Download IODINE, IODIDE, TRI-IODIDE EQUILIBRIUM (Rev`d 3/25

IODINE, IODIDE, TRI-IODIDE EQUILIBRIUM (Rev'd 3/25/06)
QUANTITATIVE TECHNIQUES
Use of an Analytical Balance
Preparation of a Solution
Use of a Pipet
INTRODUCTION
Your assignment is to study the equilibrium between the iodide ion and iodine molecule
to form the tri-iodide ion in aqueous solution (reaction 1), as described by equation 2:
I2 + I1- I31-
K EQ
=
a I−
3
a I2 a I1−
≅
[I13− ]
[I 2 ][I1− ]
(1)
(2)
where aI3- represents the activity of tri-iodide which is approximately equivalent to the
stoichiometric concentration of the tri-iodide ion, [I3-] at dilute concentrations. The
variables for iodine and iodide are defined in an analogous manner.
Equation 3 is the Beer's law relationship for absorbance of radiation by the tri-iodide ion
and will be used to determine the tri-iodide molar absorptivity and equilibrium constant
for the reaction.
A = εb[I31-]
(3)
Here A is absorbance, b is the path length of the spectrometer, and ε is the molar
absorptivity coefficient, a value that is dependent on many factors including both wave
length and temperature.
Equation 2 can be rearranged to equation 4:
K EQ
=
[I13− ]
{ ([I 2 ] o − [I13− ]) ([I1− ] o − [I13− ]) }
Here the subscript 0 denotes the initial concentrations of the iodine and iodide species
after final dilutions and before equilibrium has been attained.
( 4)
PRE-LABORATORY EXERCISES
1. Calculate the value for the equilibrium constant for the reaction represented by
equation 1 using thermodynamic data. Use the data given in Table A.
Table A: Thermodynamic Properties in Aqueous solution at Unit Activity and 25oC.
[Note that ionic properties are based on the assignment of zero value for ∆Hfo, ∆Gfo, and Sfo
for H+(aq). Values are for an effective [1M] , i.e., unit activities can be somewhat different from
1M concentration.] [1]
ION
II2
I3-
∆Hfo (kJ/mol)
-55.19
22.6
-51.5
∆Gfo (kJ/mol)
-51.57
16.40
-51.4
∆So (J/Kmol)
111.3
137.2
239.3
You will use this theoretical value later to evaluate the accuracy of your experimental
value using the test suggested in your error analyses section.
2. Why is it important to use a constant temperature bath in this exercise?
3. Calculate the concentration of tri-iodide that will give an absorbance of 0.5 where the
path length is 1.0 cm and the molar absorptivity is 30,000M-1cm-1.
4. The following data was measured from one iodine/iodide solution. Initially, the
iodine concentration was 4.044 x 10-5 M and the iodide concentration was 2.371 x
10-3 M. At equilibrium, the tri-iodide concentration was 2.53x10-5 M. What is the
experimental equilibrium constant for the reaction?
LABORATORY EXERCISE
1. Before coming to lab, read over the “Spectrometer Operating Instructions” and
prepare any questions about using the Ocean Optic Fiber Optic UV-VIS spectrometer.
Since this is the first time you’ve used the UV-VIS spectrometers in P. Chem. labs,
please summarize (in your own words) these instructions in your pre-laboratory
report.
2. Equation (3) is closely obeyed only when the absorbance of tri-iodide is between 0.2
and ~0.8. For this reason, you will need to find appropriate initial concentrations of
iodine and iodide for this exercise. To this end, you will be using two solutions: a
solution of I2 and KI that will be prepared for you and a 100 mL solution of 0.1 M KI
that you must prepare. If you are unsure how to prepare this solution and calculate its
concentration to at least 4 significant figures, please consult your teaching assistant or
the lab supervisor!
4. Since all of the iodine comes from the pre-made stock solution, a suitable baseline
level of tri-iodide (with peak absorbance close to 0.2) can be found by trial and error.
Using volumetric pipets, transfer 2-7 mL of the iodide/iodine stock solution to 100
mL volumetric flasks and dilute to the marks1. Place these solutions in a constant
temperature bath until thermal equilibrium has been reached.
5. Scan these diluted solutions to determine a spectral region where the tri-iodide
absorbs (turn on the light source in the absorbance mode). Three peaks should be
observed, one for the iodide and two for the tri-iodide (near 300 and 350 nm). Find
an amount of stock solution (2-7 mL) where the absorbance of the largest peak is
between 0.18 and 0.25. Note these wavelengths and also this amount of iodide/iodine
stock solution!
6. Place this same amount of the iodide/iodine stock solution into several 100 mL
volumetric flasks. To each flask, add between 1 mL and 10 mL of the 0.1M KI
solution and dilute each to the mark with water. This should increase the absorbance
of the weakest peak (why?). At least 7 of these solutions should be prepared. Leave
these samples in the constant temperature bath until thermal equilibrium has been
reached.
7. Scan each sample and save the spectra to a disk (floppy or CD). Re-zero the
spectrometer as needed (see the Spectrometer Operating Instructions).
8. Use both of the peak absorbances (occurring at ~285 and ~350nm) to evaluate your
results. Set up a spread sheet using the absorbance for each sample at each peak and
use a form of iteration to minimize the relative standard deviation of your calculated
Keq‘s. Do this by varying the value of the absorptivity constant (~20, 000M-1cm-1 < ε
< ~40,000M-1cm-1) for each of the peaks. Choose one absorbance spectrum to
include it in your report. Also, prepare a plot of measured absorbance as a function of
initial iodide concentration and speculate on your findings.
TIPS
Discuss assumptions and approximations made in the theory section of your
preliminary notebook.
Double-check units and significant figures for ALL reported quantities.
Your experimental procedure section of the preliminary report should contain more
than the steps outlined in this protocol. Each experimental step (numbers 2 through
7) is performed for a reason. Include these reasons in the experiment section. For
instance, consider question 2 in the pre-laboratory exercises.
Ask your teaching assistant or the lab supervisor if you have any questions.
1
To protect the integrity (i.e., concentration/purity) of the already-prepared KI/I2 stock solution, do not
pipet directly from the bottle containing the stock solution, do not return used solution to the stock, and
seal the bottle with ParafilmTM when you are finished.
REFERENCES
[1]
Gordon M. Barrow, Physical Chemistry, 6th Ed., New York, 1996, p. 878.