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S-1
EXPERIMENT S: THE RATE OF CHEMICAL REACTIONS
OBJECTIVES
The rate at which a chemical reaction occurs depends on the chemical nature of the reactants, the
concentration of reactants, the temperature of the reaction and the effect of catalysts. In this
experiment, you will study the effect of the concentration of reactants on the rate of reaction (1) at room
temperature:
2 I
-
(aq)
+ S2O
8
2-
(aq)
2 SO
4
2-
(aq)
+ I
2(aq)
(1)
In this reaction, the peroxydisulfate ion, S2O
8
2-
, is reduced to sulfate ion (SO
4
2-
) while iodide ions (I
-
)
are oxidized to iodine (I2). In the next experiment (Experiment T), you will study the effect of
temperature and of a catalyst on the rate of this same reaction at fixed concentrations of reactants.
THEORETICAL CONSIDERATIONS
Equilibrium constants indicate whether the reactants or the products for a given reaction are favored at
equilibrium. The
H
rxn
will tell you whether heat will be given off or absorbed during the course of the
reaction. However, neither the K
eq
nor
H
rxn
can tell you how fast a reaction will proceed. For
example, the reaction of hydrogen gas and oxygen gas to form water has a very large equilibrium
constant and is highly exothermic. However, the speed of reaction for a mixture of H
2(g)
and O
2(g)
at
room temperature (in the absence of a catalyst, or of a spark) is extremely slow. For a 1 L mixture of
H
2(g)
and O
2(g)
at 25
C, it has been estimated that the time for half of the molecules to react (called the
half-life of the reaction), would take longer than the present age of our solar system. In this
experiment, you will study the rate of a chemical reaction.
