CATALYTIC SALT EFFECT
Investigation of the catalytic effect of sodium
chloride on
the rate constant of the persulphate iodide reaction
THEORY
A reaction between the ions A
and B of charges ZA and ZB represented as:.
Using simplified limiting form of
the Debye-Huckel equation, Bronsted and Bjerrum derived the reaction.
Where k is the reaction rate constant,
µ is
the ionic strength of reaction medium.
The persulphate iodide reaction
|
an accelerating effect would be expected since ions
of similar charges are involved.
The reaction rate may be followed in media of
different ionic strength using NaCl.
PROCEDUR
1-
Prepare and thermostat the
following mixtures at 25 °C.
Distilled water
|
50
|
40
|
30
|
20
|
10
|
4 N NaCl
|
--
|
10
|
20
|
30
|
40
|
0.04 N KI
|
25
|
25
|
25
|
25
|
25
|
0.04 N K2S2O8
|
25
|
25
|
25
|
25
|
25
|
Total volume
|
100
|
100
|
100
|
100
|
100
|
Each mixture is then 0.01 N
with respect to each of S2O8-2 and I-.
2-
After 30 min. 25 ml mixture
1, are transferred into a flask.
3- Liberated iodine is
titrated with thiosulphate using starch as indicator.
4-
Repeat steps (2), (3) for other
mixtures.
CALCULATIONS
1. Substituting
in the simplified second order equation to calculate the rate constant for each
mixture.
Where a is the primary concentration, and its compensated by
the value of concentration (0.01 N) or volume (25 ml)
x is the volume of thiosulphate which equivalent to the iodine produced by the reaction
the value of concentration (0.01 N) or volume (25 ml)
x is the volume of thiosulphate which equivalent to the iodine produced by the reaction
2.
Calculate
the ionic strength of each mixture from
the relation:
where
c is the concentration in molar units (mol/L),
z is the charge of
each ions.
3.
Tabulate
the results for each mixture as follows:
Mix.
|
x
|
k
|
µ
|
 |
log k
|
1
2
3
4
5
|
4. Plot
log
k againstand comment on the results.
and comment on the results.
