AMERICAN JOURNAL OF SCIENTIFIC AND INDUSTRIAL RESEARCH
© 2015,Science Huβ, http://www.scihub.org/AJSIR
ISSN: 2153-649X, doi:10.5251/ajsir.2015.6.1.1.4
Reaction rate and rate constant of the hydrolysis of ethyl acetate with
sodium hydroxide
Ikhazuangbe, Prosper Monday Ohien and Oni, Aisosa Babalola
Madonna university, Faculty of Engineering
Chemical Engineering Department, P.m.b 302
Enugu, Nigeria
ABSTRACT
Hydrolysis is a chemical decomposition involving breaking of a bond and the addition of elements
of water. In this hydrolysis of ester (ethyl acetate) with an alkali (sodium hydroxide), HCl was
used as catalyst to accelerate it. 1ml and 2ml of ethyl acetate was injected separately into the
500ml reactor vessels which contains 0.05N of NaOH and thoroughly mixed. At regular time
interval, 0, 10, 20, 30, 40, and 90 minutes, 25ml of each of the samples were withdrawn into a
250ml conical flask containing 0.05N HCl, and titrated against 0.05N NaOH solution using
phenolphthalein as indicator. From the titre values, the hydrolysis involving the 1ml ethyl acetate
was faster than that of the 2ml ethyl acetate, indicating that the higher the concentration the faster
the rate of reaction. The rate constant after evaluation from the graphs was approximately
-1
-3
0.003min cm for the 1ml and 2ml ethyl acetate, signifying that while the rate of reaction is
concentration dpendent, the rate constant is not dependent on concentration
Key words: Alkali, concentration, ester, hydrolysis and reactor
product is formed. The rate at which the reactant ‘a’
is disappearing is proportional to its concentration at
any instance,
INTRODUCTION
Chemical reaction takes place when a detectable
number of molecules of one or more species have
lost their identity and assumed a new form by a
change in the kind or number of atoms in the
compound and by a change in structure or
configuration of these atoms. In this classical
approach to chemical change, it is assumed that the
total mass is neither created nor destroyed when a
chemical reaction occurs.
I.e. rate (r)
(a - x)
Rate (r) = k (a - x)
Where k = rate constant
The concept of rate of reaction is very important in
evaluating chemical reacting systems. It is the core
factor in the development of performance models to
stimulate reactor functional parameters.
Chemical kinetics is the part of physical chemistry
that studies reaction rates. The reaction rate for a
reactant or product in a particular reaction is
intuitively defined as how fast a reaction takes place.
For example, oxidation of iron under the atmosphere
is a slow reaction which can take years, but the
combustion of butane in a fire is a reaction that takes
place in seconds.
Factors which determine rate of reaction
a. Availability of reactants and its surface area.
The greater the surface area of a solid, the
greater the rate of reaction.
b. Concentration: increase in concentration
increase the rate of reaction
c. Pressure: increase in pressure results in an
increase in the rate of reaction, if the
reactants and products are gaseous
d. Catalyst: The presences of a catalyst
generally increase the rate of reaction. There
are however, negative catalysts that lower
the rate of reactions.
Rate of reaction: The rate of reaction is defined as
the change in the number of molecules of reacting
species per unit volume per unit time. It is also
defined to be proportional to the concentration of
reacting species raised to a certain power called the
order of reaction. It is usually taken as the rate at
which the reactant disappear or the rate at which the
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Am. J. Sci. Ind. Res., 2015, 6(1): 1-4
e. Temperature: increase
increase rate of reaction
in
temperature
measure of the rate of reaction, so the greater the
value of the rate constant, the faster the reaction.
Each reaction has a definite value of the rate
constant at a particular temperature and the value of
the rate constant for the same reaction changes with
temperature and the values do not depend upon the
concentration of reaction but depend upon order of
reaction.
Rate constant: Rate constant, K quantifies the
speed of a chemical reaction. For a chemical reaction
where substance A and B are reacting to produce C,
the reaction rate has the form:
Reaction:
A+B
C
m
r = K[A] [B]
n
Hydrolysis of an ester such as acetate in
presence of a mineral acid: Hydrolysis is a
chemical decomposition involving breaking of a bond
and the addition of elements of water. The use of an
acid catalyst accelerates the hydrolysis. The reaction
rate is expressed in terms of chemical composition of
the reacting species.
Where K is the rate constant that depends on
temperature
A is the concentration of substance A in moles per
volume of solution assuming the reaction is taking
place throughout the volume of the solution.
i. The hydrolysis of an ester such as ethyl acetate in
the presence of a mineral acid gives acetic acid and
ethyl alcohol.
Rate constant is the rate of reaction when the
concentration of each reaction is taken as unity. That
is why it is also known as specific reaction rate. It is a
CH3COOC2H5
+
H2O
Ethyl acetate
CH3COOH + C2H5OH
acetic acid
H+
ethyl alcohol
Therefore, the kinetics of the reaction can be studied
by taking a known quantity of ethyl acetate and
mixing it with a relatively large quantity of HCl. An
aliquot of the reaction mixture is withdrawn at
different intervals of time and titrated against a
standard alkali. Obviously, as the reaction proceeds,
the value of alkali required to neutralize the acid (HCl
present as catalyst + CH3COOH produced by
hydrolysis of the ester) progressively increases.
And verifying the constancy of the value of rate
constant, K or in this case, (a - x) ( - Vo) and a
(
Vo)
The fact that this is a first order reaction is
established by substituting the results in the first
order rate expression;
Vt = titre value at the various time intervals chosen
K=
K=
2
Where, Vo = initial titre value
= final titre value at the end of the experiment and
As the hydrolysis proceeds, there will be proportional
increase in the concentration of acetic acid formed.
log
ii. The hydrolysis of ethyl acetate illustrates a
bimolecular reaction that gives sodium acetate and
ethanol as the product from which second order rate
constant can be calculated:
1
CH3COOC2H5
Ethyl acetate
+
NaOH
sodium hydroxide
log
CH3COONa
sodium acetate
As the reaction proceeds, each hydroxide ion
removed in the formation of ethanol removes one
molecule of ethyl acetate, being the number of moles
of either OH or ethyl acetate so removed, the
concentration of the reactants decreases. If we start
with equal concentrations of the reactants, the
concentration of NaOH can be conveniently followed
at different time intervals by withdrawing an aliquot
from the reaction mixture and determining the NaOH
+
C2H5OH
ethanol
present in it direct or indirect titration with a standard
solution of an acid.
The fact that it is a second order reaction can be
established by the consistency of the values of K
determined by substituting the titration results at
different time intervals in the second order rate
equation as follows:
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Am. J. Sci. Ind. Res., 2015, 6(1): 1-4
slope, the value of the rate constant, K can be
evaluated.
K= .
3
Experimental procedure: 25ml of 0.05N HCl was
measured with a pipette into each of eight (6)
numbered 250ml conical flasks. 400ml of 0.05N
NaOH solution was placed in the reactor (500ml
conical flask immersed in a thermostatic water bath
o
set at 28 C). At time zero, 1ml of ethyl acetate was
also measured with a syringe into the reaction vessel
and the mixture thoroughly shaken for ten seconds.
25ml of the sample was withdrawn ( and at time 10,
20, 30, 40 and 90minutes after the reaction has gone
to completion) and discharged as rapidly as possible
into the respective 250ml conical flask containing the
25ml 0.05N HCl already and titrated against 0.05N of
NaOH solution using two drops phenolphthalein as
indicator. The experiment was repeated with the
volume of ethyl acetate increased to 2ml at the same
temperature.
Where Vo is the volume of the acid equivalent to the
alkali present at the beginning of the experiment and
Vt is the volume of the acid equivalent to the alkali
present at any selected time interval. Obviously, Vt is
proportional to (a - x) and Vo is proportional to ‘a’ in
the original rate equation for the second order
reaction of TYPE I, viz:
K= .
4
If we start with different concentrations of the
reactants, the second order rate equation TYPE II
should be used for establishing the reaction to be of
second order.
K=
. log
5
All reagents used in this experiment are of analar
grade.
If ‘b’ moles of ester and ‘a’ moles of NaOH are taken
initially, then
o
Table 2: Temperature = 28 C and volume of ethyl
acetate = 1ml
‘a’ Vo (the volume of acid equivalent to the amount
of NaOH present initially)
(a - x)
Vt (the volume of acid equivalent to the
amount of NaOH present at time, t)
(a - b)
(the volume of acid equivalent to the
amount of NaOH present at the end of the reaction)
X = a – (a - x) = Vo - Vt
b= a – (a - b) = Vo Therefore,
(b - x) = a – (a - b) – [a – (a - x)]
= a – (a - b) – a + (a - x)
= (a - x) – (a - b) = Vt By substitution, equation 5 becomes
K=
. log
Time
(min)
Titre
(ml)
Vt -
(Vo
)
-
Log
0
5.60
-16.70
-16.70
1.00
0.00
10
17.80
-4.50
-16.70
3.71
0.57
20
20.90
-1.40
-16.70
11.93
1.08
30
21.60
-0.70
-16.70
23.86
1.38
40
21.90
-0.40
-16.70
41.75
1.62
90
22.30
0.00
-16.70
-
-
6
K=
Log
[
]
=
o
Table 1: Temperature = 28 C and volume of ethyl
acetate = 2ml
+ log
7
Since this equation is of the form: y = mx + c, this
represents an equation for a straight line.
Therefore, a plot of log
line with a slope equal to
Time
(min)
Titre
(ml)
Vt -
(Vo
)
-
Log
0
9.70
-15.20
-15.20
1.00
0.00
10
22.80
-2.10
-15.20
7.24
0.86
20
24.00
-0.90
-15.20
16.89
1.23
vs. t gives a straight
. From the value of the
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Am. J. Sci. Ind. Res., 2015, 6(1): 1-4
30
24.40
-0.50
-15.20
30.40
1.48
40
24.70
-0.20
-15.20
76.00
1.88
gives a slope which is equal to
evaluated:
90
24.90
0.00
-15.20
-
-
Slope = 0.034min =
, from which K is
-1
Therefore,
Log
(
∞)
0.034 =
( 𝑡 ∞)
1.8
K=
1.6
K = 0.0031
1.4
K
1.2
0.8
From equation 7, Log
0.6
table 1, a plot of Log
0.4
=
0
0
10
20
30
40
+ log
. Using
against time in figure 1,
gives a slope which is equal to
evaluated:
0.2
, from which K is
-1
Slope = 0.027min =
50
Tiime (min)
Therefore,
o
Fig.2: graph for 1ml of ethyl acetate at 28 C
Log
-3
Also, for the 2ml ethyl acetate:
1
(
-1
0.003min cm
0.027 =
K=
∞2)
= 0.0028
( 𝑡 1.8
∞)
-1
K
1.4
Conclusion: In conclusion, it can be seen from this
experiment that rate of reaction is concentration
dependent while rate constant is not. Also, that the
rate constant for the hydrolysis of ethyl acetate with
o
sodium hydroxide using HCl as a catalyst at 28 C is
-1
-3
approximately 0.003min cm .
1.2
1
0.8
0.6
0.003min cm
-3
1.6
0.4
REFERENCE
0.2
E.U. Akusoba and G.O. Ewelukwa, (2006); Calculations in
chemistry for senior secondary schools, Africana first
publishers limited, Owerri, Nigeria. Pg 123 – 124
0
0
10
20
30
Time (min)
40
50
H.S. Fogler (2008); Elements of chemical reaction
engineering, Fourth edition, Pearson Education
international, Massachusetts, USA. Pg. 4-7
o
Fig.1: graph for 2ml of ethyl acetate at 28 C
For the 1ml ethyl acetate:
From equation 7, Log
table 2, a plot of Log
=
+ log
Octave Levenspiel, (2008); chemical reaction engineering,
rd
3 edition, Choudhary Press, Delhi, India. Pg 27 -28
, using
S.S. Dara (2009); A textbook of engineering chemistry, S.
Chand and company Limited, Ram Nagar, New Delhi.
pg.845 – 862
against time in figure 2,
4