Iodide Ions and Peroxodisulphate Ions

Investigating Rate of Reaction between Iodide ions and Peroxodisulphate ions with respect to

Temperature and use of a Catalyst

Introduction

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-

Hypothesis

• The use of a catalyst will speed up the reaction as long as the catalysts electrode potentials are feasible for each step in the reaction. Since a catalyst lowers the activation energy and takes the reaction through a different route, according to the Maxwell-Boltzmann diagram, at a constant temperature more particles are able to react as demonstrated by the diagrams below:

T = Temperature

• An increase in the temperature of the system will increase the rate of reaction. Again, using the Maxwell-Boltzmann distribution diagram, we can see how the temperature affects the reaction rate by seeing that an increase in temperature increases the average amount of energy of the reacting particles, thus giving more particles sufficient energy to react.