Krishna Trehan
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Many reactions do not go to completion;
instead, they reach states where the products
and reactants are both present.
When the concentrations of the reactants and
products are both equal and constant, the
reaction has reached equilibrium.
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The equilibrium constant relates the
concentrations of the products to those of
the reactants in a ratio.
For exmaple: Fe3+ + SCN- ↔FeSCN2+
If Kc is greater than 1, then the products are
favored. If Kc is less than 1, then the reactants
are favored.
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In order to find the equilibrium constant you
need the concentrations of the reactants and
products. Usually this is given to use, but if it
is not, you can use spectrophotometry to find
the concentrations.
A spectrophotometer consists of two
instruments, a spectrometer (for producing
light of any wavelength you chose) and a
photometer (for measuring the intensity of
light).
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A beam of light from the spectrometer is sent
through a solution and the photometer
receives the beam after it is sent through the
solution
Spectrometer Light  Solution  Photometer
The spectrophotometer will return a number;
this is a representation of the analytes
absorbance (the lights frequency will be
altered by the solution).
The degree of color is proportional to the
concentration.
A sample of a solution is put into the
spectrophotometer where it is analyzed.
http://www.okokchina.com/Files/uppic34/Spectrophotometer960.jpg
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The purpose of this experiment is to be able
to find the equilibrium constant of a FeSCN2+
solution using spectrophotometry.
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Spectrophotometer
Cuvet (to place solution sample into
spectrophotometer)
5 mL of 2E-3 M Fe(NO3)3
5mL of 2E-3 M KSCN.
FeSCN2+
Beaker
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First we need to find the wavelength of optimum
absorbance. This shows the frequency of visible
light which is most easily absorbed by the
solution.
Place the solution FeSCN2+between the
Spectrometer Light and the Photometer
Chose a value from 380nm to 780 nm (visible
light range on the spectrum)
The spectrophotometer will return a value, this is
the absorbance value. Then you must graph this
point on a graph with the x-axis as Light
Frequency, and the y-axis as Absorbance.
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Once this is completed, you must repeat this
step several more times (using different
frequency values) to find the frequency which
yields the highest Absorbance value, the
optimum absorbance value. This frequency
value will be used throughout the rest of the
experiment.
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This is an example of a chart with a solution
of chlorophyll
http://www.bio.davidson.edu/Courses/Bio111/Bio111LabMan/lab1fig3.gif
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In this step, we will keep the frequency of the
spectrometer constant (the value achieved in
step 1)
Prepare multiple solutions of FeSCN2+ (with
different concentrations) and add them
between the spectrometer and the
photometer
While keeping the frequency constant, test
the solutions and record the outcome.
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The outcome is the transmittance of the
solutions. In order to obtain the Absorbance,
we must use the following equation:
Once the absorbance values are obtained,
graph the results
◦ X-axis = Concentration (Molarity)
◦ Y-axis = Absorbance
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Find a line which connects both the points.
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Mix the 5 mL of 2E-3 M Fe(NO3)3 with the
5mL of 2E-3 M KSCN.
Analyze this solution in the
spectrophotometer with the same frequency
as the one obtained in Step 1.
Convert this transmittance into Absorbance.
Call this value k.
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Now there is an Absorbance value (y-value),
but there is no concentration value (x-value).
Graph the line y=k on the same graph
created in step 2.
The point at which the two lines intersect will
have the point (x, k)
Point x is the concentration of the solution.
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1. How many moles of Fe3+ and SCN- were
initially present?
2. How many moles of FeSCN2+ were in the
mixture at equilibrium?
3. How many moles of Fe3+ and SCN- were
used up in making the FeSCN2+ ?
4. How many moles of Fe3+ and SCN- remain
in the solution at equilibrium?
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5. What are the concentrations of Fe3+ , SCN, and FeSCN2+ at equilibrium?
6. Determine Kc for this reaction.