Rates of reaction - kinetics (A2)
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Please can you help me understand what the clock reaction does, and why the chemicals (such as: potassium iodide solution, potassium peroxodisulphate solution, sodium thiosulphate solution, starch solution) are needed. My understanding of this area is not very strong and has not made any progress in class (my syllabus is Salters chemistry).
Igloo writes ...
Clock processes are associated with reactions which suddenly produce colour, and can often be adapted to measure rates of reaction.
One of the best known clock reactions involves the chemicals you have mentioned in your question. When all these ingredients are mixed together the following reactions occur:
First the potassium iodide is oxidised by the potassium peroxodisulphate to form iodine:
2KI(aq) + K2S2O8(aq) -> 2K2SO4 (aq) + I2(aq)
The iodine then immediately reacts with the sodium thiosulphate:
I2(aq) + 2Na2S2O3(aq) ----> Na2S4O6(aq) + 2NaI(aq)
However, when the supply of sodium thiosulphate is exhausted, the iodine can no longer be removed, so its colour becomes immediately evident. If starch has also been added to the mixture, this event is heralded by the sudden appearance of a deep blue-black colour.
In chemistry “magic” shows, the concentrations of all the chemicals can be arranged in such a way that the mixture turns blue-black after a fixed period of time, e.g. exactly 15 seconds, and when the audience is invited to start their watches at the moment the chemicals are mixed together, sure enough, the mixture turns blue-black after exactly 15 seconds. Thus the reaction is as reliable as a “clock”!
In a school or college laboratory, however, the reaction can be adapted to determine orders of reaction, where the time of the appearance of the colour is not known beforehand, so rates can be determined by measuring the time taken for the colour to appear.
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updated: 17 January 2006
