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LECTURE Chem DEMONSTRATION #1 (CLOCK REACTION)
The aqueous reaction between two anions, peroxydisulfate (S2O2) and thiosulfate
8
(S2O2), to form sulfate (SO2) and dithionate (S4O2) anions is accelerated by iodide (I). In the
3
4
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course of the reaction, iodide is converted to triiodide (I3) temporarily, then I3 is returned to I.
Consequently, I3 is an intermediate and iodide is a catalyst. The net reaction is, therefore,
2
S2O8
+
peroxydisulfate
2 S2O2
3
thiosulfate
2 SO2 + S4O2
4
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sulfate
dithionate
while the two reactions involving the consumption and re-generation of the catalyst are
2
2
S2O8 + 3 I 2 SO4 + I3
2 S2O2 + I3 S4O2 + 3 I
3
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which, youll note, add to the net reaction.
Our goal is to find the kinetic rate law, which well assume has the general form
d [S2O2]
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Rate =
= k [S2O2]m [I]n
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dt
where k is the rate constant, and m and n are reaction order exponents. These are the
quantities we seek, and heres how well do it. Triiodide (itself colorless) forms a colored
complex with starch, but, since the step that destroys triiodide is faster than its reaction with starch,
no color can form as long as some of the thiosulfate reactant is present. As soon as
the thiosulfate is gone, the color rapidly appears (as long as some peroxydisulfate and iodide are
present, which well ensure is the case). Thus, our primary data are initial concentrations and the
time required for the color to appear after the solution is initially mixed.
Clock reaction
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If we make the initial amount of peroxydisulfate much larger than the initial of
thiosulfate, amount we will be able to measure a very good approximation to the initial rate:
d [S2O2]
[S2O2]t = [S2O2]t = 0
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8
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Rate =
dt
Well measure for three different reaction mixtures:
Reagent
Mixture A
Mixture B
Mixture C
0.01 M Na2S2O3
100 mL
100 mL
100 mL
0.2 M KI
250 mL
250 mL
125 mL
Starch Indicator
25 mL
25 mL
25 mL
0.2 M Na2S2O8
250 mL
125 mL
250 mL
0.2 M Na2SO4
0 mL
125 mL
0 mL
0.2 M KNO3
0 mL
0L
125 mL
Note that the total volume of each solution is the same, 625 mL. The KNO3 added in
mixture C is chemically inert. It is there simply to keep the total ion concentration constant in all
three mixtures. The table on the next page should be completed in advance so that experimental
data can be added in class as they are collected.
Clock reaction
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Clock Reaction Data and Calculations
The table below should be completed in advance, except for the boxes with the thicker
outline which rely on data we will measure in lecture.
MIXTURE A
MIXTURE B
MIXTURE C
[S2O2]t = 0
3
[S2O2]t = 0
8
[I]t = 0
[S2O2]t =
8
/s
Rate/ (units?)
k/ (units?)
Once has been measured for each mixture, the Rate can be calculated from the expression
on the previous page. Comparison between experiments with different initial concentrations allow
us to find the reaction orders m and n. Once we know these orders, the rates, and the initial
concentrations, we can find k and deduce its units. We end with the full rate expression. (You
should fill in the boxes.)
d [S2O2]
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Rate =
=
dt
Clock reaction
[S2O2]
8
[I]
3