Chemistry Project


This experiment involves an Iodine clock reaction, using Peroxidisulfate(VI) ions and Iodide ions in solution to form Sulfate(VI) ions and iodine: \[S_2O_8^{2-}(\textup{aq}) + 2I^-(\textup{aq})\rightarrow SO_4^{2-}(\textup{aq})+I_2(\textup{aq})\] As both of the reactants are colourless, the progress of the reaction is shown by the blue colour of the Iodine. If starch is added, this becomes clear.

To measure the rate of the reaction, the time for a set amount of Iodine to be produced can be measured. To do this, Thiosulfate(VI) ions were added to the reaction mixture. These turn Iodine back into Iodide ions, so no Iodine will be evident until all the Thiosulfate is used up. \[2S_2O_3^{2-}(\textup{aq})+I_2(\textup{aq})\rightarrow S_4O_6^{2-}(\textup{aq})+2I^-(\textup{aq})\] This will result in a sudden blue colour. The time for this colour to appear will be quantified as the rate of the reaction.


This experiment had several aims. These were:

  • Find the orders of the reactants in reaction 1.

  • Find the value of k, the constant in the rate equation

  • Find how the reaction rate is affected by temperature using a water bath

  • Use the Arrhenius equation to find the activation energy and collision factor of the reaction

Experimental Procedure



A table to show the equipment used, and to justify its usage
Equipment Justification
Thermometer (0-100\(^{\circ}\)C) The reactants must be at the correct temperatures for the experiments involving a temperature change, and a digital probe is unnecessary
Test Tubes Test tubes will be used for the reactions
Burettes Burettes are the most accurate way of measuring out volumes of liquids
100cm3 Volumetrics The volumetrics will be used to make up diluted solutions from the stock solution, and are the most accurate way of doing so
Pipettes (Graduated and 5cm3) The pipettes will be used to measure quantities of the diluted reactants, and by using these sizes, the greatest possible accuracy can be achieved
Stopwatch A stopwatch will be used to record the time taken for the blue colour to appear
Teat Pipettes The volume of starch is not crucial to the experiment, so it will be easiest to use a teat pipette
Glass Stirring Rods The reaction mixtures must be stirred to ensure the results are reliable
Water Baths Water baths are the most reliable way of heating up the solutions to the required temperature, and can be set accurately

Risk Assessment


A table to show the chemicals used and the risks associated with them
Chemical Concentration (mol dm-3) Volume (dm3) Hazard Precautions Disposal
Potassium Iodide 1 0.3 Mostly harmless (may cause irritation if in contact with eyes and skin) Wear goggles and avoid contact with skin Down the sink
Potassium Peroxidisulfate 0.04 0.2 Irritant, Harmful, Toxic Wear goggles and avoid any skin contact Cover with reducing agent
Sodium Thiosulfate 0.01 0.3 Irritant if in contact with eyes Wear goggles Down the sink
Starch 1% 0.1 Irritant and toxic if consumed Wear goggles and avoid consumption Down the sink
Iodine 0.25 0.3 Mild Irritant, stains skin Avoid contact and wear goggles Down the sink
Potassium Sulfate 0.16 0.3 Mild hazard at low concentrations Avoid contact, handle with care, and wear goggles Down the sink