9/11/2016
For the chemical equilibrium,
I 2 ( aq )  I  (aq )  I 3 ( aq )
The thermodynamic Equilibrium constant is defined as;
Chemical Equilibrium in
Solution
I2(aq) + I- (aq) = I3-(aq)
Ka 
and
Ka 
aI 
ai   i ci
3
aI 2 a I
aI 
3
aI 2 aI 

I
[ I 3 ]

3

[ I 3 ]
 I  I [ I 2 ][ I  ] [ I 2 ][ I  ]
 Kc

2
Ka  Kc
implies  I    I  ;
3
The activity coefficient in general is approximated by
DH Limiting Law; log  i 
Ka 
509 z I
1 I
2
i
 I  1 from the above approximation.
2
Ka 
[ I 3 ]
 Kc
[ I 2 ][ I  ]
I 2 ( aq )  I  (aq )  I 3 ( aq )

3
[I ]
 Kc
[ I 2 ][ I  ]
To determine the [I2] one can use the quantitative reaction with the
thiosulfate ion.
Meaning of [X]:
I 2 ( aq )  2S 2O32 (aq)  2 I  ( aq )  S4O6 2 ( aq )
c
[ X ]  X where c X , co are concentration of X in mol/L
c0
and concentration of the standard state: 1 mol/L
In effect [X] is the concentration value of X, in mol/l
and no unit is involved.
To calculate the concentrations of the three species in aqueous
solution direct reaction method is not feasible as it disturbs the
reaction equilibrium. Only the total concentration [I2(aq) + I3-(aq)] is
easily determinable, not individual values.
The strategy is to establish the reaction equilibrium in the aqueous
solution which is in contact with an organic layer.
I 2 (org )  I 2 (aq)
This establishes a second (partition) equilibrium system in
addition to the reaction equilibrium.
k
I 2 (aq )  I  (aq)  I 3 (aq)
aq
CH2Cl2
I 2 (org )  I 2 (aq)
k
I 2 (org )
I 2 (aq)
I 2 (org )
I 2 (aq)
I 2 (org )
At a given temperature partition coefficient k is a constant for a
given equilibrium between the two phases.
If k is known or established, one can find the I2 concentration in
organic layer, and use k to find the I2(aq) concentration.
1
9/11/2016
CH2Cl2 layer
titrate 10.00mL
Determining k
-------------- Mix ---------------
Run CH2Cl2 (25 mL)
Aqueous Layer
(100 mL)
No.
Molarity I2
Molarity Kl
1
2
3
4
5
0.080
0.040
0.020
6
0.080
0.0 (DI)
0.0 (DI)
0.0 (DI)
0.15
0.15
0.03
0.080
0.040
CH2Cl2 layer
titrate 10.00mL
Aqueous layer
titrate 25.00 mL
Burette
Size, mL
Molarity
-2
S2O3
Burette
Size, mL
50
50
10
0.1
0.1
0.1
0.1
0.1
0.1
10
10
10
50
10
10
50
10
10
Molarity
-2
S2O3
Run CH2Cl2 (25 mL)
Molarity I2
Molarity Kl
mL
1
2
3
0.080
0.040
0.020
0.0
0.0
0.0
10.00
10.00
10.00
Run
No.
CH2Cl2
(25 mL)
Molarity I2
4
5
6
0.080
0.040
0.080
Vol (mL)
‐2
No. 0.1M S2O3
4
5
6
Aqueous layer
titrate 25.00 mL
Vol (mL)
[I2 aq]
‐2
0.01M S2O3
k
mean k =
Aqueous layer
titrate 25.00 mL
Molarity Kl
mL
M
mL
0.15
0.15
0.03
10.00
10.00
10.00
0.10
0.10
0.10
25.00
25.00
25.00
[I2 org]
M
0.10
0.10
0.10
Aqueous layer
(use 25‐mL pipette)
Vol (mL)
[I2 aq]Total [I3‐1 aq] [I‐1 aq] ‐2
0.01M S2O3
Kc



 I (aq)    I (aq )  0   I 3 (aq ) 
Titration Reaction
CH2Cl2 layer
titrate 10.00mL
Determining Kc
I 2  2 S 2O32  2 I   S4O6 2
or
I 3  2 S2O32  3I   S 4O6 2
@ 'endpoint'
0.01
0.01
0.01
Aqueous Layer
-2
-2
(100 mL)
Vol CH2Cl2 [ S2O3 ] Aq. volume [ S2O3 ]
CH2Cl2 layer
(use ‐10mL pipette)
Run
[I2 org]
25.00
25.00
25.00
1
2
3
Water 100.00mL
CH2Cl2 layer
titrate 10.00mL
0.10
0.10
0.10
CH2Cl2 layer
Run Vol (mL)
‐2
No. 0.1M S2O3
-2
Aq. volume [ S2O3 ]
mL
titrate 10.00mL
I2 (M) CH2Cl2 25.00mL
Determining Kc
Aqueous Layer
-2
(100 mL)
Vol CH2Cl2 [ S2O3 ]
No.
0.01
0.01
0.01
0.1
0.1
0.1
Aqueous layer
titrate 25.00 mL
Titration of the aqueous layer
of the reaction mixture:
Run
 [ I 2 ( aq )]  [ I 3 (aq)] of the
equilibrium system
mol I 2
1

mol S 2O32 2
1
 S 2O32  VS O 2
2 3
2
V
1
S O 2
[ I 2 ]   S2O32  2 3
V
2
4
5
6
0.080
0.040
0.080
Aqueous Layer
-2
-2
(100 mL)
Vol CH2Cl2 [ S2O3 ] Aq. volume [ S2O3 ]
CH2Cl2 layer
(use ‐10mL pipette)
[ I 2 ]VI 2 
I2
No.
CH2Cl2
(25 mL)
Molarity I2
Vol (mL)
‐2
No. 0.1M S2O3
4
5
6
Run
Aqueous layer
titrate 25.00 mL
[I2 org]
Molarity Kl
mL
M
mL
0.15
0.15
0.03
10.00
10.00
10.00
0.10
0.10
0.10
25.00
25.00
25.00
Aqueous layer
(use 25‐mL pipette)
Kc 
Vol (mL)
[I2 aq] total [I aq]
2
‐2
0.01M S2O3
M
0.10
0.10
0.10
[ I 3 ]aq
[ I 2 ]aq [ I  ] aq
[I3‐1 aq]
[I‐1 aq]  I  ( aq)   [ I  (aq )]0   I 3 (aq ) 
2