DSE
Extended Expt
Activation Energy of the Peroxide Iodine Clock Reaction
Purpose
To determine the activation energy of the reaction between acidified hydrogen peroxide
solution and iodide solution.
Introduction
Iodine clock reaction takes various forms. One version uses oxidation of iodide solution
by acidified hydrogen peroxide and monitored by thiosulfate.
2I-(aq) + 2H+(aq) + H2O2(aq) → I2(aq) + 2H2O(l) …….. Main reaction
I2 (aq) + 2(S2O32-(aq) → 2I-(aq) + S4O62-(aq) …….. Monitor reaction
I2(aq) + starch → blue complex
…….. Indicator reaction
Iodine formed from the main reaction is immediately taken up by the thiosulfate and will
not affect the starch indicator. After a period of time, when the small amount of thiosulfate
has all been consumed, subsequent appearance of iodine will affect the indicator and will
suddenly render the solution mixture a deep blue colour. Time recorded indicates the time
required for the formation of a fixed amount of iodine by the main reaction.
Experimental set usually employs the method of mixing two solution mixtures. One
consists of a solution mixture of I-(aq) and starch and the other a solution mixture of H2O2,
acid and thiosulfate.
Four reactions at temperatures 30oC, 40oC, 50oC and 60oC are to be carried out. Time for
the sudden appearance of deep blue will be recorded. According to Arrhenius equation,
H act
H act
1
1
k = Ae- RT or ln(k) = ln(A) or a plot of ln( ) vs ( ) is a straight line. A value
t
T
RT
of Hact, i.e. activation energy, can be deduced from the slope of the straight line.
Safety
Wear safety spectacles and avoid skin contact with the chemicals. Use the
minimum quantity of materials required. Dispose of chemical waste and
excess materials according to your teacher’s instruction.
EYE PROTECTION
MUST BE WORN
Materials and Apparatus
Chemicals and consumables
Qty
FA 1*: (10 cm3 0.6M H2SO4 + 2.5
cm3 6.5% H2O2) make up to
90 cm3
90 cm3
Equipment
Mini heater set, ME 29
Qty
1
Extended Expt
FA 2*
Determination of activation energy
(10 cm3 0.6M KI + 10 cm3
0.08M S2O32- + 20 cm3
0.2% starch) make up to 90
cm3
90 cm3
Page 2
12V DC power supply
---
Boiling tube
2
600 cm3 beaker
1
25 cm3 measuring cylinder
1
0-110oC thermometer
2
Stopwatch
1
*
HARMFUL
HARMFUL/
IRRITANT
Experimental procedure
1.
Transfer 10 cm3 of FA 1 into a boiling tube, labeled A.
2.
Transfer 10 cm3 of FA 2 into another boiling tube, labeled B.
3.
Place the two boiling tubes into an empty 250 cm3 beaker which acts as a heat
insulator. Lower one min-heater probe into boiling tube A, ensuring it is
completely immersed. Similarly, lower another mini-heater probe into boiling tube
B, again allow it completely immersed. Place a thermometer separately into each
of the boiling tubes.
4.
Switch on the toggle switch of the mini-heater. Note the rise in temperature of the
two solutions. Stir the solution with the glass thermometer.
5.
When the temperature reaches about 57oC, switch off the toggle switch..Press the
red push-on ON/OFF switch repeatedly until a steady and exact temperature of
60oC has been maintained for a minute.
6.
Pour FA 1 into FA 2 and start timing when half of the content has been transferred.
7.
Record the time taken, accurate to 0.01 sec, for the sudden appearance of a deep
blue coloration.
8.
Measure the solution temperature immediately after the reaction.
9.
Repeat the experiment at 50oC, 40oC and 30oC respectively.
Extended Expt
Determination of activation energy
Fig. 1
Page 3
Mini heater console with identical
heater probes A and B
Fig. 2
Boiling tube / beaker
arrangement
A very important note: Never try to heat up the heater probe without immersing it in
water or aqueous solutions, a usual caution for any electrical
domestic direct-heating kettle.
Toggle
1P2T
switch
Push-on switch
2.0A fuse
Off
Heater A
Heater B
LED
On
12 V DC/ 3A
5.1 K
1/8 W
Fig. 3
15 
5W
15 
5W
Heater circuit
Data presentation
The rate of reaction for each experiment can be represented by the following formula.
‘rate’ =
1
reaction t ime in seconds
Complete the following table for each of your experiments. The mean temperature is the
average of the initial and final temperature for the experiment.
Expt
Initial Temp /
K
Final Temp /
K
Mean Temp /
K
Reaction
time / s
1
333.0
331.0
332.0
16
0.0625
2
323.0
322.0
322.5
29
0.0345
3
313.0
312.3
312.7
51
0.0196
4
303.0
302.7
302.9
94
0.0106
‘Rate’ =
1
t
Extended Expt
Determination of activation energy
Page 4
Data treatment and calculation
0.003012
1
t
0.0625
1
t
-2.7726
322.5
0.003101
0.0345
-3.3673
3
312.7
0.003199
0.0196
-3.9332
4
302.9
0.003302
0.0106
-4.5433
Expt
Mean Temp /
K
1/T
1
332.0
2
Plot a graph of ln
1
vs 1/T and determine the activation energy, H act (orEa), from the
t
gradient
According to Arrhenius equation
we have:
ln
ln(k) = ln(A) -
H act
RT
Slope of the best straight line of
(
k = Ae-
H act
RT
ln
1
vs 1/T
t
= -6067 K
H act
)   6067
R
H act = (R)(6067) J mol-1
= (8.314 J K-1 mol-1)(6067 K)
= 50.4 kJ mol-1
(Literature value: H act = 51.8 kJ mol-1)
Extended Expt
Determination of activation energy
Page 5
Discussion
(i)
Judging from the H act value obtained, was the reaction fast or slow? Was the
sudden appearance of blue coloration related only to the time of reaction? What was
actually measured?
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(ii) Briefly explain whether or not H act is dependent on temperature.
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(iii) What is the difference between reaction rate and rate constant, k?
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(iv)
It has been suggested that a “rule-of-thumb” in quantitative description of reaction
rate is that a reaction will double its rate if the temperature is increased by 10oC. Try
to find out and explain if this is true or not by referring to the graph plotted.
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(v) Outline three conditions which determine the rate of this reaction. (Note: state of
reactant is of course not included)
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(vi) State two procedural errors and two instrumental errors and ways to minimize these
sources of error..
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Conclusion
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