Experiment- Finding the Equilibrium Constant, Kc, for the Formation
of Iron (III) Thiocyanate
Chemicals: 0.200 M Fe(NO3)3 (aq), 2.00x10-3 M NaSCN (aq), 2.00x10-3 M Fe(NO3)3 (aq),
0.10 M HNO3 (aq)
Equipment: Spec 20 spectrophotometer, cuvettes, 10mL pipettes, 5mL pipettes
For any reaction in which the backwards or reverse reaction will occur, an equilibrium is
established when the reaction rate of the reverse reaction equals the rate of the forward reaction.
Once the equilibrium state has been reached, the concentrations of all products and reactants will
remain constant until there is a change in the reaction conditions. The presence of an
equilibrium state is shown as:
For any equilibrium, the ratio of the product and reactant concentrations is described by the
Equilibrium Constant, Kc:
The value of Kc can be calculated from the initial concentrations of the reactants as long as the
concentration at equilibrium can be measured for at least one reactant or product.
Sample Calculation: A mixture of 1.000 x 10-3 mol of H2 (g) and 2.000 x 10-3 mol of I2 (g) is
placed in a 1.000 L container and allowed to reach equilibrium. If the concentration of HI (g) at
equilibrium is 1.87 x 10-3 M, what is the value of Kc for the reaction? This problem can best be
solved by creating an Initial, Change, Equilibrium or ICE table:
The change in concentration of HI is +1.87 x 10-3 M. The change in concentration of both H2
and I2 can be found using stoichiometry:
1.87 x 10-3 M HI x 1 mol H2
2 mol HI = 0.935 x 10-3 M
By plugging in the changes in concentration, the ICE table can be completed to show the
equilibrium concentration for each species:
The equilibrium constant expression for the reaction is:
The value of Kc is found by inserting the equilibrium concentrations into the equilibrium
In today’s experiment you will combine solutions of known concentration of iron (III) nitrate and
sodium thiocyanate in order to establish the following equilibrium:
The equilibrium constant expression for this reaction is:
Once equilibrium has been established, you will measure the equilibrium concentration of
FeNCS2+ (aq) using spectrophotometry. You will then use stoichiometry and the initial
concentrations of the reactants to find the equilibrium concentrations of the remaining species
and calculate the value of Kc.
The Beer-Lambert Law states that for solutions where the concentration is relatively low a graph
of the concentration of the analyte (meaning the substance detected by the spectrophotometer)
vs. absorbance (A) will produce a straight line.
The instrument we will be using for these measurements is called a Spec 20, shown below:
This instrument measures the intensity of light at a specific wavelength passing through a sample,
the percent transmittance (%T). To calculate the amount of light absorbed by the sample (A)
from %T the following relationship is used,
A = 2 – log (%T)
Setting 0%T (Note: Be sure the instrument has been switched on and warmed up for at least 15
minutes before setting 0%T and 100%T)
With the sample chamber empty, meaning there is no cuvette in the chamber, use the On/Off
knob on the front of the instrument to adjust the reading for %T to 0.0.
Fill the cuvette with 0.10 M HNO3 and place the cuvette in the sample chamber. Be sure the
white line on the cuvette is lined up with the raised mark at the front of the sample chamber and
close the lid to the sample chamber. Use the 100%T knob on the front of the instrument to adjust
the reading for %T to 100.0.
Remove the cuvette and verify that the reading returns to 0.0%T. If the reading does not return
to 0.0, repeat the steps above to confirm that the scale has been set for 0%T and 100%T. Once
set-up is complete, rinse the cuvette with distilled water.
Each time a measurement is to be recorded, rinse the cuvette by filling it with the solution to be
tested then discarding in waste. Refill the cuvette with the solution to be tested, place the cuvette
in the sample chamber (again making sure the white line on the cuvette is aligned with the mark
at the front of the sample chamber), close the camber lid and read the %T from the display.
Waste Disposal: All waste from today’s experiment will be disposed of in Inorganic Waste.
Preparation of a Standard Curve
In order to find the concentration of FeNCS2+ at equilibrium a series of solutions of known
concentration of FeNCS2+ will be prepared and their absorption will be measured using the
Spec20. When the absorption (A) of these solutions is plotted against concentration a straight
line (the Standard Curve) should be produced. Prepare five standard solutions according to the
table below. Solutions will be prepared in 25.00mL volumetric flasks. Use a volumetric pipette
to measure the Fe(NO3)3. Use the burette provided by your instructor to measure the NaSCN.
Add 0.10 M HNO3 to each flask until the volume reaches the 25.00mL mark. By using a large
concentration of Fe(NO3)3 it is assumed that the equilibrium is forced to the right and all of the
NaSCN will be converted to FeNCS2+.
Solution mL 0.200 M Fe(NO3)3 mL 2.00x10-3 M NaSCN
1 10.00 0.00
2 10.00 1.00
3 10.00 2.00
4 10.00 3.00
5 10.00 4.00
Turn on the Spec20, set the wavelength to 447 nm, allow it to warm up for ~10 minutes and set
0%T and 100%T. Measure %T for each solution using the Spec20 and calculate the absorption,
A, for each. Calculate the concentration of FeNCS2+ in each standard solution assuming that
100% of the NaSCN has been converted to FeNCS2+. Create a graph of absorption vs.
concentration of FeNCS2+ in your lab notebook. Use a ruler to draw the best-fit line for the data,
this is your Standard Curve.
Prepare the equilibrium solution by adding 5.00mL of 2.00x10-3 M Fe(NO3)3 and 5.00mL of
2.00x10-3 M NaSCN to a 25.00mL volumetric flask. Dilute to the mark with 0.10 M HNO3.
Allow the solution to stand for approximately 10 minutes to assure that there is time for
equilibrium to be established. Measure the %T of the solution and calculate absorption. Using
the Standard Curve, find the concentration of FeNCS2+ at equilibrium. Calculate the initial
concentrations of the reactants and create an ICE table in your lab notebook to find Kc.
1. Write the Keq expression for the equilibrium reaction shown below:
2. Given that Keq for the reaction shown below is 5.0x10-9 at 25oC, what are the concentrations
of barium and carbonate in a saturated solution of BaCO3 at 25oC?
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