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Aim
To determine the stability constants and molar
absorptivities of the donor-acceptor complexes formed between iodine and butyl
alcohol from UV-vis spectrophotometric data.
Results and
calculation
Temperature
of experiment = 27.5 ˚C
[I2] provided = 0.005 M
[I2] required = 0.0005M
Volume of iodine required = (0.0005 x
25.0 x 10-3) / 0.005 = 0.0025L = 2.5mL
For
solution 1, volume of cyclohexane required = 25.0 – 2.5 = 22.5 mL
For solution 2 of t-butyl alcohol, c1V1=c2V2
Volume of alcohol required = (Concentration
of the alcohol required x Total vol of solution) / (Conc of the given alcohol) =
(0.2) x (25)/ (2.0) = 2.5mL
Volume
of cyclohexane required = 25.0 – 2.5 -2.5 = 20.0 mL
Similar calculations
are repeated for the other 5 solutions of t-butyl alcohol.
Table 1: Absorbance for t-butyl alcohol
|
||||||
Solution
|
Vol of I2 /mL
|
Vol of cyclohexane / mL
|
Vol of t-butyl alcohol /mL
|
Absorbance
|
||
l=440nm
|
l=460nm
|
l=520nm
|
||||
1
|
2.5
|
22.5
|
0.0
|
0.0504
|
0.1168
|
0.4717
|
2
|
2.5
|
20.0
|
2.5
|
0.0951
|
0.1677
|
0.3976
|
3
|
2.5
|
17.5
|
5.0
|
0.1379
|
0.2120
|
0.3737
|
4
|
2.5
|
12.0
|
10.0
|
0.1943
|
0.2644
|
0.3284
|
5
|
2.5
|
7.5
|
15.0
|
0.2280
|
0.2949
|
0.2952
|
6
|
2.5
|
2.5
|
20.0
|
0.2631
|
0.3274
|
0.2797
|
The
isosbestic point occurs at 491.8 nm with absorbance of 0.3293 A.
Since A =a/ [A]0, where a is the absorbance of the
solution and [A]o is the initial
concentration of the acceptor (I2), the value of A can be
calculated as follows:
(Solution 1 at 440nm) A = a/ [A]0
= 0.0504 / 0.0005 = 100.8 mol-1L
The absorbance, a, of the solution
equals the sum of absorbances for the acceptor (A) and the complex(C): a = eA ([A]0 – [C]) + eC [C], where eA and eC are molar absorptivities of the acceptor and complex respectively.
When no donors are added, no complexes are formed.
Thus [C] = 0 and a = eA [A]0
Molar absorptivity of acceptor I2,
eA at l = 440nm = a/ [A]0
= 0.0504 / 0.0005 = 100.8 mol-1
L cm-1
(Solution 2 at 440nm) A = a/ [A]0 = 0.0951 / 0.0005 = 190.2 mol-1 L
(eA –A)/[D]0 = (100.8 –
190.2) / 0.2 = -447
Table 2: A and (eA – A) / [D]o values of t-butyl solutions at different wavelengths
Wavelength
|
440nm
|
460nm
|
520nm
|
||||||
Solution
|
A
|
eA
|
(ea – A) / [D]o
|
A
|
eA
|
(ea – A) / [D]o
|
A
|
eA
|
(ea – A) / [D]o
|
1
|
100.8
|
100.8
|
-
|
233.6
|
233.6
|
-
|
943.4
|
943.4
|
-
|
2
|
190.2
|
100.8
|
-447.0
|
335.4
|
233.6
|
-509.0
|
795.2
|
943.4
|
741.0
|
3
|
275.8
|
100.8
|
-437.5
|
424.0
|
233.6
|
-476.0
|
747.4
|
943.4
|
490.0
|
4
|
388.6
|
100.8
|
-359.8
|
528.8
|
233.6
|
-369.0
|
656.8
|
943.4
|
358.3
|
5
|
456.0
|
100.8
|
-296.0
|
589.8
|
233.6
|
-296.8
|
590.4
|
943.4
|
294.2
|
6
|
526.2
|
100.8
|
-265.9
|
654.8
|
233.6
|
-263.3
|
559.4
|
943.4
|
240.0
|
Based on the equation A= (eA – A) / (k [D]o) + eC, the gradient of the graph of A against (eA – A) / [D]o is 1/K and the y-intercept is eC.
 |
At 440nm,
k
= 1/1.626
= 0.6150 mol-1 L
and
eC = 954.9 mol-1 L cm-1
|
 |
At 460 nm,
k
= 1/1.172
= 0.8532 mol-1 L
and
eC = 955.2 mol-1 L cm-1
|
 |
At 520nm,
k
= 1/0.477
= 2.094 mol-1 L
and
eC = 467.0 mol-1 L cm-1
|
Average stability constant K = (0.6150 +
0.8532 + 2.094) / 3 = 1.187 mol-1 L
Average molar absorptivity of iodine-t-butyl
alcohol complex,
eC = (954.9 + 955.2 + 467) / 3 = 792.4
mol-1 L cm-1
Similarly,
Table 3: Absorbance for n-butyl alcohol
|
||||||
Solution
|
Vol of I2 /mL
|
Vol of cyclohexane / mL
|
Vol of n-butyl alcohol /mL
|
Absorbance
|
||
l=440nm
|
l=460nm
|
l=520nm
|
||||
1
|
2.5
|
22.5
|
0.0
|
0.0104
|
0.0996
|
0.4579
|
2
|
2.5
|
20.0
|
2.5
|
0.0532
|
0.1707
|
0.4114
|
3
|
2.5
|
17.5
|
5.0
|
0.0837
|
0.2142
|
0.3660
|
4
|
2.5
|
12.0
|
10.0
|
0.2140
|
0.2786
|
0.3204
|
5
|
2.5
|
7.5
|
15.0
|
0.2562
|
0.3129
|
0.2771
|
6
|
2.5
|
2.5
|
20.0
|
0.2855
|
0.3374
|
0.2518
|
The
isosbestic point occurs at 490.8 nm with absorbance of 0.3146 A.
Table 4: A and (eA – A) / [D]o values of n-butyl solutions at different
wavelengths
Wavelength
|
440nm
|
460nm
|
520nm
|
||||||
Solution
|
A
|
eA
|
(ea – A) / [D]o
|
A
|
eA
|
(ea – A) / [D]o
|
A
|
eA
|
(ea – A) / [D]o
|
1
|
100.8
|
100.8
|
-
|
199.2
|
199.2
|
-
|
915.8
|
915.8
|
-
|
2
|
190.2
|
100.8
|
-447.0
|
341.4
|
199.2
|
-711.0
|
822.8
|
915.8
|
465
|
3
|
275.8
|
100.8
|
-437.5
|
428.4
|
199.2
|
-573.0
|
732.0
|
915.8
|
459.5
|
4
|
388.6
|
100.8
|
-359.8
|
557.2
|
199.2
|
-447.5
|
640.8
|
915.8
|
343.8
|
5
|
456.0
|
100.8
|
-296.0
|
625.8
|
199.2
|
-355.5
|
554.2
|
915.8
|
301.3
|
6
|
526.2
|
100.8
|
-265.9
|
674.8
|
199.2
|
-297.3
|
503.6
|
915.8
|
257.6
|
 |
At 440 nm,
k
= 1/1.626
= 0.6150 mol-1 L
and
eC = 954.9 mol-1 L cm-1
|
 |
At 460nm,
k
= 1/0.825
= 1.212 mol-1 L
and
eC = 919.0 mol-1 L cm-1
|
 |
At 520nm,
k
= 1/1.345
= 0.7435 mol-1 L
and
eC = 159.0 mol-1 L cm-1
|
Average stability constant K = (0.615 +
1.212 + 0.7435) / 3 = 0.8568 mol-1 L
Average molar absorptivity of
iodine-n-butyl alcohol complex,
eC = (954.9 + 919 + 159) / 3 = 677.6
mol-1 L cm-1
Discussion
Ultraviolet-visible spectroscopy
The absorption of Ultraviolet-visible (UV-vis) radiation may
cause electrons to transit to higher energy states. Through analysis of the
resultant spectra, the identities and concentrations of compounds may be
determined.
The possible electronic
transitions that light may cause are shown on the left diagram.
|
 |
In this experiment, when the alcohol reacts
with iodine, alcohol act as a donor and iodine act as acceptor to produce the
alcohol-iodine (donor-acceptor) complex. When a photon is absorbed, an electron
in the highest occupied molecular orbital (HOMO) of alcohol gains enough energy
to jump to the lowest occupied molecular orbital (LUMO), anti-bonding orbital (s*) of iodine.
The energy of the photon corresponds to the energy gap between the HOMO and
LUMO.
Theory behind experiment
Iodine shows a single maximum absorption at
520nm, in inert solvent cyclohexane. This wavelength of absorbed light
corresponds to the energy require to promote electrons from ground state to
excited state. When iodine forms a complex with a butyl alcohol
donor, the maximum absorption occurs at a shorter wavelength. More energy is required
to transfer an electron from the donor to an orbital associated with the
acceptor. Molar absorptivities from charge-transfer absorption are large,
proven from the results of more than 600 mol-1 L cm-1.
Spectra
From the spectroscopic data of solution 1,
it can be observed that iodine absorbs at a higher wavelength. However, as
butyl alcohol concentration increases, the concentration of the resultant
iodine-alcohol complex increases too. This causes absorption peaks to occur at
lower wavelengths and overall absorbances to decrease. The slope of the
spectrum becomes increasingly gentle too.
Precautions
Water was not allowed to be present in the
volumetric flasks. This is because water is a potential ligand and may form
complex with iodine and hence affecting the stability constant. Thus, all
apparatus used in the experiment were dry. Inert solvent, cyclohexane, was used
to ensure no side interactions with the reactants.
25.0 mL solution was prepared first by
adding iodine, followed by cyclohexane then butyl alcohol so as to prevent
iodine from reacting rapidly with the alcohol. This decreases the enthalpy of
formation and bonding forces, leading to increased accuracy o absorption
spectrum. In addition, the solutions were left to stand for about an hour for
all the reactions to reach equilibrium in order to achieve better absorption
spectrum. Thus, the concentration of complexes formed will be higher and as
absorption is dependent on concentration according to Beer-Lambert Law, the
resultant spectra will be more accurate too.
Care was taken to ensure that the cuvettes inserted
into the spectrometer were free of fingerprints on the transparent sides before
being inserted into the spectrophotometer as fingerprints may scatter the radiation
passing through, resulting to inaccuracies. The readings were taken starting
from the least concentrated solution to the higher concentrations. This was
done to prevent significant changes in the concentrations of solutions in the
cuvette which may lead to inaccuracies. In addition, the cuvettes were rinsed
three times with the solution that is to be measured before placement into the
spectrophotometer.
Stability constant
Iodine-butyl alcohol complexes differ in their stabilities which may be measured from their stability constants, K which is directly proportional to complex concentration but inversely proportional to donor and acceptor concentrations. A large K value means a larger concentration of complex in equilibrium compared to the product of the concentration of reactants, thus, the complex is more stable and less likely to dissociate into iodine and alcohol.
Theoretically, the stability constant of iodine-t-butyl alcohol complex should be larger than iodine-n-butyl alcohol complex. A tertiary alcohol, with 3 electron-donating methyl groups which can donate electron density inductively to the oxygen atom, forms a more stable complex than n-butyl alcohol which has only 1 electron-donating propyl group. The lone pair in t-butyl alcohol has a higher electron density and is more available for donation to iodine than that in n-butyl alcohol, thereby forming a more stable complex. The experimental K values – higher for iodine-t-butyl alcohol complex than iodine-n-butyl alcohol complex – reaffirm this.
Conclusion
The
average stability constant of iodine-t-butyl alcohol complex is 1.187 mol-1
L while that of iodine-n-butyl alcohol complex is 0.8568 mol-1 L.
The average molar absorptivity of iodine-t-butyl alcohol complex is 792.4 mol-1
L cm-1 while that of iodine-n-butyl alcohol complex is 677.6 mol-1
L cm-1. Isosbestic point of spectroscopic data with t-butyl alcohol
donor occurs at 491.8 nm with absorbance of 0.3293 A while that
with n-butyl alcohol donor occurs at 490.8 nm with
absorbance of 0.3146 A.
The
results indicate that t-butyl alcohol forms a more stable complex with iodine
than n-butyl alcohol.
Bibliography
1. Pavia et al.. Introduction to Spectroscopy. Brooks/ Cole. 2001.
2. Blinder, S. M. Introduction to quantum mechanics: in Chemistry, materials science, and
biology. Elsevier Academic Press. 2004. p. 177
3. “Understanding Chemistry: Complex
Ions ” www.chemguide.co.uk/inorganic/complexmenu.html
4. Inorganic Chemistry 4th
Edition, Shriver & Atkins, Oxford University Press
5. Banwell and McCash, Fundamentals of Molecular
Spectroscopy, 4th edition
6. G. D. Christian, J. E. O’Reilly. Instrumental Analysis. Allyn and Bacon.
1986.
Answers
to exercises
1. t-butyl alcohol
forms a more stable complex with iodine. A tertiary alcohol, with 3
electron-donating methyl groups which can donate electron density inductively
to the oxygen atom, should form a more stable complex than n-butyl which has
only 1 electron-donating propyl group. The lone pair in t-butyl alcohol has a
higher electron density and is more available for donation to iodine than that
in n-butyl alcohol.
On the other
hand, t-butyl alcohol being branched has higher steric hindrance than the more
linear n-butyl alcohol. Oxygen atom in t-butyl alcohol may be hindered from
interacting with iodine.
The higher K
values for the t-butyl alcohol-iodine complex indicate that t-butyl alcohol
forms a more stable complex with iodine than n-butyl alcohol. It suggests that
the significance of electronic effects outweigh that of the steric interference
for these molecular complexes.
2.
The isosbestic point is defined as the wavelength
at which two species have the same molar absorptivity during a reaction.
From Beer-Lambert Law, A= εcl (where ε is molar absorptivity, c is the
concentration of sample and l is the path length), the total absorbance of a
system, AT is given to be AT= ε1c1l
+ ε2c2l. It is assumed that
alcohol does not absorb in the wavelengths (400 nm – 580 nm) in this
experiment. At isosbestic point, ε1=
ε2, Thus it is extremely unlikely that a third species will have the
same molar absorptivity. The equation can be rewritten as AT= εl (c1+c2) where ε=ε1
+ ε2.
Hence, the isosbestic point
indicates that the total absorbance of a reaction solution depends on the total
concentration of the solution, but not on the concentrations of the individual
species in the solution. The presence of isosbestic point implies that only 2
dominant species, reactant and product, are in the solution.
3. The absorption
maximum occurs at the wavelength at which the species absorbs the most
radiation and shifts upon a change in the relative
concentrations of donor, acceptor used and complex produced. Absorbance
reflected in the spectra is the sum of the absorbance of the individual species.
The complex
absorbs more at lower wavelengths while iodine absorbs at higher wavelengths.
As the concentration of donor increases, more iodine-alcohol complex forms and
thus the wavelength of maximum absorption will decrease.
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