Question

In a clock reaction experiment, the concentration of each reactant does not change significantly over the time period of a clock reaction.

Why is this important?

Answer 1

Clock Reaction:

A clock reaction is a more convenient way of obtaining the initial rate of reaction by taking a single measurement. The time from the start of an experiment is measured for a visual change to be observed, often a colour of a precipitate. Provided there is no significant change in rate during this time, it is assumed that the average rate of reaction over this time will be the same as the initial rate. The initial rate is then proportional to 1/time.
The clock reaction is repeated several times with different concentrations and values of 1/time is calculated for each run.

Clock reactions measure the average rate during the first part of the reaction. The shorter the time period over which the average rate is measured, the less the rate changes over that period. This means the average rate calculated will be more similar to the initial rate, as the initial rate wont have changed as much. Generally accurate when less than 15% of the reaction has taken place.

Importance:

A common type of clock reaction relies on the formation of iodine. As aqueous iodine is coloured orange-brown, the time from the start of the reaction and the appearance of the iodine colour can be measured. Starch is usually added because it forms a complex with iodine, which is an intense dark blue-black colour. The colour change can be delayed by adding a small amount of another chemical (e.g. sodium thiosulfate) which removes the iodine as it forms. Once this chemical is all used up, the blue-black colour appears.