Expt 6 - Chemical Kinetics

8/4/2019 Expt 6 - Chemical Kinetics

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MAPUA INSTITUTE OF TECHNOLOGY

School of Chemical Engineering, Chemistry, Biological Engineering, and Material Science and Engineering

Physical Chemistry Laboratory 2 - 3rd

Quarter SY 2010-2011

Neil Patrick P. Tangara , 3rd

Year B.S. Chemical Engineering

Experiment 6 │ Group 5 │ March 1, 2011 1 of 5

Experiment No. 6

CHEMICAL KINETICS: THE HYDROLYSIS OF METHYL ACETATE

Meynard Austria1

, Neil Patrick Tangara, Darlene Pudolin, Emily Rose Santos, Aeiocellis Tan, Creza Loraine Talingting2

1Professor;

2Students, all from CHM171L/A31, School of Chemical Engineering, Chemistry, Biological Engineering & Material Science

and Engineering, Mapua Institute of Technology

ABSTRACT

This experiment intends to determine the order of reaction, rate law constant, and activation energy needed to

initiate the reaction. Also, the effects of temperature, concentration, catalyst, and other parameters such as

pressure on the rate of reaction have been investigated using chemical kinetics. The hydrolysis of Methyl

Acetate was carried out with HCl as the catalyst. It was found to be a 2nd order reaction with respect to the acid

and had an activation energy value of 375,254.0016 J/mole. It was found that increasing both the temperature

and the amount of catalyst present hasten the rate of reaction by an appreciable value by providing it moreenergy and an optimal reaction condition, respectively. Chemistry theories such as the kinetic molecular and

collision theories have been taken into consideration upon interpretation of the results gathered. This report

will discuss why such a phenomenon occurs.

Keywords: rate law constant, order of reaction, activation energy, catalyst

INTRODUCTION

Chemical kinetics is the study and

discussion of chemical reactions with respect toreaction rates, effect of various variables, re-

arrangement of atoms, formation of intermediates

etc.

At the macroscopic level, we are interested in

amounts reacted, formed, and the rates of their

formation. At the molecular or microscopic level,

the following considerations must also be made in

the discusion of chemical reaction mechanism.

Molecules or atoms of reactants must collide with

each other in chemical reactions.

The molecules must have sufficient energy

(discussed in terms of activation energy) to initiate

the reaction.

In some cases, the orientation of the molecules

during the collision must also be considered.

Methyl acetate hydrolyzes to form acetic acid

and methanol, according to the following reaction:

(1)

The reaction is extremely slow in pure

water, but is catalyzed by both hydronium and

hydroxide ions. In this experiment the kinetics of

the reaction catalyzed by HCl will be studied. HCl

also suppresses the ionization of acetic acid so as

not to change the concentration of hydronium ions

present.










METHODOLOGY

To determine the effect of concentration

and temperature on the reaction mechanism, tworuns of difference concentrations were made at

25oC (assuming room temperature) and one run

was made at 35oC.

The concentration of methyl acetate at a

given time and temperature was determined

through titration of samples with a standard

sodium hydroxide solution since concentration

varies with temperature (25 and 35 oC).

A test tube containing about 12 ml methylacetate was set into a thermostat at 25° C.

Approximately 250 ml of standardized 1 N

hydrochloric acid was placed in a flask clamped in

the thermostat. After thermal equilibrium has been

reached (10 or 15 min should suffice), two or three

5-ml aliquots of the acid were titrated with the

standard sodium hydroxide solution to determine

the exact molarity of the sodium hydroxide in

terms of the standardized hydrochloric acid. Then

50 ml and 100ml of acid was transferred to each of

two 250-ml flasks clamped in the thermostat and 5

min allowed for the reestablishment of thermal

equilibrium. Precisely 5 ml of methyl acetate was

next transferred to one of the flasks with a clean,

dry pipette; the timing watch was started when the

pipette is half emptied. The reaction mixture is

shaken to provide thorough mixing.

A 5-ml aliquot was withdrawn from the

flask as soon as possible and run into 50 ml of

distilled water. This dilution slows down the

reaction considerably, but the solution should be

titrated at once; the error can be further reducedby chilling the water in an ice bath. The time at

which the pipette has been half emptied into the

water in the titration flask is recorded, together

with the titrant volume. Additional samples were

taken at 10-min intervals for an hour; then at 20-

min intervals for the next hour and a half. The

average values were determined by processing

samples for 1 hour and 3 hour intervals.

In similar fashion, another run was madeon a temperature of 35°. Because of the higher

rate of reaction, three samples are first taken at 5-

min intervals, then several at 10-min intervals, and

a few at 20-min intervals. To get the average

values, a 1 hour interval run was made.

RESULTS AND DISCUSSIONS

A. At Room Temperature and Low Concentration

Reaction

Time,

minutes

Volume

NaOH

used, mL

Conc.

Acetic Acid,

M

Conc.

Methyl

Acetate

left, M

0 2.7 0.2847 0.8532

10 2.9 0.3058 0.8321

20 3.0 0.3164 0.8216

30 4.0 0.4218 0.7161

40 4.2 0.4429 0.6950

50 4.3 0.4535 0.6845

60 4.5 0.4745 0.6634

80 4.6 0.4851 0.6529

100 4.9 0.5167 0.6212

120 5.1 0.5378 0.6001

180 5.4 0.5695 0.5685

360 6.2 0.6538 0.4841

Results show that methyl acetate is being

consumed as it is in the reactant side while acetic

acid is being produced as it is in the product side.

More NaOH was used as the reaction further took

place since the concentration of acetic acid

increases with time and thus, requiring more NaOH

to reach its equivalence point during titration.

This part will give us the basis of

comparison later on in the determination of the










effect of increasing the amount of catalyst (HCl)

present and of increasing the temperature of the

reaction vessel. Two theories (kinetic and collision)

shall be the basis for comparison.

B. At Room Temperature and High Concentration

Reaction

Time,

minutes

Volume

NaOH

used, mL

Conc.

Acetic Acid,

M

Conc.

Methyl

Acetate

left, M

0 2.8 0.4060 0.3763

10 3.1 0.4495 0.3328

20 3.3 0.4785 0.303830 3.5 0.5075 0.2748

40 3.6 0.5220 0.2603

50 3.9 0.5655 0.2168

60 4.2 0.6090 0.1733

80 4.5 0.6525 0.1298

100 4.7 0.6815 0.1008

120 4.8 0.6960 0.0863

180 5.0 0.7250 0.0573

360 5.1 0.7395 0.0428

As compared to part A, part B, having a

higher concentration of catalyst (100ml versus

50ml), have had a faster rate of reaction as evident

on the higher concentration of acetic acid

produced after initiating the reaction at time 0

until time equals 6 hours.

The catalyst works by providing the

optimal conditions for a reaction to occur. The

more catalyst there is, the more contact with the

reactants there is, thus, increasing the rate of

reaction until it reaches its maximum speed.

However, adding too much catalyst on the system

might not help hasten the reaction rate especially

when it already dilutes the system from its optimal

reaction conditions with useless catalyst sites.

This proves the capacity of the catalyst to

initiate and hasten the rate of reaction by

providing the best possible reaction conditions that

require minimal activation energy.

C. At High Temperature and Low Concentration

Reaction

Time,

minutes

Volume

NaOH

used, mL

Conc.

Acetic Acid,

M

Conc.

Methyl

Acetate

left, M

0 2.1 0.4779 0.6600

10 2.5 0.5690 0.5690

20 2.7 0.61445 0.523530 3.1 0.7055 0.4324

40 3.4 0.7738 0.3641

50 3.7 0.8421 0.2959

60 4.0 0.9104 0.2276

80 4.2 0.9559 0.1821

100 4.7 1.0697 0.0683

120 4.8 1.0924 0.0455

180 4.9 1.1152 0.0228

The effect of increasing the temperature

was studied in this part. It was found that it made

the rate of reaction faster as evident on the higher

concentrations of acetic acid produced as

compared to part A (35 versus 25 oC).

Increasing the temperature increases the

kinetic energy of the molecules (kinetic theory).

The more energy there is, the more the molecules

move, thus, increasing the number of collisions

taking place in the reaction vessel. The more

collisions there is, the more the reactants react,

thus, leading to a faster rate of reaction compared

to “non-excited” molecules (collision theory).

CONCLUSION

In summary, the rate of reaction was found

to be dependent on several factors such as the










concentration, catalyst, temperature and other

parameters such as pressure and surface area

which were kept constant throughout the

experiment.

It was found that increasing the amount of

catalyst hastens the rate of reaction by providing

the optimum reaction conditions, that is, by

lowering the activation energy needed for the

reaction to initiate.

It was also found that increasing the

temperature hastens the rate of reaction as well by

providing the molecules more kinetic energy which

leads to more collisions within the system whichinitiates reactions faster as based on kinetic and

collision theories of chemistry.

APPENDICES

Preliminary Data Sheet

*Please refer to the attached document for the

preliminary data sheet.

Kinetics Graphs

*Please refer to the attached document for the

graphs.

Sample Computations

Given: Hydrolysis of Methyl Acetate

Vol. of 1.16 M HCl = 50 ml

Vol. of water = 50 ml

Vol. of MeAc = 10 ml

Required: concentration of HCl in mixture

Solution:

⁄ ()

= 0.527272727 M

Given: Hydrolysis of Methyl Acetate

K1 (low conc.) = 0.0041

K2 (high conc.) = 0.00004

K3 (high temp.) = 0.00003

Required: activation energy

Solution: Arrhenius Equation

[

]

Substituting the values,

[

]

⁄

REFERENCES

Knight, S.B, Crockford, H.B., Fundamentals of

Physical Chemistry 2nd ed., Wiley International,

1964, {Accessed: 02-26-11}

Atkins, Physical Chemistry 5th

edition, chapter 24,

page 841, W.H. Freeman & Company, {Accessed:02-26-11}

Part A: Hydrolysis Rate at Room Temperature and Lower Concentration of HCl










y = -0.002x + 0.8144R² = 0.8665

0

0.2

0.4

0.6

0.8

1

0 50 100 150

time,min

Zero Order

y = -0.0029x - 0.2019R² = 0.8925

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

00 50 100 150

l n Y b

time, min

1st Order

y = 0.0041x + 1.218R² = 0.9163

0

0.5

1

1.5

2

0 50 100 1

1 / y b

time,min

2nd order

y = -0.0004x - 2.4539R² = 0.7709

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

0 50 100 150

l n Y b

time, min

1st Order

y = 0.0042x + 1.2056R² = 0.9122

0

0.5

1

1.5

2

0 50 100 1

1 / y b

time,min

2nd order

y = -0.0003x + 2.4643R² = 0.8412

-5

-4

-3

-2

-1

0

0 50 100 150 200

1st order

y = -0.0051x + 11.634R² = 0.7638

0

0.2

0.4

0.6

0.8

1

0 50 100 150 m o l e s a c e t i c a c i d ,

Y b

time,min

Zero Order

Part B: Hydrolysis Rate at Room Temperature and Higher Concentration of HCl

Part C: Hydrolysis of Rate at High Temperature and Lower Concentration of HCl

y = -0.0037x + 11.754R² = 0.8378

-0.2

0

0.2

0.4

0.6

0.8

0 50 100 150 200

zero order

y = 3E-05x + 0.0851

R² = 0.8446

-10

0

10

20

30

40

50

0 50 100 150 2

2nd order

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Expt 6 - Chemical Kinetics