Rate of Reaction

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Investigating Rate of Reaction between Iodide ions and Peroxodisulphate ions with respect to Temperature and use of a Catalyst


The aim of this experiment was to investigate the affect of the use of a catalyst and temperature on the rate of reaction while keeping all the other factors that affect the reaction rate constant. In the reaction, potassium peroxodisulphate and potassium iodide will be used to provide the peroxodisulphate ions and iodide ions respectively. The ionic formula for the reaction is as follows:

S2O82- (aq) + 2I- (aq) → 2SO42- (aq) + I2 (aq)

However, in order to measure the rates of reaction, sodium thiosulphate and starch are added. Sodium thiosulphate is added to react with a certain amount of iodine as it is made. Without the thiosulphate, the solution would turn blue/black immediately, due to the iodine and starch. The thiosulphate ions allow the rate of reaction to be determined by delaying the reaction so that it is practical to measure the time it takes for the iodine to react with the thiosulphate. After the all the thiosulphate has reacted with the iodine, the free iodine displays a dark blue/black colour with the starch. If t is the time for the blue/black colour to appear, then 1/t is a measure of the initial rate.

While measuring the effect of the use of a catalyst and temperature on the reaction rate, several factors must be kept constant. During the reaction with a catalyst, the temperature will be kept constant (at room temperature), concentration, pressure and since the reaction involves liquids, the surface area will be kept constant at all times, however they must be mixed the same. Predictions may be made about the suitability of possible catalysts by assuming that the mechanism of catalysis consists of two stages, either of which can be first:

•The higher oxidation state of the catalyst oxidises I- (aq) •The lower oxidation state of the catalyst reduces S2O82-


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