Experiment 1- Kinetics study of the hydrolysis of methyl acetate by acid-base titration.

BACH 2114 KINETICS, MECHANISM, AND STEREOCHEMISTRY Name

: 1. Amirah Chan (15WAR08520) 2. Chey Sze Ying (15WAR09194) 3. Chong Khai En (15WAR08702)

Group

: RBS 2- Group 1

Date : 20-5-2015 Title: Experiment 1- Kinetics study of the hydrolysis of methyl acetate by acid-base titration. Aim : To determine the activation energy of the hydrolysis of methyl acetate by acid-base titration. Introduction: Hydrolysis occurs when the bond in a molecule is being broken down after addition of water. Acid – base – catalyzed hydrolyses is very common and it is used in the hydrolysis of ester such as methyl acetate. Hydrolysis occurs in a very low rate in pure water. Activation energy of the hydrolysis of methy acetate is required to be overcome by the reactants in order to complete the reaction. Thus, acid which produces hydrogen ion such as hydrochloric acid is used as a catalyst to increase the rate of reaction. As the rate of reaction is being increased, the activation energy is now being overcome. When methyl acetate reacts with water molecules, acetic acid and methyl alcohol will be formed as the products. The reaction is reversible and the rate constant, k of this reaction is relatively small. A large amount of water is present during the reaction so that there is a complete reaction towards the end of the experiment. In the hydrolysis of methyl acetate, the methyl acetate will take a hydrogen ion and the hydrogen ion will attached to one of the lone pairs on the oxygen which formed a double bond to the carbon. The carbon will then become electrophile and one of the lone pairs on the oxygen of water molecules will attack the electrophilic carbon. Then, the oxygen atom from the water molecule will be deprotonated. Methanol is now being produced. The hydrogen is being removed from the oxygen which attached to the electrophilic carbon. The products which are acetic acid and alcohol are then being produced.

Schematic mechanism of hydrolysis of methyl acetate

Results: A) Standardization of sodium hydroxide solution Average volume used (ml)

1

Volume of NaOH used (ml) 32.20

2

32.10

32.17

3

32.20

Titration

B) 1 – Room temperature V ∞ - Vt (ml)

ln (V∞ - Vt)

0

Titrant volume Vt (ml) 32.90

16.51

2.8040

10

33.00

16.41

2.7979

20

33.50

15.91

2.7669

30

33.60

15.81

2.7606

40

34.00

15.41

2.7350

50

34.20

15.21

2.7220

60

34.35

15.06

2.7120

80

35.30

14.11

2.6469

V∞ - Vt (ml)

ln (V∞ - Vt)

18.57

2.9215

t (min)

2 – Water bath, 40oC

1

Titrant volume Vt (ml) 30.50

6

32.80

16.27

2.7893

11

34.30

14.77

2.6926

16

35.30

13.77

2.6225

21

35.30

13.77

2.6225

31

37.70

11.37

2.4310

41

39.60

9.47

2.2481

51

39.60

9.47

2.2481

71

39.60

9.47

2.2481

t (min)

Calculation of V∞ : V∞ = Final volume per 5 ml aliquot of the reaction mixture at time t = Volume of NaOH required to neutralize HCl in 5 ml of the reaction mixture + volume of NaOH required to neutralize the acetic acid produced by complete hydrolysis of 1 mol of methyl acetate in 5 ml of the reaction mixture Volume required to neutralize HCl in 5 ml of reaction mixture at any time: 100 V x Vs Where Vs = volume of solution initially formed by mixing 100 ml of 1 M HCl with 5 ml methyl acetate (104.6 ml) Vx = Volume of NaOH required to neutralize a 5 ml aliquot of the original 1 M HCl

No. of moles of methyl acetate initially present in any 5 ml aliquot of the reaction mixture: 5 d2 5 25 d2 x = M2 V S M2V S Where d2 = density of methyl acetate (0.9273 at 25oC; 0.9141 at 35oC) M2 = molecular weight (74.08)

Since 1000/N ml of NaOH of molarity N is required to titrate the CH 3COOH produced by hydrolysis of 1 mol of CH3COOCH3: 1000 25 d 2 25000 d 2 x = N M 2 V S NM 2 V s

Therefore, V∞ required to titrate both the HCl and CH3COOH produced by complete hydrolysis of the CH3COOCH3 in a 5 ml sample of the reaction mixture: V ∞=

100V X 25000 d 2 + VS NM 2 V S

Graph of density of methyl acetate against temperature 0.930

f(x) = - 0x + 0.96

0.925

0.920

Density of methyl acetate, d2

0.915

0.910

0.905

0.900 20

30

40

Temperature (oC)

Graph 1 From Graph 1: At room temperature, T = 27oC, y = -0.0013(27) + 0.9603 = 0.9252 At T = 40oC, y = -0.0013(40) + 0.9603

50

= 0.9083

To find molarity of NaOH N1V1 = N2V2 Where N2 = the molarity of NaOH (1.0 M)(5 ml) = N2 (32.17 ml) ∴ N2 = 0.16 M Therefore for the reaction at room temperature, V ∞=

100V X 25000 d 2 + VS NM 2 V S

V ∞=

100(32.17) 25000(0.9252) + 104.6 (0.16)(74.08)(104.6)

∴V ∞=¿ 49.41

Graph of ln (V∞ - Vt) against time for reaction at room temperature 2.85

2.80

f(x) = - 0x + 2.81

2.75

ln (V∞ - Vt)

2.70

2.65

2.60

2.55 0

10

20

30

40

50

Time (min)

Graph 2 From Graph 2, ln C = - k1t + c ln (V∞ - Vt) = - k1t + c y = -0.0019x + 2.8116 ∴ k1 = 0.0019 M min-1

Therefore for the reaction at 40oC, V ∞=

100V X 25000 d 2 + VS NM 2 V S

V ∞=

100(32.17) 25000(0.9083) + 104.6 (0.16)(74.08)(104.6)

60

70

80

90

∴V ∞=¿ 49.07

Graph of ln (V∞ - Vt) against time for reaction at 40oC 3.00 2.90 2.80 f(x) = - 0.01x + 2.79 2.70 2.60

ln (V∞ - Vt)

2.50 2.40 2.30 2.20 2.10 2.00 0

10

20

30

40

Time (min)

Graph 3 From Graph 3, ln C = - k2t + c ln (V∞ - Vt) = - k2t + c y = -0.0099x + 2.7886 ∴ k2 = 0.0099 M min-1

ln

k 2 E a T 2−T 1 = k1 R T 1 T 2

ln

Ea 0.0099 313−300 = 0.0019 8.3145 ( 313 ) ( 300 )

(

) (

)

50

60

70

80

Ea =99 133.74 J ∴ E a=99.13 kJ

mol-1 mol-1

Discussion: When esters such as methyl acetate react with water, some are converted to alcohol and acid, and the reaction can be explained as: CH3COOCH3 + H2O + H2

↔

CH3COOH + CH3OH + H2

This is a reversible reaction, but with this reaction can be considered as a complete reaction with the presence of excess water. Hydrolysis occurs slowly in pure water, with acid as catalyst. If this reaction is carried out in a dilute solution with the presence of a known concentration of strong acid, the rate and order of the reaction depends only on the concentration of methyl acetate. This is thought to be due to the concentration of water which is considered to be constant throughout the experiment and the concentration of acid does not change. The rate of reaction, however, is proportional to the concentration of acid. As we carry out this experiment, methyl acetate is not directly titrated with sodium hydroxide. The methyl acetate is hydrolyzed and separated into acetic acid and methanol by addition of water. The resulted acetic acid is what was being titrated in this experiment. Phenolphthalein is used as the indicator for this experiment. In this experiment, the hydrolysis of methyl acetate was carried out in two temperature variables, room temperature and 40 oC. This can determine the specific rate constants. Both temperatures are titrated where the time intervals are fixed. For the room temperature, it is titrated at 10 minutes intervals for an hour, then at 20 minutes for half an hour. This allows determining how it affects the reaction rate and thus the activation energy of the experiment after the concentration has settled. In the second set of experiment in 40oC, it creates a higher temperature and therefore higher activation energy, only it recorded at 5 minutes intervals for 20 minutes, at 10 minutes intervals for half an hour and then 20 minutes intervals for half an hour. The time between titrations is increased due to the molecules ability

to achieve the activation barrier at a higher rate compared to the room temperature. The temperatures played an important role in the rate of reaction. It is a source of energy in order to have a chemical reaction occurs. In order to initiate a reaction, the reactants must be moving fast enough and have enough kinetic energy so that they collide with sufficient force for bonds to break. The minimum energy with which molecules must have in order for a collision to occur chemical reaction is known as the activation energy. When temperature increases, the reactant particles move more quickly. They have more energy and the particles collide more often, thus more of the collisions are successful. Subsequently, the rate of reaction increases. The proportion of collisions that can overcome the activation energy increases with temperature. The data were used to construct a plot of ln [V∞-Vt] versus time (Graph 2) which according to equation: ln C = -k1t + constant Thus, calculated result has a slope equal to –k 1. The slope of the best straight line through the data was calculated using linear regression to be 0.0019 min-1. This means that the apparent first order rate constant, k 1 for the reaction is 0.0019 M min-1 from which the second order rate constant, k 2 is calculated to be 0.0099 M min-1. According to the results we obtained, the activation energy of hydrolysis of methyl acetate is calculated as 99.13kJ mol -1. The experimental value we obtained is higher than the theoretical value published by Department of Chemical and Environmental Engineering, The National University of Singapore, 2003, which is 60.62kJ mol -1. This errors may caused by the incorrect concentration of the solutions prepared by the lab. Some other reasons may include the different judgment of human on the end-point of titration and also related to the environment factors. There are some precautions in this experiment. The hydrochloric acid was handled carefully by wearing glove and rinse with plenty of water if acid contact with eyes or hands. During the experiment, all the equipments like burette, beaker and pipette were rinsed properly with the solution to be filled in it before taking them in use as the presence of any other chemical can be the reason for wrong measurement. The air bubble in the nozzle of burette was removed before taking the initial reading as it can be the reason for altering the results. In addition, the flask was swirl well right after adding the

indicator and also the titration flask with addition of each drop of solution from burette to ensure complete mixing of reagents. The burette reading was taken with placing the eyes parallel to the bottom meniscus of solution to avoid parallax error. The titration was carried out slowly as the end point was approached and the flask was removed immediately when the end point of reaction was achieved. On the other hand, pipetting has to be accurate in order to avoid excess addition of the solution and thus give an inaccurate result. Conclusion: The experimental activation energy for the hydrolysis of methyl acetate by acid-base titration is 99.13kJ mol-1. The experimental value we obtained is higher than the theoretical value published by Department of Chemical and Environmental Engineering, The National University of Singapore, 2003, which is 60.62kJ mol-1.

References: 1. Laboratory Manual, Physical Chemistry, Year 1, viewed on 30 th May 2015, 2. Determination of Reaction Rate and Reaction Rate Constant Practical Report, viewed on 30th May 2015, 3. Laboratory Report, viewed on 30th May

2015,

4. Volumetric Analysis, viewed on 30th May 2015, 5. The Mechanism for the Acid Catalysed Hydrolysis of Esters, viewed on 31st May 2015, < http://www.chemguide.co.uk/physical/catalysis/hydrolyse.html>