Labmouse Chemistry AS | Chemical Kinetics AQA 3.2.2 | Practical: Iodine clock to show effect of increasing concentration on rate of reaction
Labmouse Chemistry AS
Atomic Structure AQA 3.1.1
Amount of Substance AQA 3.1.2
Introduction to Organic Chemistry AQA 3.1.5 & 3.1.6
Energetics AQA 3.2.1
Chemical Kinetics AQA 3.2.2
Rate of reaction
Energy of particles
Factors affecting the rate of reaction
Practical: Effect of temperature on rate of reaction between acid and thiosulfate ions
Evaluation/limitations and improvements
Practical: Iodine clock to show effect of increasing concentration on rate of reaction
Dynamic Equilibria AQA 3.2.3
Redox AQA 3.2.4
Haloalkanes AQA 3.2.8
Alkenes AQA 3.2.9
Analytical Techniques AQA 3.2.11
Practical Skills AQA 3.2.3
LabMouse Chemistry A2
LabMouse Physics AS
LabMouse Physics A2
Answers Crib Sheet
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Oxidation of aqueous iodide ions by peroxodisulfate(VI) ions in the presence of sodium thiosulfate and starch to show the effect
of increase of concentration on the rate of reaction. The reaction takes place in two stages. Firstly the peroxodisulfate ions
(S2O82-) oxidise the iodide ions to iodine.
S2O82-(aq) + 2I-(aq) → 2SO42-(aq) + I2(aq)
The iodine produced is immediately reduced by the small quantity of thiosulfate ions present.
2S2O32-(aq) + I2(aq) → S4O62-(aq) + 2I-(aq)
This continues until all the thiosulfate ions present are used up. The next iodine produced reacts with the starch present to
give the familiar blue-black colour of the iodine clock.
Pour x cm3 of 0.1M potassium iodide solution into a clean dry 100 cm3 measuring cylinder. Add
30 cm3 of approximately molar potassium nitrate solution, about 3 cm3 starch solution, and then de-ionised
water to give a total of 100 cm3. Mix the contents by transferring them to a clean dry 500 cm3 conical
flask. In the second cylinder pour 30 cm3 of 0.1M ammonium peroxodisulfate and add de-ionised water to give total
volume of 100 cm3. Rinse and fill a burette with 0.1M sodium thiosulfate solution. Run in exactly 1 cm3 of
this thiosulfate solution into the conical flask, mix, add the contents of the second cylinder quickly but carefully, and halfway
through the addition, start the stop clock. When all the contents of the cylinder have been added, mix the contents of the flask
thoroughly. Note the time when the blue colour appears, but do not stop the clock. As soon as possible after the appearance of
the blue colour, add a second 1 cm3 portion of thiosulfate solution. Again note the time of the appearance of the blue
colour and run in another 1 cm3 portion of thiosulfate. Keep following this procedure until you have ten readings.
N.B. Suitable values of x are 30, 20 and 10 cm3.
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Click on the 20 or 10 cm3 columns to perform the practical for that volume of KI(aq).
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