Expt 2
Kc of iron(III) thiocynate by colorimetry
Objective
To determine the equilibrium constant of the reversible reaction between iron(III) and
thiocynate using colorimetry
Theory and introduction
Equilibrium constant, Kc of the reversible reaction between iron(III) and thiocynate
can be determined readily by colorimetry.
Fe3+(aq) + SCN-(aq)
Conc. at start:
Ci mol dm-3
Ci mol dm-3
-3
Conc. at eqm: (Ci - Ce ) mol dm (Ci - Ce ) mol dm-3
Fe(SCN)2+(aq)
0 mol dm-3
Ce mol dm-3
[FeSCN 2 ]e
3
Kc = [Fe ]e [SCN ]e mol-1 dm3
Ce
2
= (C i - C e ) mol-1 dm3
Standard solutions of iron(III) and SCN- are prepared, hence Ci is known. Ce is
determined by colorimetry.
If initial concentrations of Fe3+(aq) and SCN-(aq) are not the same, their equilibrium
concentrations will also differ. Equilibrium constant expression will have a different
form. This is illustrated in the experiment.
The “DMM Display” technique
Conventional colorimeter is not used. Instead, a sensor (ME 28) incorporating an
optical inducer and an electronic device is connected to a commercial digital
multimeter (DMM) for optical intensity measurement. Blank sample output is
adjusted to have a reading of 100, carrying no units. Complete blockage of optical
path, irrespective of wavelength employed, is assumed to have a reading of 0.
Matching of optical intensity
Traditional matching of colour intensity by the eye and instrumental matching are
both used in the experiment. Equilibrium sample and the calibration sample (slightly
more red) are placed side by side. Calibration sample solution mixture is diluted
drop-wise until both exhibit the same colour intensity. Concentration of the coloured
species in the calibration sample is calculated which is assumed to be the same as that
of the equilibrium sample. Precise judgment using a colorimeter over that of using
human eye is illustrated in the experiment.
Volume transfer using drops instead of actual measurement
Concentration calculation for diluting solutions is easier to handle if drops of
solution/water are used instead of measured volume, provided volume of each drops
Expt 2
Kc of iron(III) thiocynate
Page 2
are the same.
Concentration of diluted solution
No. of drops of water added
= Original concentration x No. of drops of solution  no. of drops of water added
Note:
* For the concentration calibration part of the experiment, large excess of
iron(III) over thiocynate should be used
* Time is required to attain chemical equilibrium, especially for situations
involving dilution of the iron(III) complex ion.
Wear safety spectacles and avoid skin contact with the chemicals. Use the
minimum quantity of materials required. Dispose of chemical waste and
excess materials according to your teacher’s instruction.
EYE PROTECTION
MUST BE WORN
Materials and Apparatus
Chemicals and consumables
Qty
Equipment
Qty
0.2M Fe(NO3)3
10 cm3
Colorimeter ME 28 with connector
1
0.02M Fe(NO3)3
20 cm3
Digital multimeter
1
0.02 M KSCN
20 cm3
Disposable cuvette
(12.5 x 12.5 x 45 mm)
3
Plastic dropper (large)
4
100 cm3 beaker
1
Wash bottle with distilled water
1
Tweezers
1
Experimental procedures
Expt (A)
Set full-scale (i.e. 100 mV)
1. Connect ME 28 to a DMM and select the DC 200 mV range
2. Switch on ME 28 and select the 470 nm (blue) light source
3. Fill a clean cuvette 3/4 full with distilled water
4.
Lower the cuvette with the help of a pair of tweezers into the sample tube
cavity, ensuring the clear sides of the tube are used for the optical experiment.
Expt 2
Kc of iron(III) thiocynate
Page 3
5. Turn the knob until a display of 100.0 (or very close to) is shown
6. Fix the position of the knob. The setup is ready for experiment.
(i) Equilibrium sample reading
1. Transfer, with the help of a plastic pipette, 5 drops of 0.02M Fe(NO3)3 to a
clean cuvette, followed by 5 drops of 0.02M KSCN. Add 40 drops of
distilled water to make the final volume equal to 50 drops of red equilibrium
mixture solution. Allow 5 mins for chemical equilibrium.
2. Place the prepared solution mixture into the sample cavity, with cuvette’s
clear sides facing the light source. Record the voltage registered. Take it out
from the cavity.
(ii) Calibration sample reading
1. Add 10 drops of 0.02M Fe(NO3)3 to another clean cuvette followed by 1
drop of 0.02M KSCN. Place it sided by side with the earlier cuvette. Add
drops of distilled water until it appears to have the same colour intensity as
the equilibrium sample.
2. Lower the calibration sample cuvette into the sensor cavity. Note the voltage
reading displayed. It should be less than that of the equilibrium sample
reading. This means the calibration sample is darker than the equilibrium
sample. This otherwise cannot be detected by our eye.
3. Continue to add drops of distilled water until the DMM reading is exactly
the same (or close to) as the equilibrium sample. Allow10 mins for chemical
equilibrium. Record the total number of drops of distilled water added.
Expt (B)
Repeat Expt (A)
Part (i) becomes 4 drops of 0.02M Fe3+(aq) added to 6 drops of SCN-(aq) followed by
40 drops of distilled water
Part (ii) remains the same.
Expt (C)
Repeat Expt (A)
Part (i) becomes 6 drops of 0.02M Fe3+(aq) added to 4 drops of SCN-(aq) followed by
40 drops of distilled water
Part (ii) remains the same.
Expt 2
Kc of iron(III) thiocynate
Page 4
Results
Colour of LED: blue
Solution temperature: 29oC
Equilibrium
Calibration
Number of drops
Expt.
Number of drops
.02M
KSCN
0.02M
Fe(NO3)3
Distilled
water
A
5
5
40
B
4
6
C
6
4
DMM
reading
DMM
reading
0.02M
KSCN
0.2M
Fe(NO3)3
Distilled
water
24.5
1
10
52
24.8
40
25.0
1
10
54
24.9
40
34.4
1
10
60
34.5
Calculations (consider data obtained in Expt A)
(1) Calibration sample
Since presence of Fe3+(aq) is in large excess over that of SCN-(aq), It is assumed
that all of the SCN-(aq) ions are converted to the deep red complex ion
Fe(SCN)2+(aq). Hence concentration of the adjusted Fe(SCN)2+(aq), is the same as
1
that of the equilibrium sample, i.e. Ce = 0.02 x (1  10  53) M =3.175 x 10-4 M
(2) Equilibrium sample
Initial concentration of Fe3+(aq), Ci = Initial concentration of SCN-(aq) = 0.02M x
5
(5  5  40) = 0.002M
Fe3+(aq) +
At start:
0.002 mol dm-3
SCN-(aq)
Fe(SCN)2+(aq)
0.002 mol dm-3
0 mol dm-3
At eqm: (0.002 - 3.175 x 10-4) mol dm-3 (0.002 - 3.175 x 10-4) mol dm-3
3.175 x 10-4 M mol dm-3
[FeSCN 2 ]e
3
Kc = [Fe ]e [SCN ]e mol-1 dm3
3.175 x 10 -4
-4 2
= (0.002 - 3.175 x 10 ) mol-1 dm3
= 113 mol-1 dm3
No standard literature value for the Kc of iron(III) thiocynate. Acceptable range:
90-120 mol-1 dm3
Expt 2
Kc of iron(III) thiocynate
Page 5
[FeSCN 2 ]e
[Fe 3 ]e [SCN - ]e
[Fe3+(aq)]eqm
[SCN-(aq)]eqm
[Fe(SCN)2+(aq)]eqm
-3
/mol dm
-3
/mol dm
-3
/mol dm
1
0.00168
0.00168
3.175 x 10-4
113 mol-1dm3
2
0.00129
0.00209
3.077 x 10-4
114 mol-1dm3
3
0.00212
0.00132
2.817 x 10-4
101 mol-1dm3
Expt.
Questions for discussion
(1) Why is it necessary to use a large excess of Fe3+(aq) in making the calibration
sample solution?
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To shift the equilibrium position far to the right so that practically all the SCNions are converted to the Fe(SCN)2+ complex ion.
(2) Why use the blue LED light source? Can we use another source of light, like the
green or red LED?
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Monochromatic blue wavelength is best absorbed by the deep red Fe(SCN)2+
solution. Green monochromatic light is also workable, but less good. Not
recommended to use red monochromatic light.
(3) Briefly explain whether or not you would average the three experimental Kc
values to obtain a concluding result.
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For finer details of concentration consideration, we should not just consider the
calculated molarity, but the effective concentration instead. Effective
concentration is the portion of ions in the environment of random distribution of
positive and negative aqueous ions that are responsible for chemical reaction.
Effective concentration is expressed in activity Activity of the SCN (aq) ion and
that of the Fe3+(aq) ion do not vary in the same way with the amount of water
added, We should consider separately the three experimental Kc values. However,
when students are not taught about the concept of activity, averaging collected
results is a correct way.
Expt 2
Kc of iron(III) thiocynate
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(4) List (i) one major source of experimental error and (ii) one major theoretical error
for the experiment..
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(i)
(ii)
Inaccuracy in volume of solutions delivered
Activity should be used instead of molarity
Conclusion
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