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1 | P a g e
EXPERIMENT NO 1
Equilibrium constant of k
KI + I2 ↔ KI3
Aim of experiment:
To determine the equilibrium constant of the reaction KI + I
2
= KI
3
by distribution method.
Theory:
If to an aqueous moderately concentrated solution of iodide, iodine is added, it combines
with the iodide ion to form tri-iodide ion I
3
-
. Thus, in aqueous KI solution containing iodine, we
have the chemical equilibrium
K + I
= KI
3
(i.e., actually speaking I
2
+ I
-
= I
3
-
)
In a dilute solution the equilibrium constant of the reaction is:
K =[KI
3
] / ([KI] [I
2
]) or K =[I
3
-
] / ([ I
2
]
[I
-
])
Since molecular iodine is soluble in both aqueous and organic phases, it obeys the distribution
law. KI and KI
3
being electrolytes, are insoluble in CCl
4
.
If iodine is added to a system of CCl
4
and aqueous solution of KI of known concentration, it
gets distributed between the two phases. In aqueous phases, I
2
combine with KI to give
KI
3
and exists largely as the I
3
-
ion. The distribution law still applies to molecular iodine
(uncombined) in the two phases. Thus knowledge of partition coefficient of I
2
between CCl
4
and
pure water will enable us to determine the equilibrium concentration of free iodine in aqueous KI
solution in equilibrium with CCl
4
, provided the conc. of I
2
in CCl
4
is determined. If this is
subtracted from the total conc. of I
2
(determined by titration with Na
2
S
2
O
3
) in aqueous KI layers,
the conc. of iodine that combines and hence the conc. of KI
3
formed will be obtained. The
difference of the initial conc. of KI and the conc. of KI
3
gives the equilibrium conc. of KI.
Knowing the value of [I
2
], [KI] and [KI
3
] in aqueous KI layer, the equilibrium constant can be
determined.
Here partition co-efficient,
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D = (conc. of I
2
in CCl
4
layer) / (conc. of I
2
in aqueous layer)
i.e., D = c
org
/ c
aq
Conc. of free iodine in aqueous KI solution, [I
2
] = C
2
/ D
and [KI
3
] = C
1
- (C
2
/ D)
Where C
1
and C
2
are the total conc. of I
2
in aqueous KI layer and conc. of I
2
in CCl
4
layer (in
equilibrium with aqueous KI layer). So conc. of KI at equilibrium is:
[KI] = C - [C
1
- (C
2
/ D)]
Where C = Initial conc. of KI.
Apparatus:
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 Na
2
S
2
O
3
solution
5). 0.1 M KI, iodine and starch solutions.
Procedure:
1) A small amount of iodine is added to about 200cc. of pure CCl
4
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 CCl
4
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 I
2
and 0, 20 and 30 cc. of
CCl
4
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.
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