Experiment 29: Rates of chemical
reactions
Purpose
In this experiment you will determine the rate law
for an iodine clock reaction. In an iodine clock
reaction, the completion of the reaction is signaled
by the sudden appearance of the
dark color characteristic of the reaction of I2 with
starch.
The rate of an iodine clock reaction depends on
the concentration of the reactants. As a
result, the time required for the appearance of the
dark color can be adjusted by manipulating the
concentration of the reacatants
.
Clock reaction
• Normal chemical reactions occur so quickly
that very sophisticated techniques are
required to study the speed of reactions.
Fortunately scientists can also study a group
of reactions that occur very slowly known as
clock reactions.
Rate law
• The rate of exn will depend on the
concentration of the reactants thus
• Rate = k[A]x[B]y
• [A] and [B} are molar concentartions (M)
• X and Y describe the rate
• And k is the specific rate conatant
Rate law
• The object of the experiment is to determine the
values for the rate constant k and the reaction
orders x and y
• The overall order of the rxn is the sum of the
exponents
• The object of the experiment is to determine the
values for the rate constant k and the reaction
orders q and r.
• In this experiment you will study the rate of
the reaction between iodide ion, I–, and
• persulfate (peroxydisulfate), S2O82–. The
products of the reaction are I2 and sulfate ion.
S2O82- + IRate = k[A]x[B]y
Rate = k[S2O82-]x[I-]y
Δ [S2O82-] = k[S2O82-]x[I-]y
Δt
I2 + 2 SO4 -2
• To do this experiment you will need :
• to know or measure Δ S2O82- (the initial
change in concentration of S2O82- ions),
• and Δt, the time elapsed during the
change,
• as well as the initial concentration of
S2O82• And I- ions.
• A way to obtain the Δ S2O82- is by coupling
another reaction to the one we are studying.
• The new reaction is the reduction of I2 by
Na2S2O3 (sodium thiosulfate).
• I2 + Na2S2O3
2I- + S4O62-
When you add sodium thiosulfate a color change
will take place (blue-black)
Every time a color change takes place 2 x 10 -4 M of
S2O82- has reacted
Procedures
• Prepare 4 solutions ,one at a time as needed
Soln 1
Soln 2
Soln 3
Soln 4
25.0 ml KI soln
25.0 ml KI soln
50.0 ml 0.KI soln
12.5 ml KI soln
1.0 ml starch
1.0 ml starch
1.0 ml starch
1.0 ml starch
1.0 ml Na2S2O3 sol
1.0 ml Na2S2O3 sol
1.0 ml Na2S2O3 sol
1.0 ml Na2S2O3 sol
48.0 ml KNO3 sol
23.0 ml KNO3 sol
23.0 ml KNO3 sol
35.5ml KNO3 sol
1 drop EDTA soln
1 drop EDTA soln
1 drop EDTA soln
1 drop EDTA soln
75.0 ml
50.0 ml
75.0 ml
50.0 ml
Procedures
• Prepare 7 test tubes filled with 1 ml aliquots of
Na2S2O3
• Prepare soln 1 in a 250 ml erlenmeyer flask
• Pipet 25.0 ml of (NH4)2S2O8 into a 100 ml beaker
• Pour the 25.0 ml of (NH4)2S2O8 into soln 1 and
swirl ( note the time you began mixing to the
nearest second!!!)
• When you see the blue –black color appear add
1ml of Na2S2O3 and the color will dissappear
Procedures
• When the color reappears record the time and
add then another 1 ml aliquots of Na2S2O3 .
• The time being measured is the intervals
between the appearance of the blue black
color
• Repeat this procedure seven times for each
aliqot.
• Solutions 2,3 and 4 should be treated in the
same manner
graph
• X axis is cumulative times
• Y axis total moles consumed
– EACH interval is 2.0 x 10-4 moles of S2O82-
1.40E-03
y = 1E-06x
1.20E-03
Moles of S2O8
1.00E-03
8.00E-04
Y-Values
6.00E-04
Linear (Y-Values)
Linear (Y-Values)
4.00E-04
2.00E-04
0.00E+00
0
200
400
600
Time (s)
800
1000