Homework Help for Chemistry

Iodine Clock Experiment

The reaction of with is known as the Harcourt-Essen reaction and can be represented as follows:

(1)  

The reaction of with thiosulfate can be used to measure the rate of the preceding reaction.

(2)  

Thus, in the presence of , any produced by reaction(1) will be immediately converted back to  by reaction(2). Not until all of the thiosulfate has been consumed will become detectable in solution.

The addition of starch assists the visual detection of this 'end-point', as the starch complex forms a dark blue solution. Prior to this, the solution is colourless.

This overall process is known as the Iodine Clock Reaction.

Consider only the first reaction. The general rate of reaction (1) can be written as:

 

 

 

where k is the specific rate constant and b, c, and d are the order exponents of the respective reactants. On imposing pseudo first order conditions with respect to , the rate law can be re-written as:

 

 

 

If a is the initial concentration of in the reaction vessel, and x is the amount by which the concentr:concentration of has changed by time t (equal to the concentration of produced), integration yields:

 

 

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