The Richard Stockton College of New Jersey CHEM 2115: General ...

The Richard Stockton College of New Jersey
Chemistry Program, School of Natural Sciences and Mathematics
PO Box 195, Pomoma, NJ
CHEM 2115: General Chemistry Laboratory
Experiment 9: Determination of an equilibrium constant, Keq
The goal of this experiment is to determine the equilibrium constant for the following reaction (written in
net ionic form):
F e +3 (aq) + SCN − (aq) ⇀↽ F eSCN +2 (aq) (1)
The equilibrium constant can be written as:
Keq =
[F eSCN +2 ]eq
[F e +3 ]eq[SCN −1 ]eq
where the squbscript eq indicates the equilibrium concentration of each of the species. We can make up an
ICE table for this reaction:
Fe +3 (aq) + SCN − (aq) ⇀↽ FeSCN +2 (aq)
I [Fe +3 ]initial [SCN 1 ]initial 0
C −x −x +x
E [Fe +3 ]eq [SCN 1 ]eq [FeSCN +2 ]eq
We now need to determine each of the unknowns in the above table.
1. You can determine the initial concentrations of Fe +3 and SCN − by performing dilution calculations.
We know that the initial concentrations are 0.00200 M for Fe +3 and 0.00100 M for SCN − .
For example, in sample (test tube) 1 I placed 5 mL of the iron (III) nitrate solution and 1 mL of the
KSCN solution in a test tube and diluted to a total volume of 10 mL.
For [Fe +3 ]initial
For [SCN − ]initial
M1V1 = M2V2
(0.00200 M)(0.005 L) = [F e +3 ]initial(0.010 L) (4)
[F e +3 ]initial = 0.00100 M (5)
(0.00100 M)(0.001 L) = [SCN − ]initial(0.010 L) (6)
[SCN − ]initial = 1.00 × 10 −4 M (7)
2. We can determine [FeSCN +2 ]eq using our standard curve and the absorption measurements of samples
1–5. The equation of the line of our standard curve relates the absorbance (Abs) values (y-axis) to
[FeSCN +2 ]eq (x-axis):
y = mx + b −→ Abs = m ∗ [F eSCN +2 ]eq + b (8)
where m and b come from the trend line equation in Excel and Abs is your measured absorbance.
Rearranging we can solve for [FeSCN +2 ]eq:
[F eSCN +2 ]eq =
Abs − b
m
So for each sample, you can use the measured absorbance to determine the equilibrium concentration
of FeSCN +2 .
3. The “Change” row of the ICE table can be found using stoichiometry. According to the chemical
equation, all species in the reaction are in a 1:1 ratio. Therefore, the unknown x in the table is simply
[FeSCN +2 ]eq.
(2)
(3)
(9)

4. Since we know the initial concentration of each reactant (the I-row) and we know the change in
concentration to get to equilibrium (C-row) we can find the equilibrium concentration of each reactant
in the following manner:
[F e +3 ]eq = [F e +3 ]initial − x = [F e +3 ]initial − [F eSCN +2 ]eq
[SCN − ]eq = [SCN − ]initial − x = [SCN − ]initial − [F eSCN +2 ]eq
We can repeat this calculation to determine the equilibrium concentration of each reactant for each
sample (1–5).
5. We now know all three of the equilibrium concentrations so we can calculated Keq for each sample:
Note on Standard Solutions
Keq =
[F eSCN +2 ]eq
[F e +3 ]eq[SCN −1 ]eq
In preparing the standard solution you were assigned a volume of 0.00100 M KSCN to place in your 50 mL
volumetric flask. You then added 15 mL of 0.200 M Fe(NO3)3 and diluted to a total volume of 50mL.
Since you are adding a very large excess of iron (III) nitrate, the limiting reactant is KSCN and you can
assume that since there is a huge excess of iron ions, that the reaction goes to completion. Therefore, the
concentration of FeSCN +2 in your standard solution is equal to the initial concentration of KSCN.
Again this leads to a dilution calculation:
[F eSCN +2 ]standard =
where VKSCN is your assigned volume of KSCN in LITERS.
(0.00100 M)(VKSCN)
0.050 L
After entering the data in Excel, you will need to prepare the standard curve, which is a plot of the Absorbance
(Abs) on the y-axis and [FeSCN +2 ]standard on the x-axis.
You will need to fit a line to the data with the equation displayed on the graph. It is this equation that you
will use to convert the absorbance measurements on samples 1–5 to [FeSCN +2 ]eq (see step 2 above).
(10)
(11)
(12)
(13)