Aim of experiment: To determine the equilibrium constant of the reaction KI + I2 = KI3 by distribution method.
Theory: If to an aqueous moderately concentrated solution of iodide, iodine is added, it combine with the iodide ion to form tri-iodide ion I3- . Thus in aqueous KI solution containing iodine, we have the chemical equilibrium
KI + I2 = KI 3 (i.e., actually speaking I2 + I- = I3- )
In a dilute solution the equilibrium constant of the reaction is :
K = [KI3] / ( [KI] [I2] ) or K = [I3-] / ( [ I2 ] [I-] )
Since molecular iodine is soluble in both aqueous and organic phases, it obeys the distribution law. KI and KI3 being electrolytes, are insoluble in CCl4 .
If iodine is added to a system of CCl4 and aqueous solution of KI of known concentration, it gets distributed between the two phases. In aqueous phases, I2 combine with KI to give KI3 and exists largely as the I3- ion . The distribution law still applies to molecular iodine (uncombined) in the two phases. Thus knowledge of partition coefficient of I2 between CCl4 and pure water will enable us to determine the equilibrium concentration of free iodine in aqueous KI solution in equilibrium with CCl4 , provided the conc. of I2 in CCl4 is determined. If this is subtracted from the total conc. of I2 (determined by titration with Na2S2O3) in aqueous KI layers, the conc. of iodine that combines and hence the conc. of KI3 formed will be obtained. The difference of the initial conc. of KI and the conc. of KI3 gives the equilibrium conc. of KI. Knowing the value of [I2], [KI] and [KI3] in aqueous KI layer, the equilibrium constant can be determined.
Here partition co-efficient, D = (conc. of I2 in CCl4 layer) / (conc. of I2 in aqueous layer)
i.e. D = corg / caq
Conc. of free iodine in aqueous KI solution, [I2] = C2 / D
and [KI3] = C1- (C2 / D)
Where C1 and C2 are the total conc. of I2 in aqueous KI layer and conc. of I2 in CCl4 layer (in equilibrium with aqueous KI layer). So conc. of KI at equilibrium is:
[KI] = C - [C1- (C2 / D)]
Where C = Initial conc. of KI.
Reqirements: 1)Six glass stoppered
bottles
2)Pipette, burette
3)Carbon tetrachloride
4)500 cc. of 0.005 M and 250 cc. of 0.001 M Na2S2O3
solution
5)0.1 M KI, iodine and starch solutions.
Procedure:
1) A small amount of iodine is added to about 200cc. of pure CCl4 in a small beaker and the contents are stirred well to get nearly a saturated solution.
2) Six glass stoppered bottles are thoroughly cleaned and dried and numbered as 1 to 6 .Now about 50, 30 and 20 cc. of the saturated solution and 0, 20 and 30 cc. of pure CCl4 is added to bottles no. 1, 2 and 3 respectively. Then 200 cc. of distilled water is added to each bottle and are stoppered tightly.
3) To bottle nos. 4, 5 and 6, about 50, 30 and 20 cc. of the saturated of I2 and 0, 20 and 30 cc. of CCl4 is added . Then to each bottle 200cc. of 0.1 M KI solution is added and stoppered tightly.
4) All the bottles from 1 to 6 are shacked vigorously for half an hour.
5) 50 cc. of aqueous layer from 1st bottle is removed using 50 cc. pipette and is delivered in to a conical flask. Care was taken not to allow any contamination of CCl4 to get in the pipette . To this, 5 cc. of 10% K I solution is added and is titrated with 0.001 M Na2S2O3 solution using starch solution as indicator.
6) The tip of the pipette is then inserted in the lower CCl4 layer and blow gently in order to exclude any drops of aqueous layer. 5 cc. of CCl4 layer is then pipetted in to a conical flask containing about 20 cc. of 10% KI solution and titrated against 0,005 M Na2S2O3 solution. The contents should be shacked vigorously during titration.
Similarly the aqueous and CCl4 layer in 2nd and 3rd bottles are also titrated.
7) 20 cc. samples of aqueous layer and 5 cc. of CCl4 layer are removed separately by using a pipette from bottle Nos. 4,5 and 6 and are titrated against 0.005 M Na2S2O3 solution.
Observation and calculation:
Temperature of the experiment:.............
Table for distribution co-efficient of I2 :
|
Bottle no. |
Vol. of 0.001 M hypo with 50 cc. of aqueous layer |
vol. of 0.005 hypo with 5 cc. CCl4 layer |
Caq (VOL. 0.001)/50 |
Cor (VOL. 0.005) |
D = Corg / Caq |
|
1 |
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|
2 |
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|
3 |
Hence the value of D = .......
Table for determination of I2 between CCl4 and aqueous K :
|
Bottle no.
|
Vol. of 0.005 M hypo with 20 cc. of aqueous layer |
vol. of 0.005 hypo with 5 cc. CCl4 layer |
Conc. of I2 in aq. layer C1 |
Conc. of I2 in aq. layer C2 |
Conc. of free I2 in aq. layer C2 / D |
|
4 |
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|
5 |
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|
6 |
Determination of equilibrium constant:
For K1,
[I2] = C2 / D =........
[KI3-] = C1- (C2 / D) =........
[KI] = C - [C1 - (C2 / D)] = .......
So equilibrium constant ,K1 = [KI3] / ( [I2][K I] )
=..............................
Similarly find out K2 and K3 .
So average value of equilibrium constant is = ( K1+ K2+ K3 ) / 3 =...........