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To determine the activation energy for the reduction of peroxodisulphate(VI) ions. S2O82-, by iodide ions I-, using a 'clock' reaction. Principle
The equation for reduction of S2O82- by I- is: S2O82- + 2I- → 2SO42- + I2 The formation of iodine is 'monitored' by small & known amount of thiosulphate ions, S2O32-: 2S2O32- + I2 → S4O62- + 2I- Once the reactants are mixed, the stop-watch is started. At the time when all of the thiosulphate is reacted, any free iodine produced will turn starch solution (added before) into dark-blue, the time is then recorded. The amount of thiosulphate added monitors the time in which starch turns blue and the reaction rate is directly proportional to 1 ÷ time taken for starch changes to dark blue. By plotting a graph of log10(1/t) against 1/T (T=absolute temperature), activation energy (Ea) can be found. Chemicals
0.020M K2S2O8, 0.50M KI, 0.010M Na2S2O3, 0.2% starch solution Apparatus
400 cm3 beaker, boiling tubes x2, pipettes (10ml), thermometer x2, water bath, stop-watch Procedure
1.> Half-fill the beaker with hot water at temperature between 49 - 51℃ (as water bath). 2.> Pipette 10 cm3 of 0.020M K2S2O8 solution into the first boiling tube, place a thermometer in this solution and keep this in the water bath. 3.> Pipette 5 cm3 of both 0.50M KI and 0.010M Na2S2O3 and 2.5 cm3 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 ± 1℃), pour the contents of the second boiling tube into the first, shake to mix, and start the stop-watch. 5.> When the blue...