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SIC1004- PHYSICAL CHEMISTRYLAB REPORTTITLE Experiment 4 – Determination OfRate Constant Of A DissociativeReactionNAME Siti Rashidah binti Abdol RazidMATRIC NOObjectives: To determine the rate constant of a dissociative reaction of 4-Methyl-4- hydroxy-2-pentanone, MHP (diacetone alcohol) Introduction...

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SIC1004- PHYSICAL CHEMISTRY LAB REPORT

TITLE

NAME

Experiment 4 – Determination Of Rate Constant Of A Dissociative Reaction

Siti Rashidah binti Abdol Razid

MATRIC NO

1

Objectives: To determine the rate constant of a dissociative reaction of 4-Methyl-4hydroxy-2-pentanone, MHP (diacetone alcohol) Introduction : Rate constant is is the proportionality constant in the equation that expresses the relationship between the rate of a chemical reaction and the concentrations of the reacting substances. In this experiment the rate constant of a dissociative reaction of 4-Methyl-4- hydroxy-2-pentanone, MHP (diacetone alcohol) is determine. The substance will dissociate to form acetone in the presence of hydroxyl ion and will undergoes multiple step of reaction.

Figure (A) shows the overall reaction of dissociation of 4-Methyl-4- hydroxy-2pentanone, MHP (diacetone alcohol) while figure (B),(C) and (D) shows the steps of the reaction. By this, the rate constant, 𝑘 of the reaction is given by 𝑑[𝑀𝐻𝑃] 𝑑𝑡

= 𝑘[𝑀𝐻𝑃] ; 𝑘 =

𝑘3 𝑘1 [𝑂𝐻 −] 𝑘2 [𝐻2 𝑂]

Equation 1

From here, The equation can be rearrange as 𝑙𝑛

[𝑀𝐻𝑃] 0 [𝑀𝐻𝑃]

= 𝑘𝑡

Equation 2

It can also be represented as (𝑣−𝑣∞ )

(𝑣0 −𝑣∞ )

= 𝑒 −𝑘𝑡

Equation 3

2

Results: Table 1 :Data of experiment for reaction without acetone 𝑽′

𝑽

Time,𝒕𝒕/ s

𝑽′−𝑽 𝑽

𝐥𝐨𝐠𝟏𝟎(𝑽 𝑽′−𝑽)

0

57

78.5

21.5

1.332

30

59

79

20

1.301

60

60

79.5

19.5

1.29

90

61.5

79.5

18

1.255

120

62

79.5

17.5

1.243

150

63

79.5

16.5

1.217

180

64

79.5

15.5

1.19

210

65

79.5

14.5

1.161

240

65.5

79.5

14

1.146

270

66

79.5

13.5

1.13

300

66.5

79.5

13

1.114

330

67

79.5

12.5

1.097

360

67.5

79.5

12

1.079

390

68

79.5

11.5

1.061

420

68.5

79.5

11

1.041

450

69

79.5

10.5

1.021

480

69.5

79.5

10

1

510

70

79.5

9.5

0.978

540

70.5

79.5

9

0.954

570

71

79.5

8.5

0.929

600

71.5

79.5

8

0.903

3

Table 2:Data of experiment for reaction with acetone 𝑽′

𝑽

Time,𝒕𝒕/ s

𝑽′−𝑽 𝑽

𝐥𝐨𝐠𝟏𝟎(𝑽 𝑽′−𝑽)

0

35.5

82.8

47.3

1.675

30

38

83.1

45.1

1.654

60

44

86

42

1.623

90

46

86.5

40.5

1.607

120

49

87

38

1.58

150

50.5

87.5

37

1.568

180

52

87.5

35.5

1.55

210

54

87.5

33.5

1.525

240

56

88

32

1.505

270

58.5

88

29.5

1.47

300

60

88.5

28.5

1.455

330

61.5

89

27.5

1.439

360

63

89

26

1.415

390

64

89.5

25.5

1.407

420

65

89.5

24.5

1.39

450

67

90

23

1.362

480

68

90

22

1.342

510

68.5

90

21.5

1.332

540

70

90.5

20.5

1.312

570

70.5

91

20.5

1.312

600

71

91

20

1.301

4

Figure 1: Graph 𝐥𝐨𝐠𝟏𝟎(𝑽′−𝑽) against Time,𝒕(s) for the experiment for reaction without acetone. Graph 𝐥𝐨𝐠𝟏𝟎(𝑽′−𝑽) against Time,𝒕(s)

for the experiment for reaction without acetone. 1.4

𝐥𝐨𝐠𝟏𝟎(𝑽′−𝑽)

1.2 1 0.8 y = -0.0007x + 1.321 R² = 0.9967

0.6 0.4 0.2 0 0

100

200

300

400

500

600

700

Time,𝒕𝒕/s

Figure 2: Graph 𝐥𝐨𝐠𝟏𝟎(𝑽′−𝑽) against Time,𝒕(s) for the experiment for reaction with acetone.

Graph 𝐥𝐨𝐠𝟏𝟎(𝑽′−𝑽) against Time,𝒕(s) for the experiment for reaction with acetone. 1.8

1.6

𝐥𝐨𝐠𝟏𝟎(𝑽′−𝑽)

1.4 1.2 1

y = -0.0006x + 1.6612 R² = 0.9918

0.8 0.6 0.4 0.2 0 0

100

200

300

400

500

600

700

Time,𝒕𝒕/s

5

Graph analysis: Based on figure (1) and figure (2), graph plotted with 𝐥𝐨𝐠𝟏𝟎(𝑽′−𝑽) against Time,𝒕(s) will gives a straight line of slope -k/2.303, in which is define as : 𝑙𝑜𝑔10 (𝑉′

− 𝑉)

=

−𝑘

( 2.303) 𝑡 + 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡

Equation 4

Based on equation 4,

𝑚=

−𝑘

Equation 5

2.303

For figure 1: 𝑌 = −0.0007𝑋 + 1.321 ; m= -0.0007

Equation 6

Hence, 𝑘 = −(−0.0007) × 2.303 = 1.6121 × 10−3 𝑠−1 Based on the linest calculation in excel, the uncertainty of the slope = 0.00906 × 10 −3 ≈ 0.009 × 10−3 Thus , Rate constant of the dissociation of 4-Methyl-4- hydroxy-2-pentanone, MHP (diacetone alcohol) without acetone = (1.612 ± 0.009) × 10−3 𝑠−1

For figure 2 : 𝑌 = −0.0006𝑋 + 1.6612 ; m= -0.0006 Hence, 𝑘 = −(−0.0006) × 2.303 = 1.3818 × 10−3 𝑠−1 Based on the linest calculation in excel, the uncertainty of the slope = 0.0134 × 10−3 ≈ 0.01 × 10−3 Thus , Rate constant of the dissociation of 4-Methyl-4- hydroxy-2-pentanone, MHP (diacetone alcohol) with acetone = (1.38 ± 0.01) × 10−3 𝑠−1

6

Discussion : Based on figure 1 and figure 2, the rate constant of dissocciation without acetone is higher than the rate constant of dissociation with acetone. Based on equation 1, the rate constant is inversely proportional to the concentration of 4-Methyl-4- hydroxy-2pentanone, MHP (diacetone alcohol). When aceton is added, the concentration of the product will increase and shift the equilibirium to the left. This reaction will then favoured the reverse reaction. As a result, the concentation of 4-Methyl-4- hydroxy-2pentanone, MHP (diacetone alcohol) will increase. Thus the rate constant will also increases. This is based on Le Chatelier’s principle that stated that a system tend to shift its equilibirium positions as to counteract and minimise the effect of disturbance when the system is disturbed by changes in concentration of the component. That being said, the reaction satisfied the mechanism of equation 1 that can also be defined as 𝑑[𝑀𝐻𝑃] 𝑑𝑡

={

𝑘5 𝑘3 𝑘1 [𝑀𝐻𝑃 ][𝑂𝐻 − ] } { [𝐶𝐻 } 𝑘4 𝑘2 3 𝐶𝑂𝐶𝐻3 ]

Equation 7

Conclusion: Rate constant of the dissociation of 4-Methyl-4- hydroxy-2-pentanone, MHP (diacetone alcohol) without acetone = (1.612 ± 0.009) × 10−3

𝑠−1

Rate constant of the dissociation of 4-Methyl-4- hydroxy-2-pentanone, MHP (diacetone alcohol) with acetone = (1.38 ± 0.01) × 10−3

𝑠−1

References [email protected]. (n.d.). University of Malaya. Retrieved from Laboratory Manual: https://kimia.um.edu.my/LabManual/fizikal/expt-3-manual.pdf The effect of concentration on equilibrium. (n.d.). Retrieved from Nigerian Scholars: https://nigerianscholars.com/tutorials/chemical-equilibrium/effect-ofconcentration-onequilibrium/#:~:text=Equilibrium%20shifts%20to%20the%20left.%20That%20i s%2C%20when,will%20shift%20to%20increase%20the%20concentration%20 of%20products.

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