Hydrogen Peroxide Chemical Reaction Rate

Discussion:

In this experiment, the rate of reaction will be investigated. When hydrogen peroxide is added to a solution of potassium iodide, the iodide ions are slowly oxidized according to the equation:

H2O2 (aq) + 3I- (aq) + 2H+ (aq) I3- (aq) + 2H2O (l)

This reaction can be modified by the introduction of a small amount of S2O32- (aq) and starch indicator. The reactions are shown below:

H2O2 (aq) + 3I- (aq) + 2H+ (aq) I3- (aq) + 2H2O (l)….main reaction

2S2O32-(aq) + I3-(aq) S4O62- (aq) + 3I- (aq)

starch + I2 blue complex

A method to accurately measure the rate of reaction is by adding thiosulfate ions (S2O32- ). The addition of thiosulfate ions allows an accurate measurement of the rate at which the peroxide-iodide is taking place. The small amount of S2O32- added consumes the I3 from the main reaction. As long as there is S2O32- present in the reaction mixture, S2O32- will instantaneously consume I3 formed from the main reaction and the starch indicator will not be affected. However, the starch indicator will turn deep blue when all the S2O32- is consumed and I3 starts to appear from the main reaction. When all the thiosulphate is consumed, I3- formed is free to react with the added starch indicator and the solution will turn dark blue. The reaction rate will be:

rate = -(∆[I-])/3∆t

The reaction rate above is determined by the stoichiometric relationship between S2O32- , I3- and I-.

mol I- = mol S2O32- x (1 mol I3-)/(2 mol S2O32- ) x (3 mol I-)/(1 mol I3- )

So rate= (3 )/(2 ) x ([S2O32-] )/(3∆t ) = ½ [S2O32-]/∆t

No change will be observed at the start of the experiment, when H2O2, I-, H+ and a small amount of S2O32- and starch indicator are mixed together. But the reaction mixture will suddenly changes to deep blue colour after a period of time. The time elapsed before the development of the blue colour depends on the amount of S2O32- used. The more the amount of S2O32- being used, the longer the time needed for the colour to develop. Thus, S2O32- controls the time taken for the development of colour. The thiosulfate ions are the limiting reagent. Once all the thiosulfate ions are consumed, the iodine starts to form in the solution. Iodine is yellow in colour so when starch solution is added to the solution, a blue solution is formed by the complex of starch-iodine. The time for the solution to change to blue colour is determined by using a timer and the colour produced is sharp. The time from the addition