Experiment 8 Determination of the Rate of Chemical Reaction
and Activation Energy
Purpose
1. To master the effects of the concentration，temperature，and catalyst on the rate of chemical
reaction
2. To determinate the rate of chemical reaction that (NH4)2S2O8 oxidize KI，and to calculate
the reaction order and activation energy of the reaction
Principle
In a solution of (NH4)2S2O8 and KI，the following redox reaction takes place:
(NH4)2S2O8 + 2KI ＝ (NH4)2SO4 + K2SO4 + I2
or
S2O82- + 2I- ＝ 2SO42- + I2
(1)
In a certain temperature, the average rate of this reaction is:
2
v =
[S2O8 ]
= k [S2O82-]m[I-]n
t
In order to determinate v，we should determinate the change of the concentration of S2O82(that is  [S2O82-]) in a time interval t . We use a starch solution as an indicator. We previously
add certain volume starch and Na2S2O3 with accurately known concentration to the solution
derived from the KI mixed with (NH4)2S2O8 solutions. Then I2 released from the reaction (1)
immediately react with Na2S2O3 to form the colorless S4O62- and I- ions:
I2 + S2O32- = S4O62- + 2I-
(2)
The rate of reaction (2) is much faster than the rate of reaction (1). As the Na2S2O3 was
consumed completely, the I2 released from the reaction (1) immediately react with the starch to
turn the color of the solution into blue. Compare reaction(1) with (2)，1mol S2O82- must consumed
2mol S2O322
Δ[S2O82-]
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[S2O3 ]
=
2
1
The initial concentration of Na2S2O3 is  [S2O32-] because the Na2S2O3 was consumed
completely during the t . In the experiment, we record the time interval t from the
beginning of the reaction to the moment that the color of the solution turns into blue. The rate of
2
the reaction can be calculated from v =
[S2O8 ]
.
t
The experiment has proved that the rate of the reaction (1) is
2
[S2O8 ]
v=
= k [S2O82-]m[I-]n
t
Take logarithm to both side of v = k [S2O82-]m[I-]n,
we have
lgv = lgk + m lg[S2O82-] + n lg[I-]
When [I-] is kept constant, the plot of lgv versus lg[S2O82-] is a straight line. The slope of the
line m is the reaction order of [S2O82-]. Similarly when [S2O82-] is kept constant, the plot of lgv
versus lg[I-] is also a straight line and its slope n is the reaction order of [I-]. The total reaction
order (m + n) can be calculated from m and n. When we know m and n, the constant k of the
reaction rate at a given temperature can be determined from reaction rate equation v = k
[S2O82-]m[I-]n.
When the concentrations of all species in a reaction are kept constant, the effect of
temperature on reaction rate constant can be determined. From Arrhenius equation
lgk = A 
lg(
Ea
2.303RT
E (T  T )
k2
) a 2 1
k1
2.303RT2T1
The effect of temperature on rate of reaction can be quantitatively determined. Based
on lgk = A 
Ea
, we can find values of k under different temperatures. Plot lgk versus
2.303RT
1
yields a straight line. The activation energy of the reaction can be determined from the
T
slope( 
Ea
) of the line. In the equation, A is a constant unique to the reaction, R is the gas
2.303R
constant, T the absolute temperature, and Ea the activation energy of the reaction.
Apparatus and Reagents
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Apparatus： conical flask(50mL×5)， measuring cylinder(10mL×5) or measuring pipet
(10mL×5)， test tube， thermometer， water bath kettle， stopwatch
Reagents： 0.2mol·L-1 (NH4)2S2O8 solution, 0.2mol·L-1KI solution, 0.01mol·L-1Na2S2O3
solution, 0.2mol·L-1KNO3 solution, 0.02mol·L-1Cu(NO3)2 solution,
0.2mol·L-1 (NH4)2SO4
solution, 0.2% starch solution
Procedure
1. The effects of concentration on the rate of the reaction
Firstly measure out 10.00mL 0.2mol·L-1KI, 7.50mL 0.2mol·L-1 (NH4)2SO4, 4.00mL
0.01mol·L-1 Na2S2O3 and 2.00mL 0.2% starch solution with the aid of four different 10.00mL
volumetric pipet respectively, and place them together in a 50mL conical flask. Measure out
2.50mL (NH4)2S2O8 by a 10mL volumetric pipet, pour the solution into a test tube. Then, start the
stopwatch and stir the mixture of the first four solution in conical flask continuously while quickly
adding (NH4)2S2O8 solution to the conical flask. Finally stop the stopwatch and record the t
and room temperature as soon as the color of the solution turns into blue．
According to the table 1, please finish the experiment No.2, No.3, No.4, No.5 in the same way
with the experiment No.1．
Table 1 The effect of concentration on the rate of the reaction
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Date:
Room Temperature:
Volume
/mL
Humidity
Ⅰ
Ⅱ
Ⅲ
Ⅳ
Ⅴ
0.2mol·L-1(NH4)2S2O8
2.50
5.00
10.00
10.00
10.00
0.01mol·L-1Na2S2O3
4.00
4.00
4.00
4.00
4.00
0.2% starch solution
2.00
2..00
2.00
2.00
2.00
0.2mol·L-1KI
10.00
10.00
10.00
5.00
2.50
0
0
0
5.00
7.50
7.50
5.00
0
0
0
Experiment No.
Reagent
℃
-1
0.2mol·L KNO3
0.2mol·L-1(NH
4)2SO4
t /s
[(NH4)2S2O8]initial/mol·L-1
[KI]initial/mol·L-1
[Na2S2O3]initial/mol·L-1
 [S2O82-] /mol·L-1
2
v = [S 2 O 8 ] /mol·L-1·s-1
t
lg[S2O82-]
lg[I-]
lg v
m (Reaction order of S2O82-)
n (Reaction order of I-)
k=
v
[S 2 O8 ] [I - ]n
2- m
Reaction rate equation
The average reaction rate can be calculated by the data of table 1.
The reaction order for m and n also can be get respectively by using the data of experiment
No.1～3 and experiment No. 3～5 in the tableⅡ-15-1 to plot the diagrams of lgv versus lg[S2O82-]
and lgv versus lg[I-] .According to the result above we finally can obtain reaction rate constant k.
(2) The effects of temperature on the rate of reaction
Table 2
The effect of temperature and catalyst on the rate of reaction
Date:
Experiment No.
Temperature/℃
0.2 mol·L-1KI/mL
0.01mol·L-1Na2S2O3/mL
0.2%starch solution/mL
0.2mol·L-1(NH4)2S2O8/mL
0.2mol·L-1(NH4)2SO4/mL
0.02mol·L-1 Cu(NO3)2/drop
t /s
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℃
Temperature:
Humidity
1
2
3
4
Room
temperature
10
4
2
5
5
0
10+Room
temperature
10
4
2
5
5
0
20+Room
temperature
10
4
2
5
5
0
10+Room
temperature
10
4
2
5
5
2
4
V /mol·L-1·s-1
k
lgk
1 -1
/℃
T
Ea /kJ·mol-1
Here, the experiment No.1 in the table 2 is actually the experiment No. 2 in the table 1.
According to the table 2. Firstly measure out 10.00mL 0.2mol·L-1 KI, 5.00mL 0.2mol·L-1
(NH4)2SO4, 4.00mL 0.01mol·L-1 Na2S2O3 and 2.00mL 0.2% starch solution prepared with four
different 10mL volumetric pipet respectively, and place them together in a 50mL conical flask.
Measure out 5.00mL 0.2mol·L-1 (NH4)2S2O8 by a 10mL volumetric pipet, pour the solution into a
test tube. Then heat the conical flask containing the solution and the test tube containing
(NH4)2S2O8 solution in the constant temperature water bath kettle. As soon as the temperature is
10℃ higher than the room temperature and put the (NH4)2S2O8 solution to the conical flask，start
the stopwatch, stir the solution at the same time. Stop the stopwatch as soon as the color of the
solution turns into blue. Record the t and the temperature.
Please repeat another experiment in the same way when the temperature is 20℃ higher than
the room temperature.
According to data of tableⅡ-15-2 and equations of lgk = A 
Ea
and lg( k 2 )  Ea (T2  T1 ) ,
2.303RT
k1
2.303RT2T1
we can get the reaction rate v and reaction constant k under different temperatures and the
activation energy of the reaction Ea through plotting diagram of lgk versus
1
.
T
3. The effect of the catalyst on the rate of the reaction
Cu2+ ions can catalyze the reaction (1) mentioned above. The rate of the reaction (1) can
increase rapidly with the addition of trace Cu2+ ions. According to the No.4 group experiment
listed in the table 2 to do this experiment. Here, add 2 drop 0.02mol·L-1 Cu(NO3)2 as catalyst
before adding (NH4)2S2O8 to the conical flask.
According to the results of the experiments, please discuss the effects of concentration，
temperature and catalyst on the rata of the reaction respectively.
Question
1. What factors affect the rate of the chemical reaction? Please explain the general effect of
increasing concentration of reactants and increasing temperature on reaction rate.
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