REACTION RATES EXPERIMENT
The Iodine Clock Reaction 11B
It is very important for a chemist to understand the conditions that
affect the rate of a chemical reaction. In chemical manufacturing
processes, controlling the rate of a given reaction can make all the
difference between an economical process and an uneconomical one.
In the previous experiment (11A) you investigated the principle
that the rate of a reaction is determined by several factors, namely:
concentration of reactants, temperature, surface area of reactants (for a
heterogeneous reaction), nature of reactants, and the presence of a
catalyst. In this experiment, you will carry out a different reaction, but
you will again investigate the effect of the concentration of reactants (in
Part I) and the temperature (in Part II) on the reaction rate.
This experiment involves a reaction that is sometimes called an
iodine clock reaction. There are a number of different combinations of
chemicals that give a reaction of this type. What happens, essentially, is
that there are two different reactions: one in which iodine is produced (a
slow reaction) and one in which the iodine produced in the first reaction is
used up (a fast reaction). By carefully controlling the quantities of
reactants, you can obtain a situation in which the reactant in the second
reaction is used up first, allowing iodine to form at that point. At very low
concentrations the iodine then combines with starch to suddenly give a
deep blue-black color, at a time determined by the conditions used.
Hence the term “iodine clock”. The time elapsed from when the solutions
were first mixed together until the point when the blue-black color
appears is measured, and from this time measurement the rate of the
reaction can be determined. You will alter the conditions of
concentrations of reactants in Part I and temperature in Part II in order to
determine their effect on reaction rate.
OBJECTIVES
1. to observe and record the effect of changing the concentration of a
reactant on the rate of a reaction
2. to observe and record the effect of changing the temperature of a
system on the rate of a reaction
