Experiment I04
An investigation of the catalytic action of d-block ions on the reaction
between iodide and persulphate ions
Chemicals:
0.1 M Na2S2O3, (25 cm3)
0.1 M KI, (110 cm3)
Starch solution, (55 cm3)
0.1 M Na2S2O8, (220 cm3)
Solution (1M, 3 drops) of Cr3+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Cr6+, Mn7+, Fe3+,
Cu+.
Apparatus:
250 cm3 conical flask, (1)
250 cm3 volumetric flask, (1)
25 cm3 pipette, (1)
Boiling tube, (1)
Stopwatch, (1)
Burette. (4)
Principle:
Iodide ions are oxidized by persulphate ions to iodine and various metal ions will
catalyse this reaction.
S2O82-(aq) + 2I-(aq)  2SO42-(aq) + I2(aq)
A convenient way in which to measure the rate of this reaction is to add a fixed
volume of sodium thiosulphate solution to the reaction mixture. This reacts with
the iodine formed in the reaction as follows:
I2(aq) + 2S2O32-(aq)  2I-(aq) + S4O62-(aq) (very fast)
When the thiosulphate has been used up, free iodine is produced, and if some
starch solution has also been added, a deep blue colour will be produced. The time
that it takes for the blue colour to appear is inversely proportional to the average
rate of reaction during this time, and so, whether or not an ion catalyses the
reaction can be found by comparing the times for the reaction with and without
the metal ion.
P.1
Experiment I04
An investigation of the catalytic action of d-block ions on the reaction
between iodide and persulphate ions
Procedure
a.
Using volumetric flask, dilute 25.0 cm3 0.1 M Na2S2O3 to 0.01 M for the following
experiments.
b.
Place 10 cm3 of 0.1 M iodide solution, 10 cm3 of the 0.01 M thiosulphate solution, and 5
cm3 of starch solution in a 250 cm3 conical flask, and 20 cm3 of 0.1 M persulphate solution
in a boiling tube. Add the persulphate solution to the conical flask and start the stopwatch.
Note the time taken for the blue colour to develop.
c.
Repeat the experiment adding three drops of a solution of one of the ions listed below:
Cr3+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Cr6+, Mn7+, Fe3+
d.
and Cu+.
Compare the times from the various experiments with and without metal ion and so find out
if the addition of the metal ion affects the rate of the reaction.
Refernce
Uncatalysed reaction:
2I- + S2O82-  2SO42- + I2
* Catalysed reaction 1: 2I- + 2Fe3+  2Fe2+ + I2
Overall reaction:
(fast)
2Fe2+ + S2O82-  2Fe3+ + 2SO42-
(fast)
2I- + S2O82-  I2 + 2SO42-
(fast)
* Catalysed reaction 2: 2Fe2+ + S2O82-  2Fe3+ + 2SO42Overall reaction:
(slow)
(fast)
2Fe3+ + 2I-  2Fe2+ + I2
(fast)
2I- + S2O82-  I2 + 2SO42-
(fast)
P.2
Experiment I04
An investigation of the catalytic action of d-block ions on the reaction
between iodide and persulphate ions
Name:
Seat No.:
Date:
Grade:
Result:
Experiment
Time (second)
Without metal ion
Cr3+
Mn2+
Fe2+
Co2+
Ni2+
Cu2+
Cr6+
Mn7+
Fe3+
Cu+
Questions
1. According to the experimental results, which ions can catalyse the reaction between iodide
and persulphate ions ?
2. In this experiment, are the ions of transition metals homogeneous or heterogeneous catalysts ?
Explain briefly.
P.3