3+]e q
=
[S C N – ]i ni t i a l – [F eS C N 2+]e q =
and
[S C N – ]e q
Write these values below. (S ubtracting concentrations rather than m oles works if the volum e is the sam e for all
substances. Note that it takes one m ole of F e3+ and one m ole of S C N – to form one m ole of F eS C N 2+.)
3.
[F eS C N 2+]e q
M
M
M
M
M
3+
[F e ]e q
M
M
M
M
M
[S C N – ]e q
M
M
M
M
M
C alculate the K c for each sam ple. Use the relationship:
Note that K c requires equilibr ium concentrations.
Kc =
[FeSCN
[ Fe
3+
2+
]
] ⋅[ SCN − ]
(Use the correct units on the values for the K c )
Kc
K c average
4.
F ind the % deviation for each K value from the average: % deviation =
% deviation
3
Kc − Kc
Kc
average
average
x 100
Name_________________________________________ Grade___________ Date ___________
questions
1. What if the equation for the equilibrium were: Fe3+(aq) + 2SCN-(aq)
for the Kc be written?
Fe(SCN)2+(aq). How would the expression
2. If the reaction in question 1 is correct, the concentrations of Fe3+ at equilibrium in each sample would be the same as
you calculated in part 2 of the calculations section. But the values for the equilibrium concentrations of SCN- would
change. They would be calculated as follows:
[SCN-]initial - 2[[Fe(SCN)2+] = [SCN-]eq. Why is that? (Assume that the Spec 20 measurements still gave correct values
for the concentration of the product.)
3. Look at the % deviation section of the calculations table. Choose the two samples with the least % deviation. Write
the values from the table for the [SCN-]eq for these two: __________M, __________M. Use the equation in question
2 to show what these values would be if the reaction equation in question 1 is the actual equation for this equilibrium.
(Show your work.)
4. Use the values calculated for the [SCN-]eq in question 3 and the Kc expression from question 1 to calculate the values
of Kc for the two samples under the conditions raised by these questions. Show the units along with the numerical
value for the two Kc's. Divide the larger number by the smaller number, and multiply by 100 to see the percentage
differences between them. Are they farther apart than the two K's you used for the concentration data? If so, this shows
that the law of equilibrium does not apply if the stoichiometry used is not the actual stoichiometry of the reaction.
5. When you added the sodium hydrogen carbonate to the waste solutions, you were neutralizing the nitric acid present
in the Fe(NO3)3 solution. What is the formula for nitric acid? Show a balanced chemical equation for the reaction that
took place between nitric acid and sodium hydrogen carbonate. Remember, there was fizzing. One of the products was
a gas.
4
Sample Measurement
The sequence for sample measurement is:
a. Select wavelength using Wavelength Control.
b. With sample compartment empty and cover closed, adjust Zero Control so that the meter reads zero (on the left side
of the dial).
c. Insert reference blank into the sample compartment and set Absorbance Control to zero (on the right side of the
dial).
d. Insert unknown sample into the sample compartment and read measurement from meter in percent transmittance or
absorbance.
5