EXP 6

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PHYSICAL CHEMISTRY 2 LABORATORY
REPORT
Experiment No.6
Title: Kinetics of Chemical Reaction – Iodination of
cyclohexanone
Date of expt: 23 July 2008
Name: Cheam Yee Ping
Matrix No: SEC070015
Lab Patner: Chan Wei Kuen
Matrix No: SEC070014
Group: C
Laboratory: Physical chemistry 2nd year
Lecturer: Dr. Azizah binti Mainal
Date of submission report: 6 August 2008
1
Expeiment 6: Kinetics of chemical reaction – iodination of cyclohexanone
Objective: To determine the rate constant and the rate of reaction for the reaction of iodination
of cyclohexanone.
Background:
There are some factors that determine the rate of reaction. One of them is the
concentration of reactant. The increase of concentration causes the number of moles per unit
volume increase. So, the number of collisions per unit time in that volume increase and the
effective collisions increase. Therefore, the rate of reaction increase.
Another factor is temperature. When temperature increase, the average kinetic energy of
molecules increases. Then, the number of molecules with the energy equal or more then
activation energy increase and cause the number of effective collisions increase. Thus, the
reaction rate increase.
Pressure is one of the factors that determine the rate of reaction. When pressure increase,
volume will be decrease. The increase of number of molecules per unit volume cause more
collisions per second. The number of effective collisions increase so that the reaction rate also
increases.
The presence of catalysis will increase the rate of reaction. Catalyst provide an alternative
path way which has lower activation energy. Therefore, the rate of reaction increase.
Experimental Procedure:
The following solutions are to be prepared from the stock solutions:
Volume of stock solution used/cm3
I
II
III
Cyclohexanone
20
20
20
HCl
10
10
6
Distilled Water
14
16
20
KI3
6
4
4
-3
[Cyclohexanone] stock = 0.230 mol dm
[HCl] stock = 0.500 mol dm-3
[KI3] = 0.0360 mol dm-3
IV
14
10
22
4
The acid, ketone and water were mixed together in test tube. The test tube was placed in the
constant temperature bath. The test tube and its content were allowed to equilibrate for at least 15
minutes. The KI3 stock solution was thermostat as well.
The reaction was started by pipetting the required amount of the KI3 solution into the test tube.
As soon as the pipette has drained, the timer was started. Immediately, a sample was transferred
from the test tube to a spectrophotometer sample cell and its absorbance was measured. Further
samples were taken and measured at intervals of 2 minutes, until sufficient points were obtained
for giving a good plot of absorbance A versus time.
The same procedure was carried out for each of the mixture I, II, III, and IV. The gradient of the
graph was determined.
2
Results and calculations:
Temperature of water bath: 29.9°C
Wavelength of light: 565nm
Concentration of stock solution:
[S] stock = 0.2305 M
[HCl] stock = 0.506 M
[KI3] stock = 0.0360M
where [S] = [cyclohexanone]
For absorbance of KI3 stock solution
Readings
1
2
Absorbance
2.000
2.000
Time,t
(min)
0.45
3.10
5.05
7.00
9.00
11.05
13.05
15.20
Mixture
I
0.280
0.241
0.199
0.165
0.148
0.110
0.069
0.033
Time,t
(min)
0.50
3.55
5.45
7.45
9.40
11.45
13.35
Time,t
(min)
0.35
2.30
4.25
6.30
8.25
10.25
12.25
14.25
16.25
18.30
20.35
22.30
24.20
Mixture
III
0.241
0.230
0.208
0.182
0.162
0.148
0.130
0.104
0.080
0.063
0.043
0.020
0.005
Time,
t
(min)
0.35
2.25
4.30
6.30
8.30
10.30
12.30
14.25
16.30
18.30
20.25
22.20
3
2.000
Average
2.000
Mixture
II
0.195
0.193
0.096
0.065
0.063
0.029
0.005
Mixture
IV
0.214
0.208
0.168
0.155
0.131
0.109
0.089
0.067
0.043
0.025
0.011
0.002
3
SUMMARY
OUTPUT
Regression Statistics
Multiple R
0.997737
R Square
0.995478
Adjusted R Square
0.994725
Standard Error
0.006102
Observations
8
ANOVA
df
Regression
Residual
Total
Intercept
X Variable 1
1
6
7
Coefficients
0.288266
-0.01661
Significance
SS
MS
F
F
0.049184 0.0491845 1320.976
2.89E-08
0.000223 3.723E-05
0.049408
Standard
Error
0.004239
0.000457
t Stat
67.99627
-36.34524
P-value
6.81E-10
2.89E-08
Upper
Lower 95%
95%
0.277892 0.298639
-0.01772 -0.01549
4
SUMMARY
OUTPUT
Regression Statistics
Multiple R
0.9822963
R Square
0.96490601
Adjusted R
Square
0.95788721
Standard Error
0.0146383
Observations
7
ANOVA
df
Regression
Residual
Total
Intercept
X Variable 1
1
5
6
Coefficients
0.20583942
-0.0155401
SS
MS
F
0.029458029 0.029458 137.4745
0.001071399 0.000214
0.030529429
Standard
Error
0.011153769
0.001325387
t Stat
18.4547
-11.725
P-value
8.59E-06
7.93E-05
Significance
F
7.93E-05
Upper
Lower 95%
95%
0.177168 0.234511
-0.01895 -0.01213
5
SUMMARY OUTPUT
Regression Statistics
Multiple R
0.9991494
R Square
0.9982995
Adjusted R
Square
0.9981449
Standard Error
0.0034049
Observations
13
ANOVA
df
Regression
Residual
Total
Intercept
X Variable 1
SS
MS
F
1 0.07486724 0.074867 6457.671
11 0.000127529 1.16E-05
12 0.074994769
Standard
Coefficients
Error
t Stat
0.2489772 0.001816215 137.0858
-0.010155 0.000126367 -80.3596
P-value
3.9E-19
1.38E-16
Significance
F
1.38E-16
Upper
Lower 95%
95%
0.24498 0.252975
-0.01043 -0.00988
6
SUMMARY OUTPUT
Regression Statistics
Multiple R
0.9991494
R Square
0.9982995
Adjusted R
Square
0.9981449
Standard Error
0.0034049
Observations
13
ANOVA
df
Regression
Residual
Total
Intercept
X Variable 1
1
11
12
Coefficients
0.2489772
-0.0101548
SS
MS
F
0.07486724 0.074867 6457.671
0.000127529 1.16E-05
0.074994769
Standard
Error
t Stat
0.001816215 137.0858
0.000126367 -80.3596
P-value
3.9E-19
1.38E-16
Significance
F
1.38E-16
Upper
Lower 95%
95%
0.24498 0.252975
-0.01043 -0.00988
7
From graph:
mixture
I
II
III
IV
Gradient, dA/dt ( minˉ¹)
-0.0166
-0.0155
-0.0102
-0.0103
Concentration for
20
 0.230
[s]I
=
50
= 0.092 M
20
 0.230
[s]II
=
50
= 0.092 M
20
 0.230
[s]III =
50
= 0.092 M
14
 0.230
[s]IV =
50
= 0.0644 M
10
 0.500
50
= 0.10 M
10
=
 0.500
50
= 0.10 M
6
=
 0.500
50
= 0.06 M
10
=
 0.500
50
= 0.10 M
[H+]I =
[H+]II
[H+]III
[H+]IV
where [s] is the concentration of cyclohexanone
[I3-]I
[I3-]II
[I3-]III
[I3-]IV
6
 0.0360
50
= 0.00432 M
4
 0.0360
=
50
= 0.00288 M
4
 0.0360
=
50
= 0.00288 M
4
 0.0360
=
50
= 0.00288 M
=
According to Beer-Lambert Law,
A(I3- ) = l [I3-]
Where A
= absorbance

= molar absorption coefficient
[I3-] = concentration
l
= optical path length
8
l = A (I3- ) / [I3]
2.000
=
0.0360
= 55.56 M-1
The differential rate equation for the reaction is
-d[A]/dt =kl [s]a [I3]b [H+]c
Where k = rate constant,
a, b,and c are the orders of reaction with respect to S, I3- and H+ respectively
Determining the value of a, b and c
For a,
Take solution II and IV for constant [H+] and [I3] but different [s]
 dA 
 dA 
log    log  
 dt  II
 dt  IV
a =
log S II  log S IV
  0.0155 
log 

 0.0103 

=
 0.0920 
log 

 0.0644 
= 1.1458
 dA 
 
 dt  II
Let x =
 dA 
 
 dt  IV
 0.0155
=
 0.0103
= 1.5049
 dA 
 dA 
d 
d 
dx
 dt  IV
 dt  II


x
 dA 
 dA 
 
 
 dt  II
 dt  IV
dx
 0.0034   0.0006 



1.5049   0.0155    0.0103 
dx  0.4178
9
dx
x
0.4178
=
1.5049
= 0.2776
da 
 a = 1.15 ± 0.20
≈1
For b,
Take solution I and II for constant [H+] and [s] but different [I3].
 dA 
 dA 
log    log  
 dt  I
 dt  II
b=


log I 3 I  log I 3 II
 
 
  0.0166 
log 

 0.0155 

=
 0.0432 
log 

 0.02880 
= 0.1691
 dA 
 
 dt  I
Let y =
 dA 
 
 dt  II
 0.0166

 0.0155
 1.0710
 dA 
 dA 
d 
d 
dy
 dt  I
 dt  II


y
 dA 
 dA 
 
 
 dt  I
 dt  II
dy
 0.0001   0.0003 



1.0710   0.0166    0.0155 
dy  0.272
dy
db 
y
0.0272
=
1.0710
=0.03
10
 b = 0.1691 ± 0.03
≈0
For c,
Take solution II and III for constant [I3] and [s] but different [H+]
 dA 
 dA 
log    log  
 dt  II
 dt  III
c=


log H II  log H III
 
 
  0.0155 
log 

 0.0102 

=
 0.1 
log 

 0.06 
= 0.8192
 dA 
 
 dt  II
Let z =
 dA 
 
 dt  III
 0.0155
=
 0.0102
= 1.5196
 dA 
 dA 
d 
d 
dz
 dt  II
 dt  III


z
 dA 
 dA 
 
 
 dt  II
 dt  III
dz
 0.0003   0.0003 



1.51`96   0.0155    0.0102 
dz  0.0741
dz
z
0.0741
=
1.5196
= 0.0488
dc 
 c = 0.8192 ± 0.05
≈0
11
Determining the rate constant, k
The above equation can be written as
k = ___-d[A]/dt___
l [s] [H+]
k I = __- ( -0.0166 )________
55.56 x 0.092 x 0.10
= 0.0325
k II = _- ( - 0.0155)___________
55.56 x 0.092 x 0.10
= 0.0303
  0.0102
55.56  0.0920  0.0600
= 0.0333
k III 
  0.0103
55.56  0.0644  0.1000
= 0.0288
k IV 
0.0325  0.0303  0.0333  0.0288
4
= 0.0312 mol-1min-1dm3
Average of rate constant, k =
Determining the standard deviation of k
Solution
k i (mol-1min-1dm3)
I
II
III
IV
0.0325
0.0303
0.0333
0.0288
ki  k
ki  k
(mol-1min-1dm3)
0.0013
-0.0009
0.0021
-0.0024
(mol-2min-2dm6)
1.69 x 10-6
0.81 x 10-6
4.41 x 10-6
5.76 x 10-6
12.67 x 10-6
 ki  k
 k i  k 2 
Standard deviation, s = 

 n 1 
1
2
2
2
12.67  10 6
3
= 0.0021 mol-1min-1dm3
=
12
Standard uncertainties, σ =
=
s
n
0.0021
4
= 0.0011 mol-1min-1dm3
Uncertainty of k = 2σ
= 2 (0.0011)
= 0.0022 mol-1min-1dm3
Rate constant, k = ( 0.0312 ± 0.0022 ) mol-1min-1dm3
Discussion:
From the above experiment, it is noticed that the rate of bromination of ketone is same as
the rate of iodination. This shows that the rate of reaction depends on the concentration of acid
and cyclohexanone since acid hydrochloride acts as catalyst while cyclohexanone is the reactant
in the experiment. The rate of reaction does not depend on the concentration of iodide ion.
Scheme A from the proposed mechanisms satisfies the rate equation since the first step in
Scheme A is the slowest step, the rate determining step. Therefore, it controls and determines the
overall rate of the reaction.
During the experiment, we could observe that the colour of the mixture change from
brown to colourless. It is because the reaction is taking place to decolourise the brownish colour
of KI3. From the pictures above, the colour of the mixture change from brownish to yellowish
and finally become colourless.
Some precautions can be carried out in the experiment:
o Spectrophotometer has to be calibrated each time with distilled water before measuring the
absorbance for the next sample.
o Temperature must be kept constant throughout the experiment
o Spectrophotometer has to be switch on for at least 20 minutes before absorbance
measurements begin
13
Conclusion:
Rate constant, k = (0.0312 ± 0.0022) mol-1min-1dm3
Order of reaction for
1. cyclohexanone, a = 1
2. iodide ion, b = 0
3. hydrogen ion, c = 1
Reference:
1. Atkins, P.W (1998), Physical chemistry, 7th ed. Oxford
14
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