Oxidation State
Madison Bowe
Elana Keller
April 22, 2014
Introduction:
The objective of this lab was to determine the change in oxidation state of Cl when
KClO3 oxidizes I-. In this experiment chlorate was used to oxidize I- to I2 and the chlorate will be
reduced to a lower oxidation state. To analyze I2 thiosulfate was used to titrate. The thiosulfate
reduces I2 back to I-.
2S2O32- + I2 → 2 I- + S4O62I2 in water is brown while I- is colorless. The thiosulfate was titrated into the brown
solution until the solution turned nearly colorless. When the solution was clear this was the point
where I2 is reduced to I-. Starch was added to the solution to absorb the I2 producing a dark blue
color. S2O3 was titrated to this solution until the color disappeared signaling the endpoint of the
titration.
Dry KClO3 crystals were weighed to be .1 grams. The KClO3 was then added to 15 ml of
distilled water in an Erlenmeyer flask. The crystals were swirled in the flask till they dissolved.
The next step done was weighing out 1.63 grams of potassium iodide. The potassium iodide was
then added to the solution in the flask. Once the potassium iodide was added the flask was
stoppered. It is important to stopper the flask during this experiment to prevent the loss of I2. The
more I2 lost before titration the less thiosulfate will be needed. Consequently, the calculations of
mols of both thiosulfate and I- would be effected. Next, 15 mL of 12M hydrochloric acid was
added to flask. The flask was again stoppered and the solution was swirled for 30 seconds. The
solution was then left to stand for five minutes. After the five minutes the flask was placed in an
ice bath to allow to cool. This was done to prevent the vaporization of I2. To lower the acidity of
the solution 28 ml of 6 M NaOH was added to the solution. The solution was then titrated with
Na2S2O3 till the solution became a transparent orange. One dropper of starch was then added to
the solution. The solution turned dark blue because of the presence of I2 and starch. This solution
was titrated again until the solution was clear signifying that the end point was reached.
Data:
Trial 1
Grams of KClO₃
Gams of KI
Initial Thiosulfate
Final Thiosulfate
Mols KClO₃
Mols Thiosulfate
Mols I⁻
Oxidation State
Trial 2
0.104
1.630
0.00
18.50
8.48x10-4
4.07x10-3
4.07x10-3
4.8
0.100
1.612
0.00
17.69
8.16x10-4
3.89x10-3
3.89x10-3
4.78
Calculations: Separate page
Conclusion:
The Objective of the lab was to determine the oxidation state. Both trials concluded that
the oxidation state was 5. An oxidation state is a hypothetical charge that an atom would have if
all bonds to atoms of different elements were 100% ionic. Possible sources of error could of
occurred by losing I2 to the air. If this occurred the mols of both thiosulfate and I- would be lower
which would of decreased the oxidation state. Another source of error could of been over
titrating which would of increased the mols of both thiosulfate and I- which would of raised the
oxidation state.