Chemical Kinetics
Determining the activation energy of a catalysed 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 100cm3 of boiling water
Iron (III) chloride solution, FeCl3
~5cm3. Dissolve 1.4g of hydrated solid in
water with 5cm3 of concentrated HCl and
make up to 100cm3
Note: As this is a planning exercise, students may request the above plus other items.
! HAZARD WARNING !
Concentrated hydrochloric acid is corrosive and gives off a harmful vapour.
Iron (III) chloride is an irritant. Therefore you must:
 Wear safety glasses and gloves
 Keep stoppers on bottles as much as possible
Determining the activation energy of a catalysed reaction
Aim
The purpose of this experiment is to determine the activation energy for the oxidation of
iodide ions by peroxodisulphate (VI) ions in the presence of iron (III) ions. You then compare
the value obtained with that for the uncatalysed reaction.
Assessment
You will be assessed on Data Collection and Processing, Conclusion and Evaluation
As this is a major part of your Internal Assessment you must ensure that you carefully review
the assessment criteria to enable you to get the maximum marks available. This will count for
3 hours of your Internal Assessment requirement.
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%
 Iron (III) chloride solution, FeCl3 (refer to bottle for concentration) - IRRITANT
 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. Add 3 drops of the iron (III) chloride solution. 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 four additional temperatures between 50oC and 30oC (The
temperatures you use must, of course, be recorded carefully).
Results
Present your results in an appropriate form.
Calculate a value for the activation energy of this reaction.
Questions
1. Compare your result with the activation energy for the uncatalysed reaction.
2. Comment on the two values and discuss your results.