Chemical Kinetics
Determining the activation energy of a reaction
Requirements per student (or pair)
Notes
Beakers, 400cm3
To be used as a water bath
4 beakers, 100cm3
May be shared by several students
4 burettes, 50cm3, with stand
May be shared by several students
4 funnels, small
May be shared by several students
2 boiling tubes
2 thermometers, 0 – 100oC
Bunsen burner, tripod, gauze mat and heat
proof mat
Retort stand, clamp and boss
Safety glasses
Stopwatch
Potassium iodide solution, 0.50M KI
~25cm3. Dissolve 83g of KI in water and
make up to 1000cm3
Potassium peroxodisulphate (VI)
~50cm3. Dissolve 5.4g of K2S2O8 (or 4.8g of
(persulphate) solution, 0.020M K2S2O8
Na2S2O8) in water and make up to 1000cm 3
Sodium thiosulphate solution, 0.010M
~25cm3. Dissolve 2.48g of Na2S2O35H2O in
Na2S2O3
water and make up to 1000cm 3
Starch solution, 0.2%
~15cm3. Mix 0.2g of ‘soluble’ starch into a
paste with cold water and pour, with constant
stirring, into 100cm 3 of boiling water
Determining the activation energy of a reaction
Name:
Date:
Aim
The purpose of this experiment is to determine the activation energy, Ea, for
the reduction of peroxodisulphate (VI) ions, S2O82- (aq), by iodide ions, I- (aq),
using a ‘clock’ reaction.
Introduction
The equation for the reduction of peroxodisulphate (VI) ions by iodide ions is:
S2O82- (aq) + 2I- (aq)  2SO42- (aq) + I2 (aq)
A small, known amount of thiosulphate ions is added to the reaction mixture, which also
contains some starch indicator. The thiosulphate reacts with the iodine formed in the above
reaction as in the following equation:
2S2O32- (aq) + I2 (aq)  S4O62- (aq) + 2I- (aq)
At the instant that all the thiosulphate has reacted, free iodine is produced in the solution and
its presence is shown by the appearance of the blue-black colour of the iodine-starch
complex, i.e. the thiosulphate ions act as a ‘monitor’ indicating the point at which a certain
amount of iodine has been formed. For this reason, the reaction is often referred to as an
iodine ‘clock’ reaction. In general, for a ‘clock’ reaction:
Rate of reaction  1/t
where t is the time taken to reach a specified stage.
You carry out the experiment at five different temperatures between 20oC and 50oC. You then
find the activation energy for the reaction by plotting a suitable graph.
Assessment
You will not be assessed on this practical.
Requirements
 Safety glasses
 Beaker, 400cm3
 2 thermometers, 0 – 100oC
 Bunsen burner, tripod, gauze mat, heat proof mat
 4 burettes with stands, filling funnels and beakers
 2 boiling tubes
 Clamp and stand
 Potassium peroxodisulphate (VI) solution, 0.020 mol dm -3 K2S2O8
 Potassium iodide solution, 0.50 mol dm -3 KI
 Sodium thiosulphate solution, 0.010 mol dm -3 Na2S2O3
 Starch solution, 0.2%
 Watch or stopclock
Procedure
1. Half-fill the beaker with water and heat it to between 49oC and 51oC. This will be used
as a water bath. Alternatively, you may use a full-sized water bath.
2. Using a burette, measure out 10cm 3 of potassium peroxodisulphate (VI) solution into
the first boiling tube. Clamp this in the water bath and place a thermometer in the
solution in the boiling tube.
3. Using burettes, measure out 5cm 3 each of the potassium iodide and sodium
thiosulphate solutions and 2.5cm 3 of starch solution into the second boiling tube.
Place another thermometer in this solution and stand it in the water bath.
4. When the temperatures of the two solutions are equal and constant (to within +/-10C),
pour the contents of the second boiling tube into the first, shake to mix, and start the
stopwatch.
5. When the blue colour of the starch-iodine complex appears, stop the stopwatch and
write down the time in the Results Table below.
6. Repeat the experiment at temperatures close to 45oC, 40oC, 35oC, 30oC (The
temperatures you use may differ from those by a few degrees but must, of course, be
recorded carefully).
Results Table
Temperature / oC
Temperature, T/K
Time t/s
Calculations
Plot a suitable graph to enable you to calculate a value for the activation energy of this
reaction.
Calculation Help Sheet
1.
2.
3.
4.
5.
6.
7.
Work out ln1/t for all of your times.
Work out 1/T for all of your temperatures.
Plot a graph of ln(1/t) (y-axis) against 1/T (x-axis)
Draw a straight line of best fit
Calculate the gradient of this line
The gradient = -Ea/R
Calculate Ea including the correct units – no tips here!
Teacher’s Sheet for Determining the activation energy of a reaction
This experiment is straightforward and should give good results if the procedure is followed
carefully. Make sure that your students understand that they need not waste time adjusting
the temperature precisely to the values we have stated, but that they must record the actual
temperatures carefully. Note, however, that at temperatures higher than 60oC, the starchiodine complex is broken down (reversibly) and the blue colour does not appear!
Plotting ln t instead of ln(1/t) is simpler and only changes the sign of the slope.
Mark scheme:
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Converts temperatures into absolute temperatures correctly
Converts absolute temperatures into reciprocal values correctly
Converts time to reciprocal values correctly
Converts reciprocal times to loge values correctly
Plots ln(1/t) as y-axis and 1/T as x-axis
Plots points accurately
Labels axes correctly
Obtains best straight line
Recognises that gradient of graph is equal to –Ea/R
Calculate activation energy giving correct units
Notes on calculation:
Ea
=
- slope x R
=
somewhere around 53.0 kJ mol-1