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.

Results:

A) Standardization of sodium hydroxide solution

Titration Volume of NaOH used_(ml) Average volume used_(ml)

1 32.20

32.17

2 32.10

3 32.20

B) 1 – Room temperature

t (min) Titrant volume Vt_(ml) V ∞ - Vt (ml) ln (V∞ - Vt)

0 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 2 – Water bath, 40o_C

t (min) Titrant volume Vt_(ml) V∞ - Vt (ml) ln (V∞ - Vt)

1 30.50 18.57 2.9215 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

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 M2 x 5 VS = 25 d2 M2VS

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 CH3COOH

produced by hydrolysis of 1 mol of CH3COOCH3:

1000 N x 25 d₂ M2VS =25000 d2 NM2Vs

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 ∞=100VX VS +25000 d2 NM2VS 20 30 40 50 0.900 0.905 0.910 0.915 0.920 0.925 0.930 f(x) = - 0x + 0.96

Graph of density of methyl acetate against temperature

Temperature (oC) Density of methyl acetate, d2

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

= 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 ∞=100VX VS +25000 d2 NM2VS V ∞=100(32.17) 104.6 + 25000(0.9252) (0.16)(74.08)(104.6) ∴V ∞=¿ _49.41

0 10 20 30 40 50 60 70 80 90 2.55 2.60 2.65 2.70 2.75 2.80 2.85 f(x) = - 0x + 2.81

Graph of ln (V∞ - Vt) against time for reaction

at room temperature

Therefore for the reaction at 40o_C,

V ∞=100VX VS +25000 d2 NM2VS V ∞=100(32.17) 104.6 + 25000(0.9083) (0.16)(74.08)(104.6)

∴V ∞=¿ _49.07 0 10 20 30 40 50 60 70 80 2.00 2.10 2.20 2.30 2.40 2.50 2.60 2.70 2.80 2.90 3.00 f(x) = - 0.01x + 2.79

Graph of ln (V∞ - Vt) against time for reaction at 40oC

Time (min) ln (V∞ - Vt) 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 lnk2 k₁= E_a R

(

)

(

)

E_a=_{99 133.74 J mol}-1

∴ E_a=_{99.13 kJ 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 40o_{C. 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 40o_{C, 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 –k1. 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:

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